Differentiation of amphetamine/methamphetamine and other cross-immunoreactive sympathomimetic amines in urine samples by serial dilution testing.
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.
Woodworth
Alison
A
Department of Pediatrics, Washington University School of Medicine, St. Louis, MO 63110, USA.
Saunders
Al N
AN
Koenig
John W
JW
Moyer
Thomas P
TP
Turk
John
J
Dietzen
Dennis J
DJ
D000661:Amphetamine / Q000652:urine*; D003429:Cross Reactions; D004809:Ephedrine / Q000652:urine*; D006801:Humans; D007118:Immunoassay; D008694:Methamphetamine / Q000652:urine*; D018817:N-Methyl-3,4-methylenedioxyamphetamine / Q000652:urine*; D012372:ROC Curve; D015813:Substance Abuse Detection / Q000379:methods*; D013566:Sympathomimetics / Q000652:urine*
D013566:Sympathomimetics; D008694:Methamphetamine; D000661:Amphetamine; D004809:Ephedrine; D018817:N-Methyl-3,4-methylenedioxyamphetamine