Metabolic alkalosis
Assess a primary bicarbonate increase, its respiratory response, and the clinical clues that distinguish causes.
Recognise the metabolic pattern
A primary rise in bicarbonate supports metabolic alkalosis. The pH is usually raised; a simultaneous respiratory or metabolic acidosis may partly conceal it. Bicarbonate can also rise as compensation for chronic hypercapnia.
- Check whether the bicarbonate rise fits the PaCO₂, pH and known respiratory history.
- Assess the expected compensatory PaCO₂ band rather than labelling every CO₂ rise a second disorder.
- Review chloride, potassium, kidney function, volume status and the sequence of recent treatments.
Expected respiratory response
This app shows the formula band and identifies a further 2 mmHg screening margin separately. A value within that margin is borderline, not automatically compatible compensation. Linear rules become less dependable in severe disturbances.
For HCO₃⁻ 36 mmol/L
The expected PaCO₂ band is 47.2–49.0 mmHg. A measured 48 fits; a substantially lower or higher value suggests an additional respiratory process.
Open worked exampleInvestigate the cause
| Clue | Processes to consider |
|---|---|
| Gastric losses | Vomiting or gastric drainage, often with chloride and potassium depletion. |
| Medication or fluid history | Loop/thiazide diuretics, alkali administration and recent correction of chronic hypercapnia. |
| Hypertension or persistent renal losses | Mineralocorticoid activity and other renal causes; interpret in the context of kidney function and medications. |
Urine chloride can help when the history is unclear. A low result supports avid renal chloride retention. Recent diuretics and impaired kidney function can make a simple chloride-based label misleading. The gas does not decide whether a fluid intervention is appropriate.
A high gap can coexist with alkalosis
Do not stop because pH or bicarbonate is high. A raised corrected AG may reveal an acidifying process alongside metabolic alkalosis. Delta-adjusted bicarbonate can support that question when a delta ratio is unavailable or unstable.
Sources
MSD Manual Professional — Metabolic alkalosis · RCPA Manual — Alkalosis · 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.