Albumin-Corrected Anion Gap Calculator
Albumin-Corrected Anion Gap Calculator
Correct the anion gap for hypoalbuminaemia, which lowers the measured gap and can hide a clinically significant high-anion-gap acidosis.
Albumin-Corrected Anion Gap
Albumin correctionNa 138, Cl 108, HCO₃⁻ 16 mmol/L, albumin 2.2 g/dL
Formula
- Na⁺ − Cl⁻ − HCO₃⁻
- the standard, uncorrected anion gap
- 4
- a normal albumin, g/dL — the correction is zero at this value
- 2.5
- the approximate mmol/L the gap falls for each 1 g/dL fall in albumin below normal
Worked example
Na 138, Cl 108, HCO₃⁻ 16 mmol/L, albumin 2.2 g/dL
Uncorrected anion gap = 138 − 108 − 16 = 14 mmol/L
Correction = 2.5 × (4 − 2.2) = 2.5 × 1.8 = 4.5
14 + 4.5 = 18.5 mmol/L corrected
Correction added by albumin
| Albumin (g/dL) | Correction added (mmol/L) |
|---|---|
| 4.0 (normal) | 0 |
| 3.0 | 2.5 |
| 2.0 | 5.0 |
| 1.0 | 7.5 |
Why albumin hides the gap
Albumin is negatively charged at physiological pH and, despite not appearing in the sodium-chloride-bicarbonate equation, it is the single largest unmeasured anion contributing to the anion gap. A normal gap of 8 to 12 mmol/L assumes a normal albumin of roughly 4 g/dL. When albumin falls, that unmeasured anion falls with it, and the measured gap falls too — even though the acid-base process the gap is meant to detect has not changed at all.
The correction is a simple linear estimate: add 2.5 mmol/L to the measured gap for every 1 g/dL that albumin sits below 4. In critical illness, where an albumin of 2 g/dL is unremarkable, this is not a small effect. An uncorrected gap of 12 mmol/L — read as normal — corrects to about 17, which is clearly raised and should prompt a search for lactate, ketones, uraemia or a toxic alcohol.
The practical consequence is that a normal-gap acidosis and a masked high-gap acidosis look identical on the raw numbers whenever albumin is low, which is routine in intensive care, sepsis, malnutrition and chronic liver disease. Correcting for albumin before applying gap-based reasoning — including the delta ratio — is what keeps that reasoning valid in exactly the patients most likely to have a significant acidosis.
Frequently asked questions
Why does the anion gap need correcting for albumin?
Albumin is the dominant unmeasured anion in the gap calculation. Low albumin lowers the measured gap independently of any acid-base process, so a genuinely raised gap can look falsely normal.
How much does albumin change the anion gap?
About 2.5 mmol/L for every 1 g/dL that albumin falls below a normal value of 4 g/dL. An albumin of 2 g/dL therefore adds roughly 5 mmol/L to the corrected gap.
Can a normal anion gap still mean a high-anion-gap acidosis?
Yes, when albumin is low. This is common in critical illness, where an uncorrected gap of 12 mmol/L can correct to 17 or more once albumin is accounted for.
Should the delta ratio use the corrected or uncorrected gap?
The albumin-corrected gap, whenever albumin is low. Using the uncorrected gap in a hypoalbuminaemic patient understates the excess unmeasured anions and can misclassify the acid-base pattern.
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References
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–10.
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434–45.