Laboratory detection of marijuana use. Experience with a photometric immunoassay to measure urinary cannabinoids.

The urinary excretion of cannabinoids was determined using a photometric immunoassay technique among adolescents and young adults who had an abrupt and closely supervised cessation of drug use on entering a strict rehabilitation program. No falsely positive urine test results were encountered among 70 persons admitted consecutively to the program. Among five subjects with a history of chronic, heavy marijuana use (approximately 56 g/mo [approximately 2 oz/mo]), urinary cannabinoids were detected for an average of 13 days (range, nine to 25 days), following cessation of use. Among four subjects with a history of moderate marijuana use (approximately 28 g/mo [approximately 1 oz/mo]), test results remained positive for an average of 4.7 days (range, two to eight days). Test results were negative 48 hours after cessation in two subjects with a history of infrequent use (less than 7 g/mo [0.25 oz/mo]), who had smoked marijuana within the preceding two days. The specificity of the method for detecting recent marijuana use appears excellent; the sensitivity depends on the potency of the cannabis preparation, the time of last use, the frequency of previous use, and the specific gravity of the urine specimen. On the basis of this preliminary experience, positive test results for more than eight consecutive days suggest either surreptitious continued use or previous chronic, heavy use in a newly abstinent person. Purposeful adulteration of known-positive urine specimens with bleach, blood, vinegar, salt, and liquid soap produced falsely negative results. Unless samples are collected under direct observation, urine color, temperature, specific gravity, and dipstick tests for blood and pH should be recorded.