anion gap
Summary
The anion gap is a calculated value representing unmeasured anions in serum, used to classify metabolic acidosis. Normal range is 8-12 mEq/L. An elevated anion gap (>12) indicates accumulation of organic acids and helps narrow the differential diagnosis of metabolic acidosis.
Detail
The anion gap is calculated as [Na+] - ([Cl-] + [HCO3-]) and represents the difference between measured cations and anions in serum. It exists because unmeasured anions (proteins, phosphates, sulfates, organic acids) normally exceed unmeasured cations. In metabolic acidosis, the anion gap helps differentiate causes: normal anion gap (non-gap) acidosis results from bicarbonate loss or failure to excrete acid (diarrhea, RTA, ureteral diversions), while high anion gap acidosis results from accumulation of organic acids. High anion gap causes include ketoacidosis (diabetic, starvation, alcoholic), lactic acidosis, uremia, methanol/ethylene glycol poisoning, and salicylate toxicity (remembered by MUDPILES mnemonic). The anion gap can be falsely elevated by hypoalbuminemia (albumin is a major unmeasured anion) or laboratory errors, and falsely normal in severe hypoalbuminemia or paraproteinemias.
Sources
- Harrison's Principles of Internal Medicine
- Brenner and Rector's The Kidney
- Rose and Post's Clinical Physiology of Acid-Base and Electrolyte Disorders
- First Aid for the USMLE Step 1
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