Anion gap with albumin correction

The anion gap is the difference between serum sodium and the sum of chloride and bicarbonate. It represents the anions not routinely measured, mostly albumin.

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Calculator

mmol/l
mmol/l
mmol/l
mmol/l
Albumin unit
–mmol/l

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Formulas

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻). With potassium: Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻). Albumin-corrected according to Figge et al. (1998): corrected anion gap = anion gap + 0.25 × (normal albumin − measured albumin) with albumin in g/l, or factor 2.5 with albumin in g/dl. The calculator uses 44 g/l (4.4 g/dl) as the normal value.

In 152 critically ill patients, Figge et al. found that each gram per litre decrease in albumin makes the measured anion gap appear 0.25 mmol/l smaller. A low albumin concentration can therefore mask an increased anion gap.

Reference range: two figures

The reference range depends on the laboratory’s measurement method. For a long time, a range of 8–16 mmol/l was used. With modern electrode technology, Winter et al. (1990) found a lower range of 3–11 mmol/l, mainly because measured chloride values are higher. The calculator shows where the value lies in both ranges; the reference range of your own laboratory is decisive. Both figures refer to the formula without potassium.

SourceReference range (without potassium)
traditional range (cited by Winter 1990)8–16 mmol/l
Winter 1990, modern electrode technology3–11 mmol/l

Limitations

The normal value varies widely between laboratories and between individuals (Kraut 2007). Besides albumin, other proteins such as paraproteins also affect the anion gap. The anion gap is an auxiliary value in acid–base analysis and does not replace blood gas analysis and its interpretation.

FAQ

Why is the anion gap corrected for albumin?▾

Albumin is the most important unmeasured anion. When it is low, the anion gap becomes smaller and can mask an increase caused by other anions. The Figge correction converts the value to a normal albumin concentration.

With or without potassium?▾

Both forms are in use. The formula with potassium gives values higher by the potassium concentration; the reference ranges on this page apply to the formula without potassium.

References

  1. Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26:1807–10. · PubMed
  2. Winter SD et al. The fall of the serum anion gap. Arch Intern Med 1990;150:311–3. · PubMed
  3. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2:162–74. · PubMed

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Intended purpose: learning tool for medical education only. Not intended for use in patients or for diagnostic or treatment decisions; not a medical device. Calculations follow the cited original publications; no guarantee of completeness or correctness. Last updated: 10/2026 · Dr. Pascal Bafteh