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Acid-base disorders
Blood gases are read in four fixed steps — pH, the primary disorder, whether compensation is appropriate, and the anion gap — and every step skipped hides a second disorder.
Why it matters in the exam
Blood-gas questions are among those where candidates lose the most marks, because they require calculation. A typical question gives the pH, PaCO2, bicarbonate and electrolytes and asks for the full diagnosis: for example a high anion gap metabolic acidosis plus a metabolic alkalosis in a diabetic patient who has been vomiting. The candidate must therefore be able to calculate the anion gap (Na minus Cl minus HCO3, normal about 12), correct it for albumin, and check compensation using Winter's formula.
Then comes the differential diagnosis. With a high anion gap: lactic acidosis, ketoacidosis, kidney failure and toxins (methanol, ethylene glycol, salicylates) — and an osmolar gap above 10 points to the toxic alcohols. With a normal gap: diarrhoea or RTA, separated by the urine anion gap. In metabolic alkalosis, urine chloride separates vomiting (low, saline-responsive) from mineralocorticoid excess or diuretics (high). Advanced questions ask about the types of RTA: type 1 with urine pH above 5.5 and stones, type 2 with Fanconi syndrome, and type 4 with hyperkalaemia in a diabetic patient.
Key points
- Four steps: pH, primary, compensation, anion gap.
- Winter: expected PaCO2 = 1.5 × HCO3 + 8 (±2).
- The normal anion gap is about 12; correct it for albumin.
- High gap: lactate, ketones, kidney, toxins; normal gap: diarrhoea or RTA.
- Metabolic alkalosis: urine chloride below 20 is saline-responsive.
Summary table
| Primary disorder | Expected compensation | Main causes | Exam clue |
|---|---|---|---|
| Metabolic acidosis | PaCO2 = 1.5 × HCO3 + 8 (±2) | High gap: lactate, ketones, kidney, toxins; normal gap: diarrhoea, RTA | Osmolar gap above 10 → methanol or ethylene glycol |
| Metabolic alkalosis | PaCO2 rises 0.7 for every 1 rise in HCO3 | Vomiting, diuretics, hyperaldosteronism | Urine chloride below 20 → saline-responsive |
| Respiratory acidosis | Acute: +1 HCO3 for each 10 of PaCO2; chronic: +3.5 per 10 | Hypoventilation, COPD, opioids | Renal compensation takes days |
| Respiratory alkalosis | Acute: HCO3 falls 2 per 10; chronic: 4 per 10 | Anxiety, pulmonary embolism, pregnancy, early salicylates | Salicylates: respiratory alkalosis + metabolic acidosis |
| RTA | Type 1: urine pH above 5.5; type 2: proximal; type 4: hyperkalaemia | Type 4 in diabetes and aldosterone deficiency | Urine anion gap positive in RTA, negative in diarrhoea |
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Sources
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Acidosis and Alkalosis
- Brenner and Rector's The Kidney, 11th edition (2020) — Disorders of Acid-Base Balance
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.