ABSTRACT
The pressure to reduce the cost of analytic testing makes it tempting to discontinue routine confirmation of urine specimens positive for drugs of abuse by immunoassay. Beyond the economic motivation, the requirement for confirmation should be driven by the positive predictive value of the screening tests. We have quantitated positive predictive values of our screening immunoassays in a large metropolitan Veterans Affairs Medical Center. We reviewed the confirmatory rate of urine specimens positive for drugs of abuse with Beckman Synchron reagents from June 1998 to June 1999 and tabulated the false-positive screening rate. There were 175 instances of false-positive screens during the 13 months we analyzed. Positive predictive values ranged from 0% (amphetamine) to 100% (THC). We determined that the low positive predictive value of the amphetamine assay in our laboratory was primarily due to the use of ranitidine (Zantac). Urine specimens containing greater than 43 microg/mL ranitidine were positive in our amphetamine assay. This concentration is routinely exceeded in our patients taking ranitidine. In our clinical and analytic setting, the Beckman THC assay did not require confirmation. The positive predictive values of the Beckman opiate, cocaine, barbiturate, propoxyphene, and methadone immunoassays dictate routine confirmatory testing in specimens that screen positive for these substances. Finally, because of its extreme sensitivity to ranitidine, the Beckman amphetamine assay has little utility in our laboratory setting.
