National Institute of Forensic Toxicology, Oslo, Norway.

Drug habits were assessed by means of a questionnaire. Subjects were classified as infrequent users (cannabis use < once a week), or frequent users (cannabis use > once a week). The subjects agreed to give up to 5 urinary specimens daily until negative. The specimens were analysed semi-quantitatively by EMIT^® (cut-off 20 ng/ml, representing cannbinoids). Positive specimens and several negative ones were analysed for THCCOOH by a gas chromatography using EC-detector with a limit of detection 1.4 ng/ml, considerably lower than cut-offs according to NIDA (15 ng/ml), and our institute (10.3 ng/ml). Creatinine was measured in all specimens.

All 21 participants (1 participated twice) admitted to having used cannabis, but not all went through the whole follow-up period. A total of 78% of the EMIT positive specimens were confirmed by GC analysis (THCCOOH cut-off 10.3 ng/ml). Relating THCCOOH concentration to creatinine generally gave a smoother time curve than the THCCOOH concentration itself.

Infrequent users had a mean apparent THCCOOH urinary elimination t_1/2of about 1.3 days (pooled data). Frequent users had urinary elimination t_1/2 from 1 day up to about 10 days. In infrequent users the median time from last reported intake until the last positive observed was 12 days (cannabinoids), 5 days (THCCOOH 10.3 ng/ml) and 4 days (THCCOOH 15 ng/ml), respectively. In frequent users the median time from last reported intake until last positive observed was 27 days (cannabinoids), 22 days (THCCOOH 10.3 ng/ml) and 17 days (THCCOOH 15 ng/ml). In one subject 16 negative gaps (THCCOOH positive specimen followed by a negative and again a positive specimen) were observed.

The study of the urinary excretion profiles of illegal drugs in prison inmates is a method of obtaining data that cannot ethically be obtained from healthy volunteers. The present results may aid in the interpretation of urinary THCCOOH and cannabinoid drug testing results.