ABG interpretation in five steps
A systematic ABG walkthrough — pH, primary disorder, compensation, anion gap and Winter's formula — so mixed disorders are reasoned out, not guessed.
Read every ABG the same way
Never eyeball an ABG. Run the same five steps in order and mixed disorders reveal themselves. Normal values to anchor: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mmol/L.
Step 1 — Look at the pH
- pH < 7.35 → acidaemia; pH > 7.45 → alkalaemia.
- A "normal" pH with abnormal PaCO₂ and HCO₃⁻ means a compensated or mixed disorder — do not stop here.
Step 2 — Identify the primary disorder
Ask which value — PaCO₂ or HCO₃⁻ — moved in the same direction as the pH change.
- HCO₃⁻ tracks the pH → metabolic. PaCO₂ moves opposite to pH → respiratory.
| Primary disorder | pH | Primary change | Compensation (direction) |
|---|---|---|---|
| Metabolic acidosis | ↓ | ↓ HCO₃⁻ | ↓ PaCO₂ (hyperventilation) |
| Metabolic alkalosis | ↑ | ↑ HCO₃⁻ | ↑ PaCO₂ (hypoventilation) |
| Respiratory acidosis | ↓ | ↑ PaCO₂ | ↑ HCO₃⁻ (renal retention) |
| Respiratory alkalosis | ↑ | ↓ PaCO₂ | ↓ HCO₃⁻ (renal excretion) |
Step 3 — Check the compensation is appropriate
The body never overcompensates — if the "compensation" overshoots the normal range for pH, a second primary disorder is present.
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | Winter's: PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2) |
| Metabolic alkalosis | PaCO₂ rises ~0.7 mmHg per 1 mmol/L rise in HCO₃⁻ |
| Acute respiratory acidosis | HCO₃⁻ ↑ 1 per 10 mmHg ↑ PaCO₂ |
| Chronic respiratory acidosis | HCO₃⁻ ↑ 4 (3.5) per 10 mmHg ↑ PaCO₂ |
| Acute respiratory alkalosis | HCO₃⁻ ↓ 2 per 10 mmHg ↓ PaCO₂ |
| Chronic respiratory alkalosis | HCO₃⁻ ↓ 4–5 per 10 mmHg ↓ PaCO₂ |
Winter's trap: if measured PaCO₂ is higher than Winter's predicts, there is a concurrent respiratory acidosis; if lower, a concurrent respiratory alkalosis.
Step 4 — Anion gap (every metabolic acidosis)
AG = Na⁺ − (Cl⁻ + HCO₃⁻), normal 8–12 mmol/L. Correct for albumin: add 2.5 to the gap per 1 g/dL fall below 4.
- High-AG acidosis (MUDPILES): Methanol, Uraemia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates.
- Normal-AG (hyperchloraemic): diarrhoea, renal tubular acidosis, acetazolamide.
Step 5 — Delta ratio (in a high-AG acidosis)
Δ ratio = (AG − 12) / (24 − HCO₃⁻). <1 suggests a coexisting normal-AG acidosis; >2 suggests a coexisting metabolic alkalosis or chronic respiratory acidosis.
Pearl: A high anion gap is the one number you must compute even when the pH is normal — it exposes a hidden organic acidosis that compensation has masked.
Anchor
pH → primary → is compensation appropriate? → anion gap → delta. The overshoot that "compensation" can never do is your signal for a second disorder.
Sources
- Harrison's Principles of Internal Medicine, 21e
- Ganong's Review of Medical Physiology, 26e