Anion gap and hypoalbuminemia

Objectives: To show how hypoalbuminemia lowers the anion gap, which can mask a significant gap acidosis; and to derive a correction factor for it. Design: Observational study, Setting: Intensive care unit in a university-affiliated hospital. Subjects: Nine normal subjects and 152 critically ill patients (265 measurements). Interventions: None. Measurements and Main Results: Arterial blood samples analyzed for pH, PCO2, and concentrations of plasma electrolytes and proteins. Marked hypoalbuminemia was common among the critically ill patients: 49% of them had serum albumin concentration of <20 g/L. Each g/L decrease in serum albumin caused the observed anion gap to underestimate the total concentration of gap anions by 0.25 mEq/L (r(2) = .94). Conclusions: The observed anion gap can be adjusted for the effect of abnormal serum albumin concentrations as follows: adjusted anion gap = observed anion gap + 0.25 x ([normal albumin] [observed albumin]), where albumin concentrations are in g/L; if given in g/dL, the factor is 2.5. This adjustment returns the anion gap to the familiar scale of values that apply when albumin concentration is normal.