An adult is comfortable at rest at sea level. This is a reference exercise, not a test excluding all disease. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 95 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

No acid base disturbance is apparent. pH 7.40 lies within the teaching range 7.35–7.45. PaCO2 40 mmHg and bicarbonate 24 mmol/L are close to the reference values used in this module. There is no abnormal primary process for which to calculate compensation. PaO2 95 on room air is not concerning in this fictional context.

Mixed respiratory and metabolic alkalosis

An adult is comfortable at rest at sea level. This is a reference exercise, not a test excluding all disease. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 95 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

The acid base values are in the teaching reference ranges; this does not exclude illness. pH 7.40 lies within the teaching range 7.35–7.45. PaCO2 40 mmHg and bicarbonate 24 mmol/L are close to the reference values used in this module. There is no abnormal primary process for which to calculate compensation. PaO2 95 on room air is not concerning in this fictional context.

A normal pH excludes every mixed disorder

Normal PaO2 excludes pulmonary embolism

The bicarbonate proves chronic respiratory disease

An adult with known diabetes has vomiting, dehydration and a raised blood ketone measurement. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 98 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

High anion gap metabolic acidosis with expected respiratory compensation. pH 7.29 is acidemic and bicarbonate 12 is low. Anion gap = 140 − (102 + 12) = 26 mmol/L, above this exercise’s reference 8–12. Winter’s formula = 1.5 × 12 + 8 = 26 ± 2 mmHg. Measured PaCO2 26 fits. The gas supports the acid base pattern; the diagnosis and treatment require clinical and metabolic assessment.

An adult with known diabetes has vomiting, dehydration and a raised blood ketone measurement. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 98 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Winter’s expected PaCO2 is 26 plus or minus 2 mmHg. pH 7.29 is acidemic and bicarbonate 12 is low. Anion gap = 140 − (102 + 12) = 26 mmol/L, above this exercise’s reference 8–12. Winter’s formula = 1.5 × 12 + 8 = 26 ± 2 mmHg. Measured PaCO2 26 fits. The gas supports the acid base pattern; the diagnosis and treatment require clinical and metabolic assessment.

Expected PaCO2 is 40 mmHg in every metabolic acidosis

PaCO2 26 proves an additional respiratory alkalosis

Compensation should make the pH exactly 7.40

An adult has several days of profuse diarrhoea. Albumin is normal. pH 7.32, PaCO2 32 mmHg, HCO3 16 mmol/L, PaO2 96 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Normal anion gap metabolic acidosis with expected respiratory compensation. pH 7.32 and bicarbonate 16 identify metabolic acidosis. Anion gap = 138 − (110 + 16) = 12 mmol/L. Expected PaCO2 = 1.5 × 16 + 8 = 32 ± 2. Measured 32 fits. The context is compatible with gastrointestinal bicarbonate loss.

Metabolic alkalosis from bicarbonate loss

An adult has several days of profuse diarrhoea. Albumin is normal. pH 7.32, PaCO2 32 mmHg, HCO3 16 mmol/L, PaO2 96 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Anion gap is 12 and expected PaCO2 is 32 plus or minus 2 mmHg. pH 7.32 and bicarbonate 16 identify metabolic acidosis. Anion gap = 138 − (110 + 16) = 12 mmol/L. Expected PaCO2 = 1.5 × 16 + 8 = 32 ± 2. Measured 32 fits. The context is compatible with gastrointestinal bicarbonate loss.

A normal anion gap excludes metabolic acidosis

An adult has persistent vomiting. The scenario concerns the gas pattern, not an instruction to replace a particular fluid or electrolyte. pH 7.50, PaCO2 48 mmHg, HCO3 36 mmol/L, PaO2 92 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Metabolic alkalosis with a compatible respiratory response. pH 7.50 is alkalemic; bicarbonate 36 is raised. Bicarbonate has risen 12 above the reference 24. A PaCO2 rise of approximately 0.6–0.75 × 12 gives 47.2–49 mmHg. Measured 48 is compatible. These are approximate teaching rules, not exact physiologic boundaries.

Acute respiratory acidosis alone

An adult has persistent vomiting. The scenario concerns the gas pattern, not an instruction to replace a particular fluid or electrolyte. pH 7.50, PaCO2 48 mmHg, HCO3 36 mmol/L, PaO2 92 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

A PaCO2 around 47–49 mmHg is compatible with the raised bicarbonate. pH 7.50 is alkalemic; bicarbonate 36 is raised. Bicarbonate has risen 12 above the reference 24. A PaCO2 rise of approximately 0.6–0.75 × 12 gives 47.2–49 mmHg. Measured 48 is compatible. These are approximate teaching rules, not exact physiologic boundaries.

The expected PaCO2 must fall below 30

PaCO2 48 always proves primary respiratory acidosis

Compensation should lower bicarbonate to 24 immediately

Soon after sedation, an adult becomes drowsy and breathes slowly. The gas was obtained while help and airway support were being arranged. pH 7.26, PaCO2 60 mmHg, HCO3 26 mmol/L, PaO2 61 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Acute respiratory acidosis with hypoxaemia. pH 7.26 is acidemic and PaCO2 60 is raised. The short time course supports an acute ventilatory problem. For a 20 mmHg CO2 rise, an acute bicarbonate response of about 2–4 mmol/L is plausible; measured 26 fits the lower end. PaO2 61 on air is low. The bedside breathing problem needs immediate assessment, not just a label.

Chronic respiratory acidosis is proved by this one gas

Soon after sedation, an adult becomes drowsy and breathes slowly. The gas was obtained while help and airway support were being arranged. pH 7.26, PaCO2 60 mmHg, HCO3 26 mmol/L, PaO2 61 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

An acute bicarbonate rise of roughly 2–4 mmol/L is expected for this CO2 rise. pH 7.26 is acidemic and PaCO2 60 is raised. The short time course supports an acute ventilatory problem. For a 20 mmHg CO2 rise, an acute bicarbonate response of about 2–4 mmol/L is plausible; measured 26 fits the lower end. PaO2 61 on air is low. The bedside breathing problem needs immediate assessment, not just a label.

Bicarbonate should already be 40

The low pH is fully explained by a metabolic alkalosis

PaO2 is normal because PaCO2 is high

An adult has sudden tachypnoea and chest discomfort. The cause has not yet been established. pH 7.62, PaCO2 20 mmHg, HCO3 20 mmol/L, PaO2 100 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Acute respiratory alkalosis. pH 7.62 is alkalemic and PaCO2 20 is low. The CO2 has fallen 20 mmHg below 40. An acute bicarbonate decrease of about 2–4 mmol/L is compatible; measured bicarbonate 20 fits. Pain, hypoxaemia, pulmonary vascular disease and other causes need clinical consideration; do not infer anxiety from the gas alone.

An adult has sudden tachypnoea and chest discomfort. The cause has not yet been established. pH 7.62, PaCO2 20 mmHg, HCO3 20 mmol/L, PaO2 100 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

A small acute bicarbonate fall is compatible; the cause of hyperventilation remains unresolved. pH 7.62 is alkalemic and PaCO2 20 is low. The CO2 has fallen 20 mmHg below 40. An acute bicarbonate decrease of about 2–4 mmol/L is compatible; measured bicarbonate 20 fits. Pain, hypoxaemia, pulmonary vascular disease and other causes need clinical consideration; do not infer anxiety from the gas alone.

Bicarbonate 20 must mean a primary metabolic acidosis

A high PaO2 excludes pulmonary vascular disease

A stable adult with chronic hypercapnic lung disease has prior similar gases over several weeks. pH 7.35, PaCO2 60 mmHg, HCO3 32 mmol/L, PaO2 62 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Chronic respiratory acidosis with an expected bicarbonate response. PaCO2 60 is raised and the documented time course is chronic. A chronic rise of approximately 3–4 mmol/L per 10 mmHg gives bicarbonate 30–32. Measured bicarbonate 32 is compatible; pH rounds to 7.35 and must not be called normal solely from that rounding. PaO2 62 is low on air and still needs interpretation in the clinical context.

Normal gas because pH rounds to 7.35

A stable adult with chronic hypercapnic lung disease has prior similar gases over several weeks. pH 7.35, PaCO2 60 mmHg, HCO3 32 mmol/L, PaO2 62 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

For a 20 mmHg CO2 rise, bicarbonate around 30–32 mmol/L is expected chronically. PaCO2 60 is raised and the documented time course is chronic. A chronic rise of approximately 3–4 mmol/L per 10 mmHg gives bicarbonate 30–32. Measured bicarbonate 32 is compatible; pH rounds to 7.35 and must not be called normal solely from that rounding. PaO2 62 is low on air and still needs interpretation in the clinical context.

Chronic compensation requires bicarbonate exactly 24

A near normal pH rules out respiratory acidosis

An adult has a documented sustained hyperventilatory state over several days. No sudden change is described. pH 7.42, PaCO2 30 mmHg, HCO3 19 mmol/L, PaO2 94 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Chronic respiratory alkalosis with a compatible bicarbonate response. PaCO2 30 is low. The history supplies the sustained time course. The chronic bicarbonate response is approximately a fall of 4–5 mmol/L per 10 mmHg CO2 reduction. Bicarbonate 19 and pH 7.42 are compatible. The clinical cause still needs assessment.

An adult has a documented sustained hyperventilatory state over several days. No sudden change is described. pH 7.42, PaCO2 30 mmHg, HCO3 19 mmol/L, PaO2 94 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

A chronic bicarbonate fall to about 19–20 mmol/L fits a 10 mmHg CO2 fall. PaCO2 30 is low. The history supplies the sustained time course. The chronic bicarbonate response is approximately a fall of 4–5 mmol/L per 10 mmHg CO2 reduction. Bicarbonate 19 and pH 7.42 are compatible. The clinical cause still needs assessment.

A pH within range excludes a primary disturbance

Bicarbonate must increase with respiratory alkalosis

Chronicity can be inferred from a single low PaCO2 without history

An adult with ketoacidosis becomes exhausted and less responsive. pH 7.02, PaCO2 40 mmHg, HCO3 10 mmol/L, PaO2 70 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Metabolic acidosis with additional respiratory acidosis. pH 7.02 indicates marked acidemia. Bicarbonate 10 identifies metabolic acidosis. Winter’s expected PaCO2 = 1.5 × 10 + 8 = 23 ± 2 mmHg. Measured 40 is well above that range, indicating an added respiratory acidosis in this context. The deterioration requires urgent senior assessment.

Appropriately compensated metabolic acidosis

Normal ventilation because PaCO2 is 40

An adult with ketoacidosis becomes exhausted and less responsive. pH 7.02, PaCO2 40 mmHg, HCO3 10 mmol/L, PaO2 70 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Expected PaCO2 is 23 plus or minus 2; measured 40 is inappropriately high. pH 7.02 indicates marked acidemia. Bicarbonate 10 identifies metabolic acidosis. Winter’s expected PaCO2 = 1.5 × 10 + 8 = 23 ± 2 mmHg. Measured 40 is well above that range, indicating an added respiratory acidosis in this context. The deterioration requires urgent senior assessment.

A normal range PaCO2 always means adequate ventilation

An adult with infection and circulatory compromise is breathing rapidly. Lactate is raised. pH 7.47, PaCO2 20 mmHg, HCO3 14 mmol/L, PaO2 85 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Metabolic acidosis with additional respiratory alkalosis. Bicarbonate 14 is low despite the alkalemic pH of 7.47. Expected PaCO2 = 1.5 × 14 + 8 = 29 ± 2. Measured 20 is lower than expected, supporting an additional respiratory alkalosis. Anion gap = 140 − (102 + 14) = 24 mmol/L.

An adult with infection and circulatory compromise is breathing rapidly. Lactate is raised. pH 7.47, PaCO2 20 mmHg, HCO3 14 mmol/L, PaO2 85 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Expected PaCO2 is 29 plus or minus 2; measured 20 is lower than expected. Bicarbonate 14 is low despite the alkalemic pH of 7.47. Expected PaCO2 = 1.5 × 14 + 8 = 29 ± 2. Measured 20 is lower than expected, supporting an additional respiratory alkalosis. Anion gap = 140 − (102 + 14) = 24 mmol/L.

A pH above 7.45 excludes metabolic acidosis

Low bicarbonate must always make the overall pH low

An adult with substantial renal impairment has nausea and tachypnoea. Albumin is normal. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 93 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

High anion gap metabolic acidosis with expected respiratory compensation. pH 7.29 and low bicarbonate indicate metabolic acidosis. Anion gap = 142 − (104 + 12) = 26. Expected PaCO2 is 26 ± 2; measured 26 fits. The case is consistent with the context but a raised gap still requires clinical evaluation for its contributors.

Compensation has failed because pH is not 7.40

An adult with substantial renal impairment has nausea and tachypnoea. Albumin is normal. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 93 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Anion gap is 26 and PaCO2 26 matches Winter’s expected value. pH 7.29 and low bicarbonate indicate metabolic acidosis. Anion gap = 142 − (104 + 12) = 26. Expected PaCO2 is 26 ± 2; measured 26 fits. The case is consistent with the context but a raised gap still requires clinical evaluation for its contributors.

All acidosis in renal disease is necessarily normal gap

A prior stable gas was PaCO2 60 and bicarbonate 32. Over the last hour the patient has become drowsier. Current oxygen is a controlled FiO2 of 0.28. pH 7.25, PaCO2 80 mmHg, HCO3 34 mmol/L, PaO2 58 mmHg, FiO2 0.28. Which interpretation best fits the data and history?

Acute on chronic respiratory acidosis. The baseline already documented chronic hypercapnia. PaCO2 has acutely risen another 20 mmHg, while bicarbonate rose from 32 to 34. An acute buffering response on top of a chronic baseline fits pH 7.25. PaO2 is low despite supplemental oxygen. Interpret the whole clinical deterioration urgently.

A new fully compensated chronic state is proved

A prior stable gas was PaCO2 60 and bicarbonate 32. Over the last hour the patient has become drowsier. Current oxygen is a controlled FiO2 of 0.28. pH 7.25, PaCO2 80 mmHg, HCO3 34 mmol/L, PaO2 58 mmHg, FiO2 0.28. Which statement best checks your interpretation?

The prior baseline and acute CO2 increase explain the modest further bicarbonate rise. The baseline already documented chronic hypercapnia. PaCO2 has acutely risen another 20 mmHg, while bicarbonate rose from 32 to 34. An acute buffering response on top of a chronic baseline fits pH 7.25. PaO2 is low despite supplemental oxygen. Interpret the whole clinical deterioration urgently.

Use only the current pH to decide whether the process is chronic

A bicarbonate of 34 means ventilation is adequate

The CO2 change needs no clinical assessment

An adult with chronic hypercapnia has several days of vomiting and recent diuretic exposure. pH 7.50, PaCO2 60 mmHg, HCO3 45 mmol/L, PaO2 65 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Respiratory acidosis with additional metabolic alkalosis. PaCO2 60 is raised, but pH 7.50 is alkalemic. Chronic compensation for CO2 60 would give bicarbonate about 30–32. Bicarbonate 45 exceeds that response and supports additional metabolic alkalosis.

Simple chronic respiratory acidosis

An adult with chronic hypercapnia has several days of vomiting and recent diuretic exposure. pH 7.50, PaCO2 60 mmHg, HCO3 45 mmol/L, PaO2 65 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Expected chronic bicarbonate is about 30–32, well below the measured 45. PaCO2 60 is raised, but pH 7.50 is alkalemic. Chronic compensation for CO2 60 would give bicarbonate about 30–32. Bicarbonate 45 exceeds that response and supports additional metabolic alkalosis.

Any raised bicarbonate is normal compensation

The alkalemic pH means PaCO2 cannot be abnormal

An adult has vomiting and a concurrent hypoventilatory illness. pH 7.37, PaCO2 65 mmHg, HCO3 36 mmol/L, PaO2 55 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Metabolic alkalosis with additional respiratory acidosis. pH 7.37 is within the reference interval, but both PaCO2 and bicarbonate are abnormal. For bicarbonate 36, a compensatory PaCO2 around 47–49 is expected by the chosen teaching rule. Measured PaCO2 65 indicates an additional respiratory acidosis. PaO2 is also low.

Normal acid base balance because pH is 7.37

Simple compensated metabolic alkalosis

An adult has vomiting and a concurrent hypoventilatory illness. pH 7.37, PaCO2 65 mmHg, HCO3 36 mmol/L, PaO2 55 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

PaCO2 65 is substantially above the roughly 47–49 expected for bicarbonate 36. pH 7.37 is within the reference interval, but both PaCO2 and bicarbonate are abnormal. For bicarbonate 36, a compensatory PaCO2 around 47–49 is expected by the chosen teaching rule. Measured PaCO2 65 indicates an additional respiratory acidosis. PaO2 is also low.

PaCO2 65 is the expected response to bicarbonate 36

The reference range pH proves recovery

A normal pH makes compensation calculations unnecessary

An adult with poor nutrition and acute illness has albumin 2 g/dL. The lab’s teaching anion gap reference is 8–12 without potassium. pH 7.33, PaCO2 35 mmHg, HCO3 18 mmol/L, PaO2 90 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

Metabolic acidosis with an elevated albumin adjusted anion gap. pH 7.33 and bicarbonate 18 indicate metabolic acidosis. Unadjusted gap = 138 − (108 + 18) = 12. Albumin adjusted gap ≈ 12 + 2.5 × (4 − 2) = 17 mmol/L. This is an approximation, interpreted against the local method. Winter’s expected PaCO2 = 35 ± 2; measured 35 fits.

No acidosis because the unadjusted gap is 12

An adult with poor nutrition and acute illness has albumin 2 g/dL. The lab’s teaching anion gap reference is 8–12 without potassium. pH 7.33, PaCO2 35 mmHg, HCO3 18 mmol/L, PaO2 90 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Measured gap 12 becomes about 17 after albumin adjustment. pH 7.33 and bicarbonate 18 indicate metabolic acidosis. Unadjusted gap = 138 − (108 + 18) = 12. Albumin adjusted gap ≈ 12 + 2.5 × (4 − 2) = 17 mmol/L. This is an approximation, interpreted against the local method. Winter’s expected PaCO2 = 35 ± 2; measured 35 fits.

Albumin never changes interpretation of the gap

An adult has tinnitus, vomiting and possible salicylate exposure. Toxicology assessment is underway. pH 7.40, PaCO2 20 mmHg, HCO3 12 mmol/L, PaO2 98 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

High anion gap metabolic acidosis with respiratory alkalosis. pH is 7.40, but bicarbonate and CO2 are both very low. Anion gap = 140 − (104 + 12) = 24. Winter’s expected PaCO2 is 26 ± 2; 20 is too low for compensation alone. The mixed pattern fits the fictional exposure history but does not establish a toxin concentration or treatment plan.

Simple compensated metabolic acidosis

An adult has tinnitus, vomiting and possible salicylate exposure. Toxicology assessment is underway. pH 7.40, PaCO2 20 mmHg, HCO3 12 mmol/L, PaO2 98 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Expected PaCO2 is 26 plus or minus 2; measured 20 supports an additional respiratory alkalosis. pH is 7.40, but bicarbonate and CO2 are both very low. Anion gap = 140 − (104 + 12) = 24. Winter’s expected PaCO2 is 26 ± 2; 20 is too low for compensation alone. The mixed pattern fits the fictional exposure history but does not establish a toxin concentration or treatment plan.

Bicarbonate 12 is a normal acute response to any low CO2

An adult has ketosis and substantial diarrhoea. Assume baseline bicarbonate 24 and baseline anion gap 12 for this exercise. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 97 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

High gap metabolic acidosis with evidence suggesting an additional normal gap metabolic acidosis. Gap = 140 − (108 + 12) = 20. Gap rise = 20 − 12 = 8; bicarbonate fall = 24 − 12 = 12. The rise is smaller than the bicarbonate fall; with diarrhoea this suggests another normal gap acidosis. Delta comparisons are approximate and baseline dependent. Expected PaCO2 = 26 ± 2, matching the measured 26.

An adult has ketosis and substantial diarrhoea. Assume baseline bicarbonate 24 and baseline anion gap 12 for this exercise. pH 7.29, PaCO2 26 mmHg, HCO3 12 mmol/L, PaO2 97 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

Gap rise is 8 while bicarbonate fall is 12; the clinical context supports an additional normal gap process. Gap = 140 − (108 + 12) = 20. Gap rise = 20 − 12 = 8; bicarbonate fall = 24 − 12 = 12. The rise is smaller than the bicarbonate fall; with diarrhoea this suggests another normal gap acidosis. Delta comparisons are approximate and baseline dependent. Expected PaCO2 = 26 ± 2, matching the measured 26.

A delta comparison independently proves a diagnosis in every patient

The measured PaCO2 is above Winter’s range

An adult has prolonged vomiting plus independently documented ketonaemia. Assume baseline bicarbonate 24, gap 12 and normal albumin. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 96 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

High gap metabolic acidosis masked by a concurrent metabolic alkalosis. pH 7.40, bicarbonate 24 and PaCO2 40 look normal in isolation. Gap = 140 − (86 + 24) = 30. With the stated baseline and ketonaemia, the high gap plus preserved bicarbonate supports concurrent metabolic alkalosis from the history. Do not apply Winter’s formula mechanically to this net normal bicarbonate to diagnose an additional respiratory disorder.

The electrolytes and history can be ignored because the gas is normal

A normal bicarbonate excludes a metabolic acidosis

An adult has prolonged vomiting plus independently documented ketonaemia. Assume baseline bicarbonate 24, gap 12 and normal albumin. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 96 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

The gap is 30; a raised gap with unchanged bicarbonate supports opposing metabolic processes in this context. pH 7.40, bicarbonate 24 and PaCO2 40 look normal in isolation. Gap = 140 − (86 + 24) = 30. With the stated baseline and ketonaemia, the high gap plus preserved bicarbonate supports concurrent metabolic alkalosis from the history. Do not apply Winter’s formula mechanically to this net normal bicarbonate to diagnose an additional respiratory disorder.

Use Winter’s formula on normal bicarbonate to prove a respiratory disorder

The gas alone diagnoses the exact cause of the raised gap

An adult receives a measured FiO2 of 0.50 through a controlled system. The gas is arterial. Imaging, PEEP and timing are not supplied. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 55 mmHg, FiO2 0.5. Which interpretation best fits the data and history?

No acid base disturbance apparent, but substantial oxygenation impairment. pH, PaCO2 and bicarbonate are at their teaching reference values. PaO2 55 despite FiO2 0.50 is abnormal. P/F ratio = 55 ÷ 0.50 = 110 mmHg. ARDS classification requires additional clinical, imaging and respiratory support criteria; these are not supplied.

This gas alone proves an ARDS diagnosis

An adult receives a measured FiO2 of 0.50 through a controlled system. The gas is arterial. Imaging, PEEP and timing are not supplied. pH 7.40, PaCO2 40 mmHg, HCO3 24 mmol/L, PaO2 55 mmHg, FiO2 0.5. Which statement best checks your interpretation?

PaO2 divided by FiO2 is 110 mmHg; this ratio alone does not establish ARDS. pH, PaCO2 and bicarbonate are at their teaching reference values. PaO2 55 despite FiO2 0.50 is abnormal. P/F ratio = 55 ÷ 0.50 = 110 mmHg. ARDS classification requires additional clinical, imaging and respiratory support criteria; these are not supplied.

The ratio is 1.1 because FiO2 should be entered as 50

Normal pH means oxygenation is adequate

A copied worksheet reports arterial pH 7.40, PaCO2 60 and bicarbonate 24. You are asked to validate the transcription before interpreting it. pH 7.40, PaCO2 60 mmHg, HCO3 24 mmol/L, PaO2 90 mmHg, FiO2 0.21 (room air). Which interpretation best fits the data and history?

The three acid base values are internally inconsistent and need verification. Check pH ≈ 6.1 + log10[24 ÷ (0.03 × 60)] ≈ 7.22. The reported 7.40 is incompatible with the other two values beyond rounding. Verify the original report, units, sample identity and whether bicarbonate came from a separate sample. Obtain clinical review or repeat sampling when indicated. This case intentionally contains a transcription error. Do not silently fix a real report.

Fully compensated chronic respiratory acidosis

A copied worksheet reports arterial pH 7.40, PaCO2 60 and bicarbonate 24. You are asked to validate the transcription before interpreting it. pH 7.40, PaCO2 60 mmHg, HCO3 24 mmol/L, PaO2 90 mmHg, FiO2 0.21 (room air). Which statement best checks your interpretation?

The Henderson Hasselbalch check predicts pH about 7.22, not the reported 7.40. Check pH ≈ 6.1 + log10[24 ÷ (0.03 × 60)] ≈ 7.22. The reported 7.40 is incompatible with the other two values beyond rounding. Verify the original report, units, sample identity and whether bicarbonate came from a separate sample. Obtain clinical review or repeat sampling when indicated. This case intentionally contains a transcription error. Do not silently fix a real report.

Any three gas values are compatible if PaO2 is normal

Change the pH silently and use it clinically

ABG Interpretation Practice: Worked Cases With Answers

Educational scope: Fictional arterial gases for learning interpretation; not a diagnostic tool. Free, with no account or email needed.

Written by Dr. Ezekiel Aluda Osolika, emergency physician (MBChB, FEBEM). Clinical content reviewed by the author on 24 October 2026; review covered the case histories, answers, explanations, calculations, image interpretation, annotation guides and teaching diagrams. This is author review, not an independent second review. All patients and orders are fictional. Content version 2026-10-24-1. Spotted an error? Report a correction and name the case ID.

Start ABG practice

Choose a set, then work one case at a time. Each case has two scored questions: the interpretation, then the check that supports it. Bicarbonate is shown as the calculated value, and every sample is arterial.

Pressure units mmHg kPa

kPa = mmHg ÷ 7.50062. Switching changes the display only; answers are checked against the original mmHg values.

The interactive practice needs JavaScript. Every case, answer and explanation is also written out in the worked answers below.

How do you interpret an ABG step by step?

First confirm the sample and units. Check pH, identify the process that explains its direction, and compare the other values with the response expected for that process. Interpret oxygenation separately using the clinical setting and oxygen delivery information.
Teaching reference ranges are pH 7.35–7.45, PaCO₂ 35–45 mmHg and bicarbonate about 22–26 mmol/L. Reference intervals vary by laboratory. The equations use reference PaCO₂ 40 and bicarbonate 24. Compensation rules are approximate; history and time course matter.
For metabolic acidosis, expected PaCO₂ = 1.5 × bicarbonate + 8 ± 2 mmHg. A measured value above that range suggests an additional respiratory acidosis; one below suggests an additional respiratory alkalosis.
For metabolic alkalosis, this module uses an approximate CO₂ rise of 0.6–0.75 mmHg for each 1 mmol/L bicarbonate rise above 24. For respiratory disorders, distinguish acute from chronic responses using the history and the source table linked below.
Anion gap here excludes potassium: sodium − (chloride + bicarbonate). Use the laboratory’s reference and consider albumin. A normal pH can coexist with important opposing disturbances. Do not use a single score or formula as a clinical decision on its own.

Five-step ABG method: check the data, read the pH, find the primary process, compare the expected response, then look at the anion gap and oxygenation separately. — The five-step order used in every worked answer.

Common ABG interpretation mistakes

Reading the PaO₂ without the FiO₂. A PaO₂ of 100 mmHg is unremarkable on room air and a serious gas-exchange problem on an FiO₂ of 0.6. Interpret oxygenation against the oxygen being delivered.
Calling a normal pH “fully compensated”. Compensation rarely returns the pH all the way to normal. A normal pH with an abnormal PaCO₂ and an abnormal bicarbonate should make you look for two opposing processes (case 16 shows this).
Skipping the expected-response check. Without the expected PaCO₂ or bicarbonate for the primary process, a second disorder is easy to miss.
Forgetting albumin in the anion gap. Low albumin lowers the measured gap, so a raised-gap acidosis can hide behind a “normal” number. A common approximation adds about 2.5 mmol/L to the gap for every 1 g/dL that albumin falls below 4 g/dL (case 15).
Treating the gas, not the patient. The same numbers mean different things in a fit adult, a patient with chronic CO₂ retention and a patient who is tiring. The history and the trend decide what the result means.

Worked ABG cases with answers

Every case in full, with the correct answers and the reasoning. Open one when you have committed to your own answer.

Beginner: cases 1–6

Case 1 A reference gas Beginner

An adult is comfortable at rest at sea level. This is a reference exercise, not a test excluding all disease.

Arterial sample
pH	7.40
PaCO₂	40 mmHg
HCO₃ (calculated)	24 mmol/L
PaO₂	95 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: No acid base disturbance is apparent

Question 2: Which statement best checks your interpretation?

Answer: The acid base values are in the teaching reference ranges; this does not exclude illness

Worked answer

pH 7.40 lies within the teaching range 7.35–7.45.
PaCO₂ 40 mmHg and bicarbonate 24 mmol/L are close to the reference values used in this module.
There is no abnormal primary process for which to calculate compensation. PaO₂ 95 on room air is not concerning in this fictional context.

Common mistake: A reference gas is not a rule out test for a serious diagnosis.

Case 2 Ketones and fast breathing Beginner

An adult with known diabetes has vomiting, dehydration and a raised blood ketone measurement.

Arterial sample
pH	7.29
PaCO₂	26 mmHg
HCO₃ (calculated)	12 mmol/L
PaO₂	98 mmHg
FiO₂	0.21 (room air)
Sodium	140 mmol/L
Chloride	102 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: High anion gap metabolic acidosis with expected respiratory compensation

Question 2: Which statement best checks your interpretation?

Answer: Winter’s expected PaCO₂ is 26 plus or minus 2 mmHg

Worked answer

pH 7.29 is acidemic and bicarbonate 12 is low.
Anion gap = 140 − (102 + 12) = 26 mmol/L, above this exercise’s reference 8–12.
Winter’s formula = 1.5 × 12 + 8 = 26 ± 2 mmHg. Measured PaCO₂ 26 fits.
The gas supports the acid base pattern; the diagnosis and treatment require clinical and metabolic assessment.

Common mistake: Low PaCO₂ can be appropriate compensation, rather than a separate primary disorder.

Case 3 Losses from the gut Beginner

An adult has several days of profuse diarrhoea. Albumin is normal.

Arterial sample
pH	7.32
PaCO₂	32 mmHg
HCO₃ (calculated)	16 mmol/L
PaO₂	96 mmHg
FiO₂	0.21 (room air)
Sodium	138 mmol/L
Chloride	110 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: Normal anion gap metabolic acidosis with expected respiratory compensation

Question 2: Which statement best checks your interpretation?

Answer: Anion gap is 12 and expected PaCO₂ is 32 plus or minus 2 mmHg

Worked answer

pH 7.32 and bicarbonate 16 identify metabolic acidosis.
Anion gap = 138 − (110 + 16) = 12 mmol/L.
Expected PaCO₂ = 1.5 × 16 + 8 = 32 ± 2. Measured 32 fits.
The context is compatible with gastrointestinal bicarbonate loss.

Common mistake: Normal gap and normal acid base balance mean different things.

Case 4 Persistent vomiting Beginner

An adult has persistent vomiting. The scenario concerns the gas pattern, not an instruction to replace a particular fluid or electrolyte.

Arterial sample
pH	7.50
PaCO₂	48 mmHg
HCO₃ (calculated)	36 mmol/L
PaO₂	92 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Metabolic alkalosis with a compatible respiratory response

Question 2: Which statement best checks your interpretation?

Answer: A PaCO₂ around 47–49 mmHg is compatible with the raised bicarbonate

Worked answer

pH 7.50 is alkalemic; bicarbonate 36 is raised.
Bicarbonate has risen 12 above the reference 24.
A PaCO₂ rise of approximately 0.6–0.75 × 12 gives 47.2–49 mmHg. Measured 48 is compatible.
These are approximate teaching rules, not exact physiologic boundaries.

Common mistake: Judge PaCO₂ in relation to the primary metabolic change.

Case 5 Reduced ventilation after sedation Beginner

Soon after sedation, an adult becomes drowsy and breathes slowly. The gas was obtained while help and airway support were being arranged.

Arterial sample
pH	7.26
PaCO₂	60 mmHg
HCO₃ (calculated)	26 mmol/L
PaO₂	61 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Acute respiratory acidosis with hypoxaemia

Question 2: Which statement best checks your interpretation?

Answer: An acute bicarbonate rise of roughly 2–4 mmol/L is expected for this CO₂ rise

Worked answer

pH 7.26 is acidemic and PaCO₂ 60 is raised.
The short time course supports an acute ventilatory problem.
For a 20 mmHg CO₂ rise, an acute bicarbonate response of about 2–4 mmol/L is plausible; measured 26 fits the lower end.
PaO₂ 61 on air is low. The bedside breathing problem needs immediate assessment, not just a label.

Common mistake: Do not delay assessment of ventilation while completing a calculation.

Case 6 Sudden rapid breathing Beginner

An adult has sudden tachypnoea and chest discomfort. The cause has not yet been established.

Arterial sample
pH	7.62
PaCO₂	20 mmHg
HCO₃ (calculated)	20 mmol/L
PaO₂	100 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Acute respiratory alkalosis

Question 2: Which statement best checks your interpretation?

Answer: A small acute bicarbonate fall is compatible; the cause of hyperventilation remains unresolved

Worked answer

pH 7.62 is alkalemic and PaCO₂ 20 is low.
The CO₂ has fallen 20 mmHg below 40.
An acute bicarbonate decrease of about 2–4 mmol/L is compatible; measured bicarbonate 20 fits.
Pain, hypoxaemia, pulmonary vascular disease and other causes need clinical consideration; do not infer anxiety from the gas alone.

Common mistake: A physiologic pattern is not a diagnosis of its cause.

Intermediate: cases 7–13

Case 7 A documented chronic baseline Intermediate

A stable adult with chronic hypercapnic lung disease has prior similar gases over several weeks.

Arterial sample
pH	7.35
PaCO₂	60 mmHg
HCO₃ (calculated)	32 mmol/L
PaO₂	62 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Chronic respiratory acidosis with an expected bicarbonate response

Question 2: Which statement best checks your interpretation?

Answer: For a 20 mmHg CO₂ rise, bicarbonate around 30–32 mmol/L is expected chronically

Worked answer

PaCO₂ 60 is raised and the documented time course is chronic.
A chronic rise of approximately 3–4 mmol/L per 10 mmHg gives bicarbonate 30–32.
Measured bicarbonate 32 is compatible; pH rounds to 7.35 and must not be called normal solely from that rounding.
PaO₂ 62 is low on air and still needs interpretation in the clinical context.

Common mistake: Use the prior gas and time course; one sample does not establish chronicity.

Case 8 A sustained low CO2 Intermediate

An adult has a documented sustained hyperventilatory state over several days. No sudden change is described.

Arterial sample
pH	7.42
PaCO₂	30 mmHg
HCO₃ (calculated)	19 mmol/L
PaO₂	94 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Chronic respiratory alkalosis with a compatible bicarbonate response

Question 2: Which statement best checks your interpretation?

Answer: A chronic bicarbonate fall to about 19–20 mmol/L fits a 10 mmHg CO₂ fall

Worked answer

PaCO₂ 30 is low. The history supplies the sustained time course.
The chronic bicarbonate response is approximately a fall of 4–5 mmol/L per 10 mmHg CO₂ reduction.
Bicarbonate 19 and pH 7.42 are compatible. The clinical cause still needs assessment.

Common mistake: A pH within the reference interval can coexist with a compensated disorder.

Case 9 The expected breathing response is missing Intermediate

An adult with ketoacidosis becomes exhausted and less responsive.

Arterial sample
pH	7.02
PaCO₂	40 mmHg
HCO₃ (calculated)	10 mmol/L
PaO₂	70 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Metabolic acidosis with additional respiratory acidosis

Question 2: Which statement best checks your interpretation?

Answer: Expected PaCO₂ is 23 plus or minus 2; measured 40 is inappropriately high

Worked answer

pH 7.02 indicates marked acidemia.
Bicarbonate 10 identifies metabolic acidosis.
Winter’s expected PaCO₂ = 1.5 × 10 + 8 = 23 ± 2 mmHg.
Measured 40 is well above that range, indicating an added respiratory acidosis in this context. The deterioration requires urgent senior assessment.

Common mistake: Normal range CO₂ can be dangerous when a low CO₂ is needed for compensation.

Case 10 An alkalemic patient with a metabolic acidosis Intermediate

An adult with infection and circulatory compromise is breathing rapidly. Lactate is raised.

Arterial sample
pH	7.47
PaCO₂	20 mmHg
HCO₃ (calculated)	14 mmol/L
PaO₂	85 mmHg
FiO₂	0.21 (room air)
Sodium	140 mmol/L
Chloride	102 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: Metabolic acidosis with additional respiratory alkalosis

Question 2: Which statement best checks your interpretation?

Answer: Expected PaCO₂ is 29 plus or minus 2; measured 20 is lower than expected

Worked answer

Bicarbonate 14 is low despite the alkalemic pH of 7.47.
Expected PaCO₂ = 1.5 × 14 + 8 = 29 ± 2.
Measured 20 is lower than expected, supporting an additional respiratory alkalosis.
Anion gap = 140 − (102 + 14) = 24 mmol/L.

Common mistake: Name all supported processes rather than letting the direction of pH hide one.

Case 11 Acidosis with renal impairment Intermediate

An adult with substantial renal impairment has nausea and tachypnoea. Albumin is normal.

Arterial sample
pH	7.29
PaCO₂	26 mmHg
HCO₃ (calculated)	12 mmol/L
PaO₂	93 mmHg
FiO₂	0.21 (room air)
Sodium	142 mmol/L
Chloride	104 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: High anion gap metabolic acidosis with expected respiratory compensation

Question 2: Which statement best checks your interpretation?

Answer: Anion gap is 26 and PaCO₂ 26 matches Winter’s expected value

Worked answer

pH 7.29 and low bicarbonate indicate metabolic acidosis.
Anion gap = 142 − (104 + 12) = 26.
Expected PaCO₂ is 26 ± 2; measured 26 fits.
The case is consistent with the context but a raised gap still requires clinical evaluation for its contributors.

Common mistake: A plausible context does not exclude other contributors to a raised anion gap.

Case 12 Acute deterioration from a chronic baseline Intermediate

A prior stable gas was PaCO₂ 60 and bicarbonate 32. Over the last hour the patient has become drowsier. Current oxygen is a controlled FiO₂ of 0.28.

Arterial sample
pH	7.25
PaCO₂	80 mmHg
HCO₃ (calculated)	34 mmol/L
PaO₂	58 mmHg
FiO₂	0.28

Question 1: Which interpretation best fits the data and history?

Answer: Acute on chronic respiratory acidosis

Question 2: Which statement best checks your interpretation?

Answer: The prior baseline and acute CO₂ increase explain the modest further bicarbonate rise

Worked answer

The baseline already documented chronic hypercapnia.
PaCO₂ has acutely risen another 20 mmHg, while bicarbonate rose from 32 to 34.
An acute buffering response on top of a chronic baseline fits pH 7.25.
PaO₂ is low despite supplemental oxygen. Interpret the whole clinical deterioration urgently.

Common mistake: Do not apply an isolated chronic equation while ignoring the known baseline.

Case 13 More bicarbonate than compensation predicts Intermediate

An adult with chronic hypercapnia has several days of vomiting and recent diuretic exposure.

Arterial sample
pH	7.50
PaCO₂	60 mmHg
HCO₃ (calculated)	45 mmol/L
PaO₂	65 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Respiratory acidosis with additional metabolic alkalosis

Question 2: Which statement best checks your interpretation?

Answer: Expected chronic bicarbonate is about 30–32, well below the measured 45

Worked answer

PaCO₂ 60 is raised, but pH 7.50 is alkalemic.
Chronic compensation for CO₂ 60 would give bicarbonate about 30–32.
Bicarbonate 45 exceeds that response and supports additional metabolic alkalosis.

Common mistake: Compensation does not usually overshoot to reverse the direction of pH.

Advanced: cases 14–20

Case 14 Two processes with a nearly normal pH Advanced

An adult has vomiting and a concurrent hypoventilatory illness.

Arterial sample
pH	7.37
PaCO₂	65 mmHg
HCO₃ (calculated)	36 mmol/L
PaO₂	55 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: Metabolic alkalosis with additional respiratory acidosis

Question 2: Which statement best checks your interpretation?

Answer: PaCO₂ 65 is substantially above the roughly 47–49 expected for bicarbonate 36

Worked answer

pH 7.37 is within the reference interval, but both PaCO₂ and bicarbonate are abnormal.
For bicarbonate 36, a compensatory PaCO₂ around 47–49 is expected by the chosen teaching rule.
Measured PaCO₂ 65 indicates an additional respiratory acidosis. PaO₂ is also low.

Common mistake: A near normal pH can be the net result of opposing primary disturbances.

Case 15 A gap hidden by low albumin Advanced

An adult with poor nutrition and acute illness has albumin 2 g/dL. The lab’s teaching anion gap reference is 8–12 without potassium.

Arterial sample
pH	7.33
PaCO₂	35 mmHg
HCO₃ (calculated)	18 mmol/L
PaO₂	90 mmHg
FiO₂	0.21 (room air)
Sodium	138 mmol/L
Chloride	108 mmol/L
Albumin	2 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: Metabolic acidosis with an elevated albumin adjusted anion gap

Question 2: Which statement best checks your interpretation?

Answer: Measured gap 12 becomes about 17 after albumin adjustment

Worked answer

pH 7.33 and bicarbonate 18 indicate metabolic acidosis.
Unadjusted gap = 138 − (108 + 18) = 12.
Albumin adjusted gap ≈ 12 + 2.5 × (4 − 2) = 17 mmol/L. This is an approximation, interpreted against the local method.
Winter’s expected PaCO₂ = 35 ± 2; measured 35 fits.

Common mistake: Use g/dL in this correction; albumin 20 g/L equals 2 g/dL.

Case 16 The normal pH trap Advanced

An adult has tinnitus, vomiting and possible salicylate exposure. Toxicology assessment is underway.

Arterial sample
pH	7.40
PaCO₂	20 mmHg
HCO₃ (calculated)	12 mmol/L
PaO₂	98 mmHg
FiO₂	0.21 (room air)
Sodium	140 mmol/L
Chloride	104 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: High anion gap metabolic acidosis with respiratory alkalosis

Question 2: Which statement best checks your interpretation?

Answer: Expected PaCO₂ is 26 plus or minus 2; measured 20 supports an additional respiratory alkalosis

Worked answer

pH is 7.40, but bicarbonate and CO₂ are both very low.
Anion gap = 140 − (104 + 12) = 24.
Winter’s expected PaCO₂ is 26 ± 2; 20 is too low for compensation alone.
The mixed pattern fits the fictional exposure history but does not establish a toxin concentration or treatment plan.

Common mistake: Normal pH is not the same as a normal gas.

Case 17 Two metabolic acid sources Advanced

An adult has ketosis and substantial diarrhoea. Assume baseline bicarbonate 24 and baseline anion gap 12 for this exercise.

Arterial sample
pH	7.29
PaCO₂	26 mmHg
HCO₃ (calculated)	12 mmol/L
PaO₂	97 mmHg
FiO₂	0.21 (room air)
Sodium	140 mmol/L
Chloride	108 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: High gap metabolic acidosis with evidence suggesting an additional normal gap metabolic acidosis

Question 2: Which statement best checks your interpretation?

Answer: Gap rise is 8 while bicarbonate fall is 12; the clinical context supports an additional normal gap process

Worked answer

Gap = 140 − (108 + 12) = 20.
Gap rise = 20 − 12 = 8; bicarbonate fall = 24 − 12 = 12.
The rise is smaller than the bicarbonate fall; with diarrhoea this suggests another normal gap acidosis. Delta comparisons are approximate and baseline dependent.
Expected PaCO₂ = 26 ± 2, matching the measured 26.

Common mistake: Use delta comparisons as supporting evidence, not rigid diagnostic cutoffs.

Case 18 Normal gas numbers with an abnormal gap Advanced

An adult has prolonged vomiting plus independently documented ketonaemia. Assume baseline bicarbonate 24, gap 12 and normal albumin.

Arterial sample
pH	7.40
PaCO₂	40 mmHg
HCO₃ (calculated)	24 mmol/L
PaO₂	96 mmHg
FiO₂	0.21 (room air)
Sodium	140 mmol/L
Chloride	86 mmol/L
Albumin	4 g/dL

Question 1: Which interpretation best fits the data and history?

Answer: High gap metabolic acidosis masked by a concurrent metabolic alkalosis

Question 2: Which statement best checks your interpretation?

Answer: The gap is 30; a raised gap with unchanged bicarbonate supports opposing metabolic processes in this context

Worked answer

pH 7.40, bicarbonate 24 and PaCO₂ 40 look normal in isolation.
Gap = 140 − (86 + 24) = 30.
With the stated baseline and ketonaemia, the high gap plus preserved bicarbonate supports concurrent metabolic alkalosis from the history.
Do not apply Winter’s formula mechanically to this net normal bicarbonate to diagnose an additional respiratory disorder.

Common mistake: Interpret electrolytes and clinical context even when pH looks normal.

Case 19 Oxygenation is a separate question Advanced

An adult receives a measured FiO₂ of 0.50 through a controlled system. The gas is arterial. Imaging, PEEP and timing are not supplied.

Arterial sample
pH	7.40
PaCO₂	40 mmHg
HCO₃ (calculated)	24 mmol/L
PaO₂	55 mmHg
FiO₂	0.5

Question 1: Which interpretation best fits the data and history?

Answer: No acid base disturbance apparent, but substantial oxygenation impairment

Question 2: Which statement best checks your interpretation?

Answer: PaO₂ divided by FiO₂ is 110 mmHg; this ratio alone does not establish ARDS

Worked answer

pH, PaCO₂ and bicarbonate are at their teaching reference values.
PaO₂ 55 despite FiO₂ 0.50 is abnormal.
P/F ratio = 55 ÷ 0.50 = 110 mmHg.
ARDS classification requires additional clinical, imaging and respiratory support criteria; these are not supplied.

Common mistake: Enter FiO₂ as a fraction, and do not equate an oxygenation ratio with a complete diagnosis.

Case 20 Check the numbers before naming the disorder Advanced · transcription check

A copied worksheet reports arterial pH 7.40, PaCO₂ 60 and bicarbonate 24. You are asked to validate the transcription before interpreting it.

Arterial sample
pH	7.40
PaCO₂	60 mmHg
HCO₃ (calculated)	24 mmol/L
PaO₂	90 mmHg
FiO₂	0.21 (room air)

Question 1: Which interpretation best fits the data and history?

Answer: The three acid base values are internally inconsistent and need verification

Question 2: Which statement best checks your interpretation?

Answer: The Henderson Hasselbalch check predicts pH about 7.22, not the reported 7.40

Worked answer

Check pH ≈ 6.1 + log10[24 ÷ (0.03 × 60)] ≈ 7.22.
The reported 7.40 is incompatible with the other two values beyond rounding.
Verify the original report, units, sample identity and whether bicarbonate came from a separate sample. Obtain clinical review or repeat sampling when indicated.
This case intentionally contains a transcription error. Do not silently fix a real report.

Common mistake: An elegant interpretation is useless if the source data are wrong.

Intentional transcription exercise. The values in this case are deliberately inconsistent. It tests whether you check the numbers before interpreting them; the question bank does not correct them.

Questions about this practice

Is this an ABG quiz with answers?

Yes. Each case includes two questions, the correct answers, visible calculations where relevant and a common interpretation error.

Can I use kPa?

Yes. Use the pressure units switch above the cases. It changes pressure display units while preserving the original calculation values.

Does a normal pH mean the blood gas is normal?

No. Several advanced cases show how compensation or opposing disturbances can bring the pH into the reference interval.

Educational use only. These arterial blood gas cases are fictional teaching exercises. Oxygenation is interpreted separately from acid–base status, and a PaO₂/FiO₂ ratio alone does not diagnose ARDS. Educational fictional practice only. Not patient-specific advice or an assessment of clinical competence. Scores are informal feedback on one practice attempt; no pass mark, certificate or competence decision is made from them. Follow your supervision, scope of practice and local protocols.

Sources and further reading

Reference ranges, formulae and pathways vary between laboratories, services and countries. Use your own local guidance in practice.

Start ABG practice

How do you interpret an ABG step by step?

Common ABG interpretation mistakes

Worked ABG cases with answers

Beginner: cases 1–6

Intermediate: cases 7–13

Advanced: cases 14–20

Questions about this practice

Educational disclaimer

Sources and further reading