Anion Gap

Formula

Anion gap  =  Na  −  (Cl  +  HCO₃)
Normal  =  8–12 mEq/L (without potassium)
Albumin-corrected gap  =  Anion gap  +  2.5 × (4 − albumin)

The anion gap reflects the unmeasured anions in serum. When a strong acid like ketoacid or lactate accumulates, it consumes bicarbonate and is replaced in the gap by an unmeasured anion — so HCO₃ drops, Cl stays the same, and the gap rises.

MUDPILES — high-gap differential

• Methanol
• Uremia (kidney failure)
• DKA — diabetic ketoacidosis
• Propylene glycol
• INH / iron
• Lactic acidosis (sepsis, ischemia)
• Ethylene glycol
• Salicylates

NCLEX-favorite high-gap causes: DKA, lactic acidosis (sepsis or shock), and salicylate toxicity (look for tinnitus + tachypnea).

Normal-gap acidosis differential

Mnemonic HARDASS: Hyperalimentation, Addison's, Renal tubular acidosis, Diarrhea, Acetazolamide, Spironolactone, Saline (0.9% can cause hyperchloremic acidosis at large volumes).

Worked example

Na 138, Cl 100, HCO₃ 12, albumin 2.5.

1. Raw gap: 138 − (100 + 12) = 26 mEq/L → high.
2. Albumin-corrected: 26 + 2.5 × (4 − 2.5) = 26 + 3.75 = 29.75 → even higher than the raw value suggested.
3. Differential: MUDPILES. Check lactate, ketones, salicylate level, BUN.