The serum anion gap (AG) is an important tool in assessing the acid-base status of a patient and narrowing the differential diagnosis of etiologies responsible for acid-base disturbance. The anion gap is a measure of serum electrical neutrality. Total serum cations always equal serum anions. Thus, sodium + potassium + calcium + magnesium = bicarbonate + chloride + phosphate + sulfate + protein + organic acid anions. To simplify the equation, potassium, calcium, and magnesium are considered unmeasured cations (UC), and phosphate, sulfate, protein, and organic acids are considered unmeasured anions (UA). Thus, UA – UC = sodium – chloride – bicarbonate = serum anion gap. The normal range of anion gap is between 3 to 11 mEq/L, with modern techniques of measuring electrolytes. A reduction of 2.5 to 3 mEq in AG should be made for every 1-gram reduction in the serum albumin below 4.