Anion Gap Calculator

Understanding the Anion Gap

The anion gap is a calculated value from a basic metabolic panel that helps diagnose acid-base disorders. Blood is electrically neutral, so total positive charges (cations) equal total negative charges (anions). However, we don't measure all ions routinely.

We measure sodium as the main cation, and chloride plus bicarbonate as the main anions. The formula AG = Na - (Cl + HCO3) reveals the 'gap' created by unmeasured anions like albumin, phosphate, sulfate, and organic acids.

In healthy people, this gap is 3-11 mEq/L, mostly from albumin. When the gap widens above 11 mEq/L, it signals accumulation of abnormal acids—lactic acid from shock, ketoacids from diabetes, or toxins like methanol. This narrows the differential diagnosis of metabolic acidosis significantly.

Medical disclaimer: This calculator is for educational purposes only. Anion gap interpretation requires clinical context and should only be performed by qualified healthcare providers. Acid-base disorders can be life-threatening and require immediate medical attention.

High Anion Gap Metabolic Acidosis

High anion gap acidosis means an acid has accumulated in the blood, lowering pH and bicarbonate. The MUDPILES mnemonic lists the main causes: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid, Lactic acidosis, Ethylene glycol, and Salicylates.

Lactic acidosis is the most common cause, occurring in shock, sepsis, severe hypoxia, and seizures. Tissues deprived of oxygen switch to anaerobic metabolism, producing lactic acid faster than the body can clear it. Anion gap can reach 20-30 mEq/L in severe cases.

Diabetic ketoacidosis produces beta-hydroxybutyrate and acetoacetate, raising the anion gap to 20-40 mEq/L. Patients present with hyperglycemia, dehydration, and a fruity breath odor. Alcoholic ketoacidosis causes similar findings but with normal or low glucose.

Toxic ingestions like ethylene glycol (antifreeze) and methanol produce lethal metabolites. Anion gap is often above 20 mEq/L with severe acidosis. These require immediate treatment with fomepizole or dialysis to prevent permanent organ damage or death.

Albumin Correction and Clinical Pearls

Albumin is a negatively charged protein accounting for roughly half the normal anion gap. When albumin is low (from malnutrition, liver disease, or nephrotic syndrome), the measured anion gap drops even if acid accumulation is present.

The correction formula adds 2.5 mEq/L to the anion gap for every 1 g/dL that albumin falls below 4.0 g/dL. For example, if measured AG is 8 mEq/L but albumin is 2.0 g/dL, the corrected AG is 8 + (2.5 × 2) = 13 mEq/L, revealing hidden acidosis.

Normal anion gap acidosis (non-gap acidosis) occurs when bicarbonate loss is matched by chloride gain. Causes include diarrhea, renal tubular acidosis, and hyperchloremic saline infusions. The HARDUP mnemonic helps: Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureterosigmoidostomy, Pancreatic fistula.

Always interpret the anion gap alongside arterial blood gas results, electrolytes, renal function, and clinical history. A patient in shock with an AG of 25 mEq/L and low bicarbonate clearly has lactic acidosis. Context makes the calculation meaningful.