Indole compounds do not cause false positives with the TDx cannabinoid assay.

Drugs-of-abuse screening is becoming more prevalent, and a major technical and legal concern is that individuals may be mistakenly identified as drug usersbecause of a false-positive test result. Marijuana use can be detectedby screening a urine specimen with the Abbott (North Chicago, IL) TDx cannabinoid assay, which gives a positive reaction with several delta-9-tetrahydrocannabinol (THC) derivatives. The manufacturer has tested an extensive number of drugs and other compounds, at concentrations up to 100 mg/ L, and shown that they give results less than the sensitivity of the THC assay (10 jg/L). Nevertheless, it has been suggested that various endogenous indole compounds can cross-react in THC assays and cause false-positive results and the manufacturer has not reported that the TDx THC assay is free from interference by these compounds. 5-Hydroxyindole-3-acetic acid (5-HIAA), hydroxy-2-indole carboxylic acid (5-HICA), indole-3-acetic acid (IAA), and indole-3-butyric acid (IBA) were tested for cross-reactivity. The indole compounds were dissolved in methanol and minimal volumes of the methanolic solutions (to preclude matrix effects) were added to a THC-negative urineto give concentrations of 40-50 mg/L. The THC-negative control urine and supplemented urines were assayed in triplicate by the TDx THC procedure, set at a detection threshold value of 25 g/L. Results are as follows: