Anion Gap Calculator

Methodology

Anion gap; Figge albumin correction

AG = Na − (Cl + HCO₃); corrected AG = AG + 2.5 × (4.0 − albumin g/dL)

The reference range depends on the laboratory's analysers (often about 3–11 mmol/L with modern ion-selective electrodes). Each 1 g/dL fall in albumin lowers the AG by about 2.5 mmol/L.

Who it is for

Adults with a basic metabolic panel.

Version

1.0.0

How this calculator works

Where the formula comes from

• The serum anion gap, sodium minus the sum of chloride and bicarbonate, is used to evaluate metabolic acidosis; a version adding potassium is also used and has a different reference range.^[1]
• Figge and colleagues (1998) derived an albumin correction from 9 healthy subjects and 152 critically ill patients: the observed anion gap is about 0.25 mmol/L lower for each 1 g/L fall in albumin, which gives a correction of 2.5 per g/dL.^[2]

The inputs

• Electrolytes are entered in mmol/L; the albumin correction uses albumin in g/dL with a factor of 2.5 (or g/L with 0.25), relative to a normal albumin of 4.0 g/dL.^[2]

Versions and alternatives

• With modern ion-selective electrode methods, anion gap reference ranges can be lower than older ranges (for example about 3–11 rather than 8–16 mmol/L); the range depends on the laboratory's method, so the laboratory's own range should be used.^[3][1]

Where accuracy falls (published evidence)

• Low albumin lowers the observed anion gap and can mask a high anion gap acidosis.^[2]
• Lithium, bromide and paraproteins can produce a low or even negative anion gap.^[1]
• In one study, the anion gap was lower in hypercalcaemia caused by hyperparathyroidism but not in hypercalcaemia caused by cancer; values overlapped, so the anion gap cannot identify the cause in an individual patient.^[4]

What guidelines say

• A 2019 French expert-panel guideline suggests using the albumin-corrected anion gap rather than the uncorrected value when evaluating metabolic acidosis.^[5]

Sources

1. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174. doi:10.2215/CJN.03020906
2. 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
3. Lolekha PH, Vanavanan S, Teerakarnjana N, et al. Reference ranges of electrolyte and anion gap on the Beckman E4A, Beckman Synchron CX5, Nova CRT, and Nova Stat Profile Ultra. Clin Chim Acta. 2001;307(1-2):87–93. doi:10.1016/s0009-8981(01)00437-5
4. Oster JR, Gutierrez R, Schlessinger FB, et al. Effect of hypercalcemia on the anion gap. Nephron. 1990;55(2):164–169. doi:10.1159/000185946
5. Jung B, Martinez M, Claessens YE, et al. Diagnosis and management of metabolic acidosis: guidelines from a French expert panel. Ann Intensive Care. 2019;9(1):92. doi:10.1186/s13613-019-0563-2

Compiled from the published sources above. Every statement was agreed independently by three AI research tools (Claude, ChatGPT, DeepSeek) and its source verified in PubMed or Crossref by the Hakeemly editorial team. Not reviewed by a doctor unless stated. Agreed on 2026-10-11

Worked example: how to calculate anion gap

Calculation:

140 − 128 = 12

An illustration with sample values, worked out with the same method the tool above uses.

Limitations

• Use your laboratory's own reference range.
• Lithium, bromide, paraproteins and severe hypercalcaemia can distort the AG.

References

1. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162–174. doi:10.2215/CJN.03020906
2. 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