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Published:
Journal of Analytical Toxicology,
ISSN 0146-4760,
Volume 25,
Number 3, April 2001, pp. 174-178
Positive
Predictive Values of Abused Drug Immunoassays on the Beckman Synchron in a Veteran
Population
Dennis J.
Dietzen[1],[2],*, Kitty Ecos[1], David Friedman[1], and Sandra Beason[1]
[1] Pathology and Laboratory Medicine Service, Veterans Affairs Medical Center,
Memphis, Tennessee and
[2] Department of Pathology, University of Tennessee School of Medicine, Memphis,
Tennessee 38104
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 µg/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.
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