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Factors influencing the testing for driving under the influence.

To the Editor. —The recent letter by Saylor et al 1 about driving under the influence of drugs (DUID) prompts us to issue a word of caution. As the authors point out, 14% to 81% of impaired drivers with low or negligible blood ethanol levels test positive for other mind- or performance-altering drugs. The wide range observed may be due to several factors. Drug testing for DUID is not uniform. Because of limited resources, police-related toxicology laboratories frequently focus their efforts on the prosecution of offenders and not on complete toxicologic testing. For example, in states where ethanol intoxication is legally defined as a specific concentration of ethanol in blood, a higher concentration in a driver's blood may prompt no further toxicologic testing since the blood ethanol concentration alone is sufficient for prosecution. If a driver's blood ethanol concentration is low or negligible, tests for other intoxicating drugs may be