The missing gap. A pitfall in the diagnosis of alcohol intoxication by osmometry.

An osmolal gap (a disparity between measured and calculated serum osmolality) may accompany alcohol intoxication. We encountered a patient with methanol toxicity in whom no such gap was present, despite a markedly elevated serum methanol level. Further investigation revealed that serum osmolality had been measured with a vapor pressure osmometer; this technique may not detect volatile solutes such as alcohols. In vitro testing confirmed the insensitivity of vapor pressure osmometry to toxic ranges of both methanol and ethanol. When alcohol toxicity is suspected, an osmolal gap should be sought only if freezing point osmometry is available.