For the average analytic laboratory, confirmed false negatives occur at a rate of 10-30% and confirmed false positives occur at a rate of 0 – 10%. The most common reason for an analytic false negative is ordering a drug screen which does not test for the suspected drug. False positives are low due to high concentrations of the detected drug and confirmation of the presumptive positive by an alternative method. These statements are true for drug testing in which an initial analytic method identified presumptive positives and a confirmation test of a different analytic method confirmed or disproved the presence of the drug.
At Vanderbilt, urine is screened by a color test, gas chromatography, or immunoassay. Presumptive positives are confirmed by GCMS. (gas chromatography-mass spectrometry) Urine is used as the screening medium for the following reasons: noninvasive collection, large volumes, high drug concentration, metabolites present, metabolites present, and well studied and tested legally.
One should be able to obtain presumptive positives from an urine drug screen within a hour. There are some false positives of which one should be aware on the initial screening immunoassay test:
TCA False positives (i.e., presence of the following drugs will cause the presumptive urine drug screen to be positive for TCA. The confirmation test should be negative for TCA): chlorpromazine, thioridazine, promethazine, orphenadrine, diphenhydramine (Benadryl) cyclobenzaprine (Flexeril), carbamazepine (Tegretol).
Amphetamine False Positives (i.e., presence of the following drugs will cause the presumptive urine drug screen to be positive for amphetamine. The confirmation test should be negative for amphetamines): Over-the-
Counter cold preparations, bupropion, carbamazepine, labetalol,
Confirmation testing occurs within 24 hours for most drugs.
Next Week (Part III)
As always, if there are any questions, call the MTPC.
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