Anion Gap and Acid-Base Interpretation
The anion gap (AG = Na - Cl - HCO3) separates metabolic acidoses that accumulate unmeasured anions from those that do not.
Gap classification
- High anion gap acidosis: lactate, ketoacids, renal failure (urate/sulfate/phosphate), toxins (methanol, ethylene glycol, salicylate).
- Normal (hyperchloremic) anion gap acidosis: GI bicarbonate loss (diarrhea), renal tubular acidosis, early renal disease, acetazolamide; urine anion gap distinguishes renal (positive) from GI (negative) ammonium handling.
- Delta ratio: AG/24 - HCO3 — a ratio near 1-2 suggests a simple high-gap acidosis; a ratio under 1 suggests a coexisting hyperchloremic acidosis; over 2 suggests a coexisting alkalosis.
Respiration
Hypocapnia is the normal compensation for metabolic acidosis (expected pCO2 ~ 1.5 x HCO3 + 8). Failure to compensate suggests a mixed disorder.
Correction
Hypoalbuminemia lowers the gap (albumin is a negatively charged protein); add 2.5 to the gap for every 1 g/dL albumin below 4.0 g/dL.