Clinical Chemistry
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Clinical Chemistry 52: 743-746, 2006. First published February 2, 2006; 10.1373/clinchem.2005.060616
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(Clinical Chemistry. 2006;52:743-746.)
© 2006 American Association for Clinical Chemistry, Inc.

Technical Briefs

Differentiation of Amphetamine/Methamphetamine and Other Cross-Immunoreactive Sympathomimetic Amines in Urine Samples by Serial Dilution Testing

Alison Woodworth2, Al N. Saunders3, John W. Koenig3, Thomas P. Moyer4, John Turk2 and Dennis J. Dietzen1,2,a

(Departments of1 Pediatrics and 2 Pathology and Immunology, Washington University School of Medicine, St. Louis, MO;3 Drug Analysis Laboratory, Barnes-Jewish Hospital, St. Louis, MO;4 Department of Laboratory Medicine and Pathology, Mayo Clinic and Mayo Foundation, Rochester, MN;

aaddress correspondence to this author at: Department of Pediatrics, Box 8208, Washington University School of Medicine, 660 S. Euclid Ave., St. Louis, MO 63110; fax 314-286-2892, e-mail Dietzen_D{at}kids.wustl.edu)


Abstract

Background: Immunoassay-based screening for amphetamines has a variable positive predictive value (PPV) for detecting amphetamine abuse. The lack of immunoassay specificity necessitates confirmatory testing by gas chromatography–mass spectrometry (GC/MS), but the technical complexity and expense of GC/MS limit its availability. Physicians may make decisions regarding patient disposition based on unverified results. In this study we assessed the utility of using dose–response properties to distinguish urine samples containing amphetamines from samples containing cross-immunoreactive species.

Methods: Urine was supplemented with known concentrations of amphetamine, methamphetamine, methylenedioxymethamphetamine (MDMA), or pseudoephedrine. Using a series of dilutions, we determined the maximum change in rate over the fractional change in concentration for each compound in the Emit® II amphetamine/methamphetamine immunoassay. Patient urine samples that screened positive for amphetamines were diluted 1:1, 1:10, and 1:20, and maximum slope estimates within the dynamic assay range were determined. An optimal slope cutoff that differentiated samples containing (meth)amphetamine from those containing cross-reacting species was determined by ROC analysis.

Results: The slope of the dose response was largest for amphetamine and methamphetamine, followed by MDMA and pseudoephedrine. The optimum slope cutoff for identifying patient specimens containing (meth)amphetamine was 320 (sensitivity, 96%; specificity, 90%; PPV, 92%). High concentrations of less reactive compounds may mask low concentrations of amphetamines.

Conclusions: Use of the slope of the dose–response relationship in patient urine specimens can enhance the PPV of presumptive positive immunoassay results but does not exclude the presence of low amphetamine concentrations in samples containing high concentrations of cross-reactive species.






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