Medical AIAnion gap (albumin-corrected) calculator
The anion gap (sodium minus chloride and bicarbonate) screens for and characterises metabolic acidosis. Because albumin is the main unmeasured anion, a low albumin masks a real gap, so this tool also reports an albumin-corrected value.
How to measure each input
- Use a single matched panel
- Take sodium, chloride, and bicarbonate from the same blood draw. Bicarbonate is usually reported as total CO2 on a basic metabolic panel; it is an acceptable surrogate here.
- Potassium variant
- This tool uses the more common formula without potassium. If you use the (Na + K) variant, the normal range shifts up by about 4 mmol/L, so do not compare the two interchangeably.
- Albumin for correction
- Each 1 g/dL fall in albumin below 4.0 lowers the measured gap by about 2.5 mmol/L. Enter albumin (g/dL or g/L) to recover the true gap. The reference albumin of 4.0 g/dL equals 40 g/L.
- Interpret with the clinical picture
- Pair the gap with the bicarbonate, pH, lactate, glucose, and an osmolar gap when a toxic alcohol is possible. The anion gap is a screen, not a diagnosis.
Interpretation
| Band | Meaning |
|---|---|
| < 6 mmol/L, low anion gap | A low gap is most often hypoalbuminaemia, but consider lab error, lithium, bromide, or paraproteins (for example multiple myeloma). Correct for albumin before reading the gap as truly low. |
| 6 to 12 mmol/L, common modern normal range | Within a commonly used modern range for ion-selective-electrode methods. Reference ranges are analyser-dependent, so use your own laboratory's range. |
| > 12 mmol/L, elevated anion gap | Suggests a high anion gap metabolic acidosis. Work through MUDPILES or GOLDMARK: methanol, uraemia, diabetic or other ketoacidosis, paraldehyde, iron or isoniazid, lactate, ethylene glycol, salicylates (and the GOLDMARK additions, including D-lactate and propylene glycol). Correct for albumin, since a normal gap can mask acidosis when albumin is low. |
Pitfalls, exclusions and caveats
- Reference ranges vary by analyser. Modern ion-selective electrode methods read chloride higher, so a 'normal' gap may be 6 to 12 rather than the textbook 8 to 12. Always use your laboratory's range.
- Hypoalbuminaemia masks a high anion gap acidosis: a critically ill patient with albumin of 2 g/dL can have a corrected gap several points higher than the measured value. Correct before reassuring.
- Severe hyperglycaemia, hypertriglyceridaemia, or paraproteins can cause pseudohyponatraemia or artefactual electrolyte shifts that distort the gap.
- A normal anion gap does not exclude a mixed disorder. Use the delta ratio (delta anion gap over delta bicarbonate) to detect a coexisting normal-gap acidosis or metabolic alkalosis.
- Bromide and iodide intoxication can falsely raise measured chloride and lower the calculated gap.
- The corrected formula is an estimate. In acute, rapidly changing states it should support, not replace, direct measurement of lactate, ketones, and toxic alcohols.
Anion gap = Na - (Cl + HCO3). Albumin-corrected gap = anion gap + 2.5 x (4.0 - albumin in g/dL). The classic potassium-inclusive variant adds potassium to the cations: AG = (Na + K) - (Cl + HCO3), which runs about 4 mmol/L higher.The anion gap is a standard clinical-chemistry calculation; the albumin correction is commonly attributed to Figge and colleagues. This implementation is an educational tool and is not affiliated with any author or guideline body. Use your own laboratory's reference ranges.
Frequently asked
What is a normal anion gap?
A commonly cited range is 8 to 12 mmol/L without potassium, but modern analysers often run lower (roughly 6 to 12). The range is analyser-dependent, so use your own laboratory's reference values.
Why correct the anion gap for albumin?
Albumin is the largest unmeasured anion, so a low albumin lowers the measured gap and can hide a high-gap acidosis. Each 1 g/dL fall in albumin below 4.0 reduces the gap by about 2.5 mmol/L, so the corrected value restores the true picture.
What causes a high anion gap?
Use MUDPILES or GOLDMARK: methanol, uraemia, diabetic and other ketoacidosis, paraldehyde, iron or isoniazid, lactic acidosis, ethylene glycol, and salicylates, plus the GOLDMARK additions such as D-lactate and propylene glycol.
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