Reduced interference by phenothiazines in amphetamine drug of abuse immunoassays.
BACKGROUND
Emergency department physicians frequently request urine drug screens, but many are unaware of their limitations, including the potential for false-positive results. Promethazine, a phenothiazine derivative, is used for the treatment of allergies, agitation, nausea, and vomiting. Many patients taking promethazine are subject to urine drug screens and any potential interferences are important to recognize.


METHODS
During an 11-month period, all patients presenting to the Massachusetts General Hospital emergency department who had a finding of promethazine in their serum drug screen, and who also had a urine drug screen performed, were selected for inclusion in the study. The urine drug screen results (n = 22 patients/samples) were then studied.


OBJECTIVE
To determine if promethazine use can cause false-positive urine amphetamine results in widely used drug of abuse immunoassays.


RESULTS
Thirty-six percent of patients taking promethazine had false-positive test results for urine amphetamines using the EMIT II Plus Monoclonal Amphetamine/Methamphetamine Immunoassay. Sixty-four percent of patients showed cross-reactivity greater than 20% higher than the blank calibrator rate. In a separate, related study, no promethazine-induced false-positive results were seen with the EMIT II Plus, Triage, and TesTcard 9 amphetamine assays, or the Triage methamphetamine assay. Reduced chlorpromazine interference was also seen with these other assays.


CONCLUSIONS
False-positive urine amphetamine results can be obtained in patients taking promethazine. Promethazine metabolite(s), and not the parent compound, are the likely cause of these urine false-positive results obtained with EMIT II Plus Monoclonal Amphetamine/Methamphetamine Immunoassay. Immunoassays from different manufacturers can have very different "interference" profiles, which the pathologist and laboratory scientist must understand and relay to clinicians.
Melanson
Stacy E F
SE
Clinical Laboratory Division, Department of Pathology, Brigham and Women's Hospital and Harvard Medical School, Boston, Mass, USA.
Lee-Lewandrowski
Elizabeth
E
Griggs
David A
DA
Long
William H
WH
Flood
James G
JG
D000661:Amphetamine / Q000652:urine*; D000911:Antibodies, Monoclonal / Q000652:urine; D000697:Central Nervous System Stimulants / Q000652:urine*; D003429:Cross Reactions; D017248:Enzyme Multiplied Immunoassay Technique; D005189:False Positive Reactions; D006801:Humans; D011398:Promethazine / Q000652:urine*; D015813:Substance Abuse Detection / Q000379:methods*
D000911:Antibodies, Monoclonal; D000697:Central Nervous System Stimulants; D000661:Amphetamine; D011398:Promethazine