Anion Gap Calculator

Anion Gap: Clinical Guide for Healthcare Professionals

The anion gap is one of the most fundamental calculations in clinical medicine, used daily in emergency medicine, intensive care, internal medicine, and nephrology. It reflects the concentration of unmeasured anions in serum — primarily albumin, phosphate, sulphate, and organic acids — and deviations from the normal range point directly to specific metabolic disorders.

As a clinical educator who has taught acid-base physiology to medical students and residents for over a decade, I’ll walk through the complete anion gap interpretation framework that the calculator above implements automatically.

The Anion Gap Formulas

Why Albumin Correction Matters

Albumin is the primary unmeasured anion contributing to the normal anion gap. Hypoalbuminaemic patients (common in hospitalised, critically ill, and malnourished patients) have a lower baseline anion gap. Failing to correct for albumin in these patients causes clinicians to miss high anion gap metabolic acidosis — a potentially life-threatening diagnostic error. The corrected anion gap calculator above adjusts automatically when albumin is entered.

Clinical rule of thumb: for every 1 g/dL decrease in albumin below 4.0, the normal anion gap falls by approximately 2.5 mEq/L. A patient with albumin of 2.0 g/dL has a normal AG of approximately 7 mEq/L — so a measured AG of 10 in this patient is actually high (corrected AG ≈ 15).