Total THC-COOH concentrations include both the free THC-COOH and THC-COOH–glucuronide conjugate concentrations determined after alkaline or enzymatic hydrolysis. Substantial intra- and inter-subject variability occurs in patterns of THC-COOH excretion. THC-COOH concentration in the first specimen after smoking is indicative of how rapidly the metabolite can appear in urine. Mean first urine THC-COOH concentrations were 47±22.3 and 75.3±48.9 ng/ml after smoking one cigarette containing 1.75 or 3.55% of THC, respectively . Some 50% of the subjects’ first urine specimen after the low dose, and some 83% of the first urine specimens after the high dose were positive by GC/MS at a THC-COOH cutoff concentration of 15 ng/ml.
Normalization of cannabinoid concentration to urine creatinine concentration aids in the differentiation of new from prior cannabis use, and reduces the variability of drug measurement due to urine dilution. Random urine specimens contain varying amounts of creatinine, depending on the degree of concentration of the urine. Hawks first suggested creatinine normalization of urine test results to account for variations in urine volume in the bladder . Whereas urine volume is highly variable due to changes in liquid, salt, and protein intake, exercise, and age, creatinine excretion is much more stable. To date, there are too few urinary data on THC and 11-OH-THC levels to guide interpretation of positive urine cannabinoid tests; however, data are available for guiding interpretation of total urinary THC-COOH concentrations.
- Urinary THC-COOH concentrations drop rapidly until reaching a value of ca.
- Similar mean maximum THC concentrations were reported in specimens collected immediately after cannabis smoking was completed.
- No significant pharmacokinetic differences between chronic and occasional users have been substantiated .
- Similarly, when sensitive analytical procedures and sufficient sampling periods were employed for determining the terminal urinary excretion half-life of THC-COOH, it was estimated to 3−4 d .
- The mean peak THC concentrations were 94.3, 107.4, and 155.1 ng/ml after smoking single cigarettes of 1.32, 1.97, or 2.54% THC, respectively .
- Mean plasma THC-COOH elimination half-lives were 5.2±0.8 and 6.2±6.7 d for frequent and infrequent cannabis users, respectively.
The most accurate ratio (85.4%) was 50%, with a sensitivity of 80.1%, a specificity of 90.2%, and false positive and false negative predictions of 5.6 and 7.4%, respectively. When the previously recommended increase of 150% was used as CBD a threshold for new use, sensitivity of detecting new use was only 33.4%, and specificity was high (99.8%), for an overall accuracy prediction of 74.2%. Sensitivity, specificity, accuracy, and false positives and negatives were 71.9, 91.6, 83.9, 5.4, and 10.7%, respectively, when the 50% criteria was applied. These data indicate that selection of a threshold to evaluate sequential creatinine-normalized urine drug concentrations can improve the ability to distinguish residual excretion from new drug usage.
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Mean peak urine THC-COOH concentrations averaged at 89.8±31.9 ng/ml (range 20.6−234.2 ng/ml) and 153.4±49.2 ng/ml (range 29.9−355.2 ng/ml) following smoking of ca. The mean times of peak urine concentration were 7.7±0.8 h and 13.9±3.5 h for the low and high dose, respectively. Although peak concentrations appeared to be dose-related, there was a twelvefold variation between individuals.
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We evaluated the ratios of creatinine normalized THC-COOH concentrations from more than 1800 pairs of urine specimens collected during a controlled smoked THC administration study. The ratio times 100 of the creatinine normalized later specimen divided by the creatinine normalized earlier specimen were evaluated for determining the best ratio to predict new cannabis use. A relative operating characteristic curve was constructed from sensitivity and specificity data for 26 different cutoffs, ranging from 10 to 200%.