A normal anion gap is generally 3–12 mEq/L, though the reference range varies by laboratory and the method used to measure electrolytes. An elevated anion gap (typically >12 mEq/L) indicates the presence of unmeasured anions in the blood, most commonly organic acids. The classic causes are remembered by the mnemonic MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates.

If you provided an albumin value, the albumin-corrected anion gap adjusts for hypoalbuminemia. Each 1 g/dL decrease in albumin below 4.0 can mask approximately 2.5 mEq/L of the anion gap. In critically ill or malnourished patients with low albumin, the uncorrected anion gap may appear normal even when significant unmeasured acids are present. Always interpret the corrected value in hypoalbuminemic patients.

Calculate the anion gap whenever a metabolic acidosis is identified on [arterial blood gas](/tools/abg-interpreter) or basic metabolic panel (low bicarbonate, low pH). It is the essential next step in the workup of metabolic acidosis, as it determines the differential diagnosis: an elevated anion gap points toward acid-producing conditions (ketoacidosis, lactic acidosis, toxic ingestions, renal failure), while a normal anion gap (hyperchloremic) acidosis points toward bicarbonate-losing conditions (diarrhea, renal tubular acidosis, normal saline administration).

The anion gap should be calculated routinely in emergency department and ICU settings for patients presenting with altered mental status, suspected overdose, sepsis, diabetic emergencies, or unexplained metabolic derangements. It is also useful for monitoring the response to treatment in known anion gap acidosis.

The anion gap is an indirect calculation, not a direct measurement, and is affected by the accuracy of the underlying electrolyte measurements. Different laboratory analyzers can produce slightly different results, which is why normal ranges vary between institutions. The traditional normal range of 8–12 mEq/L was established with older assays; newer ion-selective electrode methods often yield a lower normal range of 3–10 mEq/L.

The anion gap does not identify the specific cause of acidosis — it only narrows the differential. Additional testing (lactate, ketones, osmolar gap, toxicology screen) is required to determine the etiology. A normal anion gap does not exclude serious illness, as hyperchloremic acidosis from diarrhea, RTA, or other causes can be clinically significant.

In patients with multiple concurrent acid-base disorders, the delta-delta ratio (comparing the change in anion gap to the change in bicarbonate) provides additional information but adds complexity. The anion gap is a screening tool that works best as part of a systematic acid-base analysis, not in isolation.