Anion gap calculator with albumin correction

The anion gap is the difference between the main measured cations and anions. Albumin is the largest unmeasured anion, so a low albumin lowers the gap; Figge and colleagues found about 2.5 mmol/L per 1 g/dL of albumin.

Anion gap = Na − (Cl + HCO₃)        with potassium: Na + K − (Cl + HCO₃)
Albumin-corrected gap = anion gap + 2.5 × (reference albumin − albumin)   (g/dL; 1 g/dL = 10 g/L)

Assumptions and limits

• Normal ranges depend on the laboratory’s analysers, and on whether potassium is included; compare with your laboratory’s range.
• mmol/L and mEq/L are the same for these singly charged ions.

Worked example

Na 140, Cl 104 and HCO₃ 24 mmol/L give an anion gap of 12 mmol/L. With albumin 2.5 g/dL and a 4.0 g/dL reference, the corrected gap is 12 + 2.5 × 1.5 = 15.75 mmol/L, shown to one decimal as 15.8.

References

• Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007;2:162–174
• Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med 1998;26:1807–1810