Blood gas, electrolyte and acid-base interpretation — formative SAQs

SAQ 1 — The five-step method on a high-anion-gap metabolic acidosis (15 marks, 15 minutes)

A three-year-old is brought in breathing deeply and rapidly with the smell of ketones on the breath. A capillary gas shows pH 7.07, PaCO2 14 mmHg, bicarbonate 4 mmol per litre, sodium 132, chloride 96, and glucose 24 mmol per litre. ^{[6] [11]}

Question. Interpret the gas using the five-step systematic method. Name the primary disturbance, judge the compensation with the appropriate formula, calculate the anion gap and classify the metabolic acidosis, and list the differential of a high-gap acidosis.

Model answer

Step 1 — pH (2 marks). The pH of 7.07 is well below 7.35, so this is a severe acidaemia. The primary disturbance is an acidosis. ^[11]

Step 2 — PaCO2 (2 marks). PaCO2 is 14 mmHg, which is below the normal range of 35 to 45. A low PaCO2 is a compensatory alkalosis, not the primary driver, because it moves against the acidaemia. ^[4]

Step 3 — Bicarbonate (2 marks). Bicarbonate is 4 mmol per litre, far below 22 to 26. A low bicarbonate with a low pH is a metabolic acidosis, and this is the primary disturbance. ^[6]

Step 4 — Compensation, Winter's formula (4 marks). For metabolic acidosis the expected PaCO2 is 1.5 times the bicarbonate plus 8, within plus or minus 2. Here that is 1.5 times 4 plus 8, equals 14, within the band of 12 to 16. The measured PaCO2 of 14 falls inside the band, so the respiratory compensation is appropriate and there is no additional respiratory disorder. ^[11]

Step 5 — Anion gap and classification (3 marks). The anion gap is sodium minus chloride plus bicarbonate: 132 minus 96 plus 4, equals 32 mmol per litre. The normal gap is 8 to 12, so this is a high-gap metabolic acidosis. The differential of a high gap is KULT — ketones, uraemia, lactate, toxins. Given the ketotic breath and the glucose of 24, this is diabetic ketoacidosis. ^[6]

Reasoning (2 marks). The five-step method holds the interpretation together: a high-gap metabolic acidosis with appropriate respiratory compensation, in a child whose history and glucose confirm DKA. The corrected sodium would guide the fluid plan, and the structured DKA protocol now drives management. ^[7]