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Clinical calculator
Anion gap calculator with albumin adjustment
Calculates the serum anion gap, Na − (Cl + HCO₃), and, with an albumin, the albumin-adjusted anion gap (Figge 1998).
Anion gap
When this estimate is not valid
- The normal anion gap depends on the laboratory and analyser; compare with your laboratory's reference range.
- This formula leaves out potassium. Formulas that add potassium have higher normal ranges.
- The albumin adjustment was derived in critically ill adults. Other unmeasured ions (phosphate, paraproteins, lithium, bromide) also change the gap.
Estimate only. Interpret it with the patient's clinical picture and the product label.
How it is calculated
AG = Na − (Cl + HCO₃)
In mmol/L (the same number in mEq/L). Potassium is not included.
Albumin-adjusted AG = AG + 2.5 × (normal albumin − measured albumin)
With albumin in g/dL (0.25 per g/L). Each 1 g/dL fall in albumin lowers the measured gap by about 2.5 mmol/L.
The normal albumin is 4.0 g/dL (40 g/L) unless you enter your laboratory's normal value (3.0–5.0 g/dL); Figge and colleagues did not fix one value.
The normal anion gap depends on the laboratory and analyser; compare the result with your laboratory's reference range. Limits: sodium 100–180, chloride 60–140, bicarbonate 2–50 mmol/L, albumin 1–6 g/dL.
Worked example
Sodium 140, chloride 104 and bicarbonate 24 mmol/L:
140 − (104 + 24) = 12 mmol/L
Anion gap 12 mmol/L.
The same values with an albumin of 2.0 g/dL:
12 + 2.5 × (4 − 2) = 17 mmol/L
Albumin-adjusted anion gap 17 mmol/L.
Questions
Why adjust the anion gap for albumin?
Albumin is the main unmeasured anion. When it is low the anion gap falls, which can hide a raised gap from another acid. Figge and colleagues (1998) found that the gap falls by about 2.5 mmol/L for each 1 g/dL (0.25 per g/L) fall in albumin, in critically ill adults.
Should potassium be included?
This calculator uses Na − (Cl + HCO₃) without potassium, the form most laboratories report. Formulas that add potassium have higher normal ranges, so compare like with like.
What is a normal anion gap?
It depends on the laboratory method and analyser. Compare the result with your laboratory's own reference range rather than a value from a textbook.
What does a very low or negative anion gap mean?
It is uncommon and is often a laboratory or data-entry error, so check the values and units first. Low albumin, paraproteins, lithium and bromide can also lower the gap.
References
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807–1810. DOI 10.1097/00003246-199811000-00019 PubMed 9824071
Formula version: Anion gap · albumin adjustment (Figge 1998) · MediHelp calc v1.0.1
Formula and sources checked on 2026-10-29
