The reliability of immunoassay for determining the presence of opiates in the forensic setting.

Urine immunoassays are commonly used as a rapid screen for drugs of abuse in emergency room, hospital, clinic, and forensic settings. The authors were concerned whether or not a negative screen of the urine for opiates was of significance and indicative that analysis of blood for opiates was not necessary. Specifically, we wished to determine whether a negative test for opiates by immunoassay absolutely rules out an acute overdose, and if not, what percentage of cases with negative results have opiates in the blood. A retrospective analysis was performed using the toxicology results for cases ruled an acute narcotic overdose at the Bexar County Medical Examiner's Office between 1998 and 2003. One hundred eighty-three cases met the criteria for the study. A false-negative rate of approximately 15% was found using an immunoassay as compared with blood analysis for narcotics. The authors feel that while this rate may be acceptable in a clinical setting, it is unacceptable in a forensic setting.