Determination of illicit drug cutoff values in a pain patient population.

Background: When properly selected, cutoff levels minimize the reporting of false negative and false positive test results and allow the laboratory to accurately determine the prevalence of marijuana, cocaine and methamphetamine use. Selecting the ideal cutoff requires the collection of drug excretion data for a large patient population to determine the expected range of drug concentrations. The cutoff can then be set to capture a high percentage of positives at a concentration within the dynamic range of the method. We used quantitative urine drug excretion data to calculate cutoffs needed to best determine the presence of these illicit drugs in urine.

Methods: This study used liquid chromatography tandem mass spectrometry (LC-MS/MS) as the analytical method. The study group was the pain patient population which is well-known to have a significant incidence of use of these illicit drugs. Frequency distributions were plotted for the creatinine normalized and raw data for all positive specimens with values greater than or equal to the method limit of quantitation. A non-parametric 2.5% estimator was applied to each data set to establish the cutoff for each drug.

Results: The urinary excretion data for the three drugs studied suggest cutoffs of approximately 30 ng/ml (benzoylecgonine), 10 ng/ml (carboxy-THC), and 50 ng/ml (methamphetamine) to identify 97.5% of the users of these drugs in this population.

Conclusions: Evaluation of urinary excretion data provides an objective method to validate the selection of cutoffs. These data provide additional support for the revised SAMHSA cutoffs which could increase the positivity rates for both benzoylecgonine and methamphetamine by 7%.