Anion gap calculator

How is the anion gap calculated?

Anion gap = sodium − (chloride + bicarbonate), all in mEq/L. Some labs add potassium to the sodium (StatPearls).

Example

Sodium 140, chloride 104 and bicarbonate 24: 140 − (104 + 24) = 12 mEq/L. With potassium 4.0 included, 16.

What is a normal anion gap?

It depends on the lab and the method. StatPearls gives about 8–12 mEq/L without potassium and 12–16 with it; the Merck Manual describes a normal gap of about 10–12.

Why correct for albumin?

Albumin carries negative charge, so low albumin lowers the gap and can hide a real rise. The correction adds 2.5 for every 1 g/dL that albumin is below 4.0: with albumin 2.0 the gap of 12 becomes 17.

What does a high anion gap mean?

It can point to certain kinds of metabolic acidosis. Interpreting it needs the full clinical picture — talk with your clinician.

Worked example: the same low bicarbonate, two different gaps

Take two patients with a sodium of 135 and a bicarbonate of 12 mEq/L. Patient A has a chloride of 98: the gap is 135 − (98 + 12) = 25 mEq/L, above the ranges quoted above. Patient B has a chloride of 112: 135 − (112 + 12) = 11 mEq/L, inside them. Both have a low bicarbonate, but they fall into different groups. The Merck Manual lists ketoacidosis, lactic acidosis, renal failure and toxins such as ethylene glycol, methanol and salicylates among the causes of a high gap, and diarrhea, ileostomy or colostomy losses and renal bicarbonate loss among the causes of a normal gap with a high chloride.

Why do labs quote different normal ranges?

Older continuous-flow analyzers reported mean gaps of roughly 11 to 15 mEq/L, while ion-selective electrodes report lower ones, such as 6.4 in one study, mainly because they give higher chloride values (Kraut and Nagami, Clinical Journal of the American Society of Nephrology, 2013). Textbook ranges differ too: Salem and Batlle give 10–12 mEq/L, or 6–10 with ion-selective electrodes. Within a population the normal gap can span 8 to 10 mEq/L, so someone whose usual gap sits at the low end can have a clear rise that still looks normal. Compare the result with your own lab’s range and, if you have them, your earlier values.

How well does the anion gap detect acidosis?

Not perfectly. In an emergency-department study quoted by Kraut and Nagami, only 58% of patients with a serum lactate above 2.5 mmol/L had an abnormal anion gap; in some studies correcting for albumin raised detection to 75% and 94%, though results varied. A normal gap therefore does not rule out lactic acidosis or ketoacidosis. The gap also reacts differently to different acids: the rise in the gap compared with the fall in bicarbonate was about 1:1 in ketoacidosis but averaged about 1.5 in lactic acidosis.

Common mistakes with the anion gap

Mixing formulas: a gap that includes potassium runs higher by about the potassium value (4.0 mEq/L raises 12 to 16), so compare it only with a range quoted for the same formula. Entering albumin in the wrong unit: the correction 2.5 × (4.0 − albumin) expects g/dL, and an albumin of 20 g/L typed as 20 would give a “corrected” gap of -28 mEq/L, an impossible value that signals the mix-up. Treating a normal gap as proof that no acid is present, as described above. And reading the number alone: a high gap says an unmeasured acid is likely present, not which one — that takes the history and further tests.

Sources and related calculators

Sources: J. A. Kraut and G. T. Nagami, “The serum anion gap in the evaluation of acid-base disorders: what are its limitations and can its effectiveness be improved?” Clinical Journal of the American Society of Nephrology 8 (2013); Merck Manual Professional Version, “Metabolic Acidosis”; StatPearls, “Anion Gap.” Related: serum osmolality calculator, corrected sodium calculator and corrected calcium calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.