The anion gap: A simple, helpful tool in the differential diagnosis of metabolic acidosis

Metabolic acidosis is a primary disturbance in acid-base physiology characterized by either a gain of H+ or a loss of HCO-3. The distinction is not trivial, since a gain in H+ is an emergency which may be life threatening. The anion gap, Na+-(Cl+HCO-3), is very helpful in this regard, since gain of H+ disturbances are almost invariably non-chloride in nature. Thus, for every 1 mmol/L decrease in HCO-3 an increase of 1 mmol/L in the anion gap would be expected. On the other hand, disturbances which involve loss of HCO3 via the gastrointestinal tract or which feature failure of the kidney to reclaim or generate HCO-3 appropriately are characterized by increased renal Cl- reabsorption and result in a normal anion gap. We recently encountered a patient with severe metabolic acidosis who was misdiagnosed because his physicians could not make this distinction. The first steps in evaluating any metabolic acidosis should include calculating the anion gap. Further, since acid-base disturbances are frequently multifactorial, the anion gap should be calculated in all acid-base disturbances, irrespective of cause.

Duke Scholars

Author Ruediger Wilhelm Lehrich Medicine, Nephrology