Your urine tox screen was positive for cocaine...

Motivation: Awkwardness arises when a patient with bacterial endocarditis denies use of IV drugs, but his or her urine tox screen comes back positive for cocaine. Urine tox screens come with the disclaimer from the lab that these tests are not definitive, and further confirmatory testing is required. When a urine tox screen comes back positive for cocaine, how accurate is this result? Surprisingly, few studies have investigated this common issue, and it is difficult to compare study results to the exact test used in the hospital, as there are many different urine tox screen assays...but I did manage to find one article that dealt with this question:

Study: Linder MW, Bosse GM, Henderson MT, Midkiff G, Valdes R. Detection of cocaine metabolite in serum and urine: frequency and correlation with medical diagnosis. Clin Chim Acta. 2000 May;295(1-2):179-85.

Study design and results: A retrospective chart review was performed at a level 1 trauma center. 500 sequential subjects with urine tox screens in the ED were included. Urine specimens positive for the cocaine metabolite (benzoylecgonine-BE) were confirmed with GC/MS technique. 54 patients were positive for BE in the urine; of these patients, 10 had a medical diagnosis of acute cocaine intoxication. 25% of these 54 patients reported using cocaine in the past, and 1 reported use of cocaine on day of testing. Based on their chart review, the authors estimated sensitivity of 100%, specificity of 90.6%, NPV of 100%, but a PPV of only 18.5% for identifying acute cocaine intoxication in the urine test. (Linder et al., 2000)

Thoughts: This study addresses the relationship between cocaine positivity in the urine test and acute cocaine intoxication. I would have been interested in the relationship between cocaine positivity in the urine test and recent cocaine intake in general-but this is difficult because it would rely on actually knowing whether or not a patient took cocaine (which could be practically impossible to know with 100% certainty) or comparing to a gold standard (which is possible, but once again, no test is perfect). From this study, I take home the point that a negative result is useful, but a positive result may not be accurate and requires further testing. This is a key point to remember in keeping the trust in a therapeutic patient-physician relationship, when a patient lets a physician know in good faith that he or she has not taken cocaine, but a positive cocaine test results. One should then verify with further tests if a true positive result would change medical management.