Anion Gap Calculator
Calculate anion gap and albumin-corrected anion gap from basic metabolic panel values. Interpret results for metabolic acidosis evaluation.
Quick Answer
Anion Gap = Na+ - (Cl- + HCO3-). Normal range is 8-12 mEq/L. An elevated anion gap (>12) suggests metabolic acidosis from unmeasured anions (e.g., lactic acid, ketoacids, toxins). Correct for low albumin by adding 2.5 for each 1 g/dL albumin below 4.0.
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About This Tool
The anion gap is a calculated value derived from routine blood chemistry tests that helps clinicians evaluate acid-base disorders, particularly metabolic acidosis. It represents the difference between the measured cations (positively charged ions) and the measured anions (negatively charged ions) in the blood. In reality, the blood is electrically neutral with equal positive and negative charges, but because standard lab panels do not measure all ions, there is an apparent "gap" that reflects the unmeasured anions.
The formula is straightforward: Anion Gap = Sodium - (Chloride + Bicarbonate). The normal range is typically 8 to 12 mEq/L, though some laboratories use 3 to 11 mEq/L depending on how the assays measure chloride. The unmeasured anions that make up the normal gap include albumin (the largest contributor), phosphate, sulfate, and organic acids. Understanding this is critical because changes in any of these components can alter the anion gap independently of a primary acid-base disturbance.
Albumin Correction
Albumin is the single largest contributor to the normal anion gap, accounting for roughly half of the unmeasured anions. When albumin is low (hypoalbuminemia), which is extremely common in hospitalized patients, the baseline anion gap decreases. This means a "normal" anion gap in a hypoalbuminemic patient may actually be masking an elevated anion gap acidosis. The correction formula adds 2.5 mEq/L to the anion gap for each 1 g/dL decrease in albumin below the normal level of 4.0 g/dL. For example, a patient with an albumin of 2.0 g/dL would have a correction of 2.5 x (4.0 - 2.0) = 5.0 mEq/L added to their measured anion gap. This corrected value is essential for accurate diagnosis in critically ill patients.
Elevated Anion Gap Metabolic Acidosis
An elevated anion gap indicates the presence of excess unmeasured anions in the blood, typically from the accumulation of organic acids. The classic causes are remembered by the MUDPILES mnemonic: Methanol poisoning, Uremia (kidney failure), Diabetic ketoacidosis (DKA), Propylene glycol toxicity, Isoniazid or Iron toxicity, Lactic acidosis (the most common cause), Ethylene glycol poisoning, and Salicylate (aspirin) overdose. Among these, lactic acidosis and DKA are by far the most frequently encountered in clinical practice. Lactic acidosis can result from any condition causing tissue hypoperfusion or hypoxia, including sepsis, cardiogenic shock, severe anemia, and carbon monoxide poisoning.
Normal Anion Gap Metabolic Acidosis
A metabolic acidosis with a normal anion gap (also called non-anion-gap metabolic acidosis or hyperchloremic metabolic acidosis) occurs when bicarbonate is lost or chloride is retained without a corresponding increase in unmeasured anions. Common causes include diarrhea (loss of bicarbonate from the GI tract), renal tubular acidosis (failure of the kidney to properly acidify urine or reclaim bicarbonate), excessive administration of normal saline (dilutional acidosis), and certain medications like acetazolamide. In these conditions, chloride rises to replace the lost bicarbonate, keeping the anion gap normal.
Delta Gap and Delta Ratio
The delta gap (also called the delta-delta) is the difference between the patient's anion gap and the upper limit of normal (12). It is compared to the decrease in bicarbonate (24 minus the measured bicarbonate) to form the delta ratio. A delta ratio between 1 and 2 suggests a pure anion gap metabolic acidosis. A ratio less than 1 suggests a mixed anion gap and non-anion gap metabolic acidosis. A ratio greater than 2 suggests the coexistence of an anion gap metabolic acidosis with a metabolic alkalosis, or a pre-existing elevated bicarbonate. This analysis is a cornerstone of the systematic approach to complex acid-base disorders in critical care medicine.
Low Anion Gap
A low anion gap (below 8 mEq/L) is less common but can be clinically significant. The most frequent cause is hypoalbuminemia, which reduces the unmeasured anion pool. Other causes include multiple myeloma (where cationic immunoglobulin paraproteins increase unmeasured cations), lithium toxicity, hypercalcemia, hypermagnesemia, and laboratory error. Bromide intoxication can cause a falsely low anion gap because some chloride assays measure bromide as chloride, artificially elevating the measured chloride value.