Notes · Renal · 9 min read
Acid–base in five steps
A structured way through any blood gas.
Key points
- Step 1: is the pH acidaemic (below 7.35) or alkalaemic (above 7.45)?
- Step 2: is the primary process metabolic (bicarbonate) or respiratory (PaCO₂)?
- Step 3: check compensation; for metabolic acidosis, expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winter's formula).
- Step 4: calculate the anion gap: Na⁺ − (Cl⁻ + HCO₃⁻); normal is about 8–12 without potassium.
- Step 5: in a high-gap acidosis, compare the change in gap with the change in bicarbonate (delta ratio).
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