Na − (Cl + HCO₃) — screens for high-AG metabolic acidosis (MUDPILES).

What it is and when to use it

The anion gap is a calculation derived from serum electrolytes that estimates the difference between measured cations and anions, and is used to detect and classify metabolic acidosis. It is computed as Na⁺ − (Cl⁻ + HCO₃⁻), with a variant that includes potassium [(Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻)]. Because albumin is the principal unmeasured anion, in hypoalbuminemic patients an albumin correction should be applied (adding roughly 2.5 mEq/L for every 1 g/dL of albumin below 4 g/dL) to avoid underestimating the gap. It is a foundational acid–base diagnostic tool rather than a prognostic score endorsed by a single society.

How to interpret it

The usual reference range for the anion gap without potassium is approximately 8–12 mEq/L (around 12–16 mEq/L when potassium is included), though it varies by laboratory and current measurement methods. An elevated anion gap points to high-anion-gap metabolic acidosis (ketoacidosis, lactic acidosis, renal failure, toxic ingestions such as methanol, ethylene glycol, or salicylates). A normal gap in the setting of acidosis suggests hyperchloremic (normal-gap) acidosis (diarrhea, renal tubular acidosis). A low gap is uncommon and may reflect hypoalbuminemia, paraproteinemia, or laboratory error. The albumin-corrected value should always be interpreted within the overall blood-gas context.

Limitations and when not to use it

Use simultaneous, reliable electrolyte values; each laboratory has its own reference range depending on methodology. Albumin correction is essential in critically ill, hepatic, or malnourished patients, where hypoalbuminemia can mask a high-gap acidosis. The calculation may be distorted by hyperlipidemia, hyperglobulinemia, lithium, bromide, or preanalytical error. The anion gap by itself is NOT a diagnosis or a measure of pH or severity: it does not replace arterial blood-gas analysis or the osmolar-gap calculation when intoxication is suspected, and it provides no direct prognostic information.

Frequently asked questions

Why does the anion gap need to be corrected for albumin?

Albumin is the main unmeasured anion; when it is low, the calculated gap is artificially low and can mask a high-gap acidosis. It is corrected by adding roughly 2.5 mEq/L for every 1 g/dL of albumin below 4 g/dL.

What is a normal anion gap value?

It is typically around 8–12 mEq/L when potassium is not included, and around 12–16 mEq/L when it is, but the exact range depends on the laboratory and measurement method.

What does a high anion gap mean?

It suggests a high-anion-gap metabolic acidosis, typically from ketoacidosis, lactic acidosis, renal failure, or toxic ingestions (methanol, ethylene glycol, salicylates). It should be confirmed with blood-gas analysis and the clinical context.