Anion Gap Calculator
Calculates the anion gap from serum sodium, chloride, and bicarbonate to help identify the cause of metabolic acidosis. Clinicians use it whenever a patient presents with unexplained acidosis or altered mental status.
About this calculator
The anion gap (AG) represents the concentration of unmeasured anions in plasma—such as phosphate, sulfate, and organic acids—and is calculated from three routinely measured electrolytes. The formula is: AG = Sodium − (Chloride + Bicarbonate), with all values in mEq/L. The normal anion gap is approximately 8–12 mEq/L (without albumin correction). An elevated anion gap (>12 mEq/L) signals the presence of unmeasured acids such as lactate, ketones, uremia, or ingested toxins—often summarized by the MUDPILES mnemonic. A normal anion gap acidosis, by contrast, suggests bicarbonate loss (e.g., diarrhea) or renal tubular acidosis. The AG is a critical first step in the systematic diagnosis of metabolic acidosis and guides further workup efficiently.
How to use
A patient's labs show: Sodium = 140 mEq/L, Chloride = 100 mEq/L, Bicarbonate = 15 mEq/L. Apply the formula: AG = 140 − (100 + 15) = 140 − 115 = 25 mEq/L. Since 25 mEq/L is well above the normal range of 8–12 mEq/L, this is a high anion gap metabolic acidosis. The clinician would next investigate causes such as lactic acidosis, diabetic ketoacidosis, or toxic ingestion. Enter the three electrolyte values and the calculator computes the anion gap instantly.
Frequently asked questions
What does a high anion gap indicate in metabolic acidosis?
A high anion gap (>12 mEq/L) indicates the accumulation of unmeasured anions in the blood, most commonly due to excess acid production or impaired acid excretion. Common causes are remembered with the MUDPILES mnemonic: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates. Identifying a high AG narrows the differential diagnosis significantly and guides targeted testing, such as lactate levels or ketone measurement. Prompt recognition is essential because many causes are life-threatening.
What is the normal anion gap range and how does albumin affect it?
The traditional normal anion gap is 8–12 mEq/L when measured without albumin correction. However, albumin is the major unmeasured anion, so hypoalbuminemia lowers the expected AG by approximately 2.5 mEq/L for every 1 g/dL drop in albumin below 4 g/dL. In critically ill patients with low albumin, an AG of 10 mEq/L might actually represent a significantly elevated corrected AG. Clinicians should apply albumin correction when interpreting the anion gap in any patient with hypoalbuminemia to avoid missing a high-gap acidosis.
How is the anion gap different from the delta-delta ratio in acid-base interpretation?
The anion gap identifies whether unmeasured anions are elevated, confirming a high-gap metabolic acidosis. The delta-delta ratio (or delta ratio) takes the analysis further by comparing the rise in AG to the fall in bicarbonate: Δ AG / Δ HCO₃. A ratio of 1–2 suggests a pure high-gap acidosis, while values outside this range suggest a mixed disorder—such as a concurrent normal-gap acidosis or metabolic alkalosis. The anion gap is the essential first step; the delta-delta is the follow-up tool for uncovering mixed disturbances.