Clinical Overview
The Serum Anion Gap is one of the first calculations performed when evaluating a patient with metabolic acidosis. It helps narrow down the differential diagnosis into two major categories: High Anion Gap Metabolic Acidosis (HAGMA) or Normal Anion Gap Metabolic Acidosis (NAGMA).
Pathophysiology & Evidence
The principle of electroneutrality dictates that the total positive charges (cations) in the blood must equal the total negative charges (anions). However, routine lab panels only measure the major cation (Sodium) and major anions (Chloride and Bicarbonate). The "Gap" represents the unmeasured anions (like albumin, phosphate, sulfate, or exogenous toxins like lactate and ketones).
Formula Breakdown
Anion Gap = Sodium - (Chloride + Bicarbonate)
If the gap exceeds 12, it indicates the presence of unmeasured toxic anions, pointing the clinician toward diagnoses like DKA, lactic acidosis, or toxic alcohol ingestion.