Anion gap and hypoalbuminemia

Crit Care Med. 1998 Nov;26(11):1807-10. doi: 10.1097/00003246-199811000-00019.


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 (r2 = .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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acidosis / blood
  • Acidosis / diagnosis
  • Anions
  • Arteries
  • Carbon Dioxide / blood
  • Electrolytes / blood*
  • Humans
  • Hydrogen-Ion Concentration
  • Reference Values
  • Serum Albumin / analysis
  • Serum Albumin / metabolism*


  • Anions
  • Electrolytes
  • Serum Albumin
  • Carbon Dioxide