Anion Gap Calculator (with albumin correction)

Understanding the anion gap calculator — concept and principle

The anion gap estimates the amount of negatively charged substances in blood that are not routinely measured. It is the first step in working out the cause of a metabolic acidosis: a raised gap points to added acids such as lactate or ketones, while a normal gap points to loss of bicarbonate. Because albumin is the largest unmeasured anion, the calculator also gives an albumin-corrected value.

The measured anions fall 12 mmol/L short of sodium; that gap is made of unmeasured anions such as albumin.

How it is calculated

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

Corrected anion gap = anion gap + 2.5 × (4 − albumin in g/dL)

Sodium, chloride and bicarbonate in mmol/L.
Albumin in g/dL; 4 g/dL is taken as the normal reference.
Potassium is left out in this version.

The principle

Blood is electrically neutral: total positive charge equals total negative charge. Routine panels measure only the main cation (sodium) and the main anions (chloride and bicarbonate), so the difference is filled by unmeasured anions, mainly albumin, plus phosphate, sulphate and organic acids. When an acid such as lactic acid is added, its hydrogen ions consume bicarbonate and its anion stays behind, so the gap widens. Each 1 g/dL fall in albumin lowers the gap by about 2.5 mmol/L, which is why low albumin can hide a raised gap.

Worked example

Cl⁻ + HCO₃⁻ = 104 + 24 = 128.
Anion gap = 140 − 128 = 12 mmol/L.
Albumin is 4 g/dL, so the correction is 2.5 × (4 − 4) = 0.
Corrected anion gap = 12 + 0 = 12 mmol/L, at the upper end of the typical reference.

Each point is the calculator re-run with only “Sodium (mmol/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Sodium (mmol/L)	Anion gap
35	-93
70	-58
105	-23
140	12
175	47
210	82
280	152

How to read the result

Where does your result fall?

With modern ion-selective electrodes, a typical reference is about 8–12 mmol/L without potassium, but each laboratory sets its own range. A high gap suggests lactic acidosis, ketoacidosis, kidney failure or toxins such as methanol, ethylene glycol or salicylate. A normal-gap acidosis suggests diarrhoea, renal tubular acidosis or large volumes of saline. A low gap is most often due to low albumin, and rarely to paraproteins such as in myeloma or to lithium or bromide.

Common mistakes

Comparing the result with a reference range that includes potassium.
Ignoring low albumin in critically ill patients, which can mask a real high-gap acidosis.
Interpreting the gap without a blood gas; the gap alone does not prove acidosis.

Keep in mind: The anion gap is not a diagnosis. Reference ranges vary between analysers, and the 2.5 correction is an approximation. Interpretation needs the full clinical picture and a clinician.

Frequently asked questions

What does a high anion gap mean?

It suggests accumulated unmeasured acids, such as lactate, ketones or toxic alcohol metabolites.

Should potassium be included?

Some labs add it, which raises the normal range by about 4 mmol/L. Use the formula your lab uses.

What is the delta ratio?

It compares the rise in the gap with the fall in bicarbonate to detect a second acid-base disorder.

Sources: Figge J et al. Anion gap and hypoalbuminemia. Crit Care Med 1998. · Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol 2007.

Checked 8 Nov 2026. Figge et al., Crit Care Med 1998.

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