Metabolic acidosis
Identify the metabolic disturbance, assess ventilation, then explain the corrected anion gap.
Start with the pattern
A primary reduction in bicarbonate is the conventional metabolic-acidosis pattern. The pH is often low, but an opposing process may bring it into the reference interval. A low bicarbonate also occurs with respiratory alkalosis, so it must be interpreted with PaCO₂ and the time course.
- Check the gas. Read pH and PaCO₂ together; confirm sample type and measurement compatibility.
- Assess compensation. Compare observed PaCO₂ with Winter’s expected range.
- Classify the gap. Calculate the no-potassium AG, correct for albumin when measured, and compare with the selected interval.
- Look for another process. Use a delta screen when appropriate, then test the interpretation against the history.
Expected respiratory response
Below the expected band suggests an additional respiratory alkalosis; above it suggests an additional respiratory acidosis. A PaCO₂ inside its routine reference interval can still be too high for a severe metabolic acidosis.
For HCO₃⁻ 12 mmol/L
Expected PaCO₂ is 24–28 mmHg. PaCO₂ 26 fits; PaCO₂ 40 does not.
Open worked exampleUse the gap to focus the next checks
| Pattern | Useful next information |
|---|---|
| Raised corrected gap | Measured lactate; blood ketones and glucose; renal function; medication and toxin exposure when clinically relevant. |
| Gap within range | Diarrhoea or other gastrointestinal losses; recent chloride-rich fluids; kidney function, potassium and medications affecting renal acid handling. |
An increased gap does not name an acid. A normal lactate does not exclude other gap anions. A normal raw AG can conceal an increase when albumin is low.
Pitfalls and unresolved causes
- A low bicarbonate during chronic respiratory alkalosis can be an expected secondary response.
- During rapidly changing illness, the compensatory response may not be at steady state.
- Use selective investigations guided by the history. A urinary anion gap is not a routine addition to every metabolic-acidosis panel.
- For the gap calculation and mixed metabolic patterns, see Anion gap and delta analysis.
Sources
MSD Manual Professional — Metabolic acidosis · SRLF/SFMU expert panel — Metabolic acidosis guideline (2019) · MSD Manual Professional — Primary changes and compensation
Adult educational interpretation at 37°C. Apply arterial rules to arterial samples, use the laboratory’s reference intervals, and review the result alongside the clinical history.