The Poison Review

Falsely Normal Anion Gap in Severe Salicylate Poisoning Caused by Laboratory Interference.  Jacob J, Lavonas EJ. Ann Emerg Med 2011 April 12 [Epub ahead of print]

Abstract

Salicylate toxicity typically causes a high-anion-gap metabolic acidosis. This short paper presents 2 cases of moderate to severe salicylate toxicity with normal-low anion gaps.  The authors suggest that the absence of an elevated anion gap in these cases is explained by the fact that high salicylate levels can interfere with the specificity of certain laboratory electrodes, showing up as falsely elevated chloride levels.  There have been more extreme examples of this laboratory error reported in the past, with measured chloride levels of 174 and 146 mEq/L and anion gaps of -49 and -26. (Am J Kidney Dis 2008;51:346)

The important take-home lesson for clinicians is that if a patient has a metabolic acidosis but a normal (or negative) anion gap, salicylate toxicity should not be ruled out.

Other causes of a negative anion gap include increased bromide levels, hyperlipidemia, iodide intoxication, and paraproteinemia in multiple myeloma.