A forensic patient tested positive for amphetamine on eight occasions over an 8-week period using a Dade Behring amphetamine/methamphetamine assay. Low concentrations of urinary creatinine in two samples suggested specimen dilution.
Telephone advice from the laboratory emphasised that a positive result from a dilute sample was highly significant. The same advice stated that according to the literature from Dade Behring, chlorpromazine and other medication that the patient was on could not account for the test result. This led us to conclude that amphetamine consumption was recent.
An extensive search for the source of amphetamine proved negative. Subsequent testing of the original samples at an alternative laboratory was negative using both the Cedia kit for an Olympus analyser and the gold standard method of gas chromatography/mass spectrometry (GC/MS).
Closer reading of the literature from Dade Behring revealed a footnote confirming that false-positive results for amphetamine may occur with patients taking chlorpromazine. The advice is that all positive results require confirmation with an alternative method, preferably GC/MS.
The conclusion of Acostsa-Armas (2003) that a positive result on LSD (lysergic acid diethylamide) immunoassay should be confirmed by at least one alternative method can be generalised to amphetamine immunoassays.
National guidelines for testing employees (Steering Group, 2004, section 4.9.3) extend this conclusion to all positive drug tests:
‘Only drugs which have been confirmed by a recognised confirmation test can be reported as positive.’
The consequences of a positive test for certain patient groups makes following these guidelines of paramount importance.
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