Anion Gap Calculator

What the anion gap is

Blood is electrically neutral: the positive and negative charges balance. But routine labs only measure some of them. The anion gap is the difference left over when you subtract the main measured anions from the main measured cation, and it stands in for the anions that are not routinely measured, chiefly albumin along with a handful of others.

Its main job is to help sort out a metabolic acidosis. When acid builds up in the blood, the pattern of the anion gap points toward the cause: a raised gap suggests the acid brought its own unmeasured anion along, while a normal gap suggests a different mechanism. This calculator computes the gap from three electrolytes. Below are the formula, how to read it, and one correction it does not apply that you need to keep in mind.

The formula this calculator uses

This tool uses the common form, without potassium:

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

All three values go in as mEq/L. Some clinicians include potassium in the calculation, which shifts the expected normal range upward by roughly its value, but the potassium-free version here is the one most laboratories and textbooks use. The result is a number in mEq/L.

A worked example

Take a sodium of 140, chloride of 104 and bicarbonate of 24, all in mEq/L:

Anion gap = 140 − (104 + 24)
Anion gap = 140 − 128
Anion gap = 12 mEq/L

At 12, this sits at the upper edge of the traditional normal range. Whether that counts as normal depends on the lab and, crucially, on the patient's albumin, which the next sections cover.

How to interpret the result

The traditional normal range is about 8 to 12 mEq/L, though this varies by laboratory, and modern analysers often run a little lower. The pattern matters more than the exact figure:

• High anion gap. Suggests a high anion gap metabolic acidosis, where an acid has added unmeasured anions. The classic causes are captured by mnemonics such as GOLD MARK (glycols, oxoproline, L-lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis) or the older MUDPILES.
• Normal anion gap. In the setting of an acidosis, points instead toward a hyperchloraemic (normal gap) cause, such as diarrhoea or a renal tubular acidosis, where bicarbonate is lost and chloride rises to fill the gap.
• Low anion gap. Less common, and usually reflects low albumin, sometimes a paraprotein, bromide, or simply lab error.

The gap is a clue that directs the next step, not a diagnosis in itself. It is read together with the pH, the bicarbonate, the clinical history and, often, further tests to pin down the specific cause.

The albumin correction this tool does not do

This is the pitfall worth stopping on. Albumin is negatively charged and is the largest single contributor to the normal anion gap. So when albumin is low, the anion gap drops with it, and a genuinely raised gap can be masked into the normal range. In critically ill and hospitalised patients, where low albumin is common, this matters a great deal.

This calculator reports the raw, uncorrected gap. To correct for albumin, the usual adjustment adds about 2.5 mEq/L to the gap for every 1 g/dL that albumin sits below normal (around 4 g/dL). A patient with a low albumin and an apparently normal gap may, once corrected, have a high anion gap acidosis that would otherwise be missed. Since the tool does not know the albumin, that correction is on you to apply.

As with the other lab tools here, this is a calculation and an educational aid, not a diagnosis. It supports the assessment of an acid-base disturbance; it does not replace clinical interpretation. Read it in context, correct it for albumin where needed, and let the full picture guide the conclusion.

Questions people ask

What is a normal anion gap?

Traditionally about 8 to 12 mEq/L for the potassium-free formula, but it depends on the laboratory and the analyser, with many modern methods running a bit lower. Because of that variation, and the effect of albumin, the trend and the clinical context matter more than a hard cutoff.

Should potassium be included?

Both forms are used. Including potassium raises the expected normal range by roughly the potassium value. This tool uses the more common potassium-free version, so compare your result against a normal range calculated the same way.

What does a high anion gap mean?

In the context of a metabolic acidosis, it points to an acid that has added unmeasured anions, with common causes including lactic acidosis, ketoacidosis, kidney failure and certain toxic ingestions. It narrows the differential rather than naming the cause, which needs the rest of the clinical and laboratory picture.

References

Where the figures come from. The uses and limits of the anion gap, including the albumin effect, draw on the review by Kraut and Madias. The broader framework for reading acid-base disturbances draws on Berend and colleagues.

1. Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clinical Journal of the American Society of Nephrology. 2007;2(1):162-174.
2. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. New England Journal of Medicine. 2014;371(15):1434-1445. https://doi.org/10.1056/NEJMra1003327