Estimating the Time of Last Cannabis Use from Plasma

Summary of this paper

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Section 1

First samples were collected at 1 min after smoking and at frequent intervals up to 168 h. From these data, the authors developed 2 models to predict time of last cannabis use within 95% confidence intervals (CIs). The first model computed the elapsed time between smoking cannabis and blood collection based on plasma THC concentration alone, whereas the second model used the plasma THCCOOH/THC concentration ratio. They applied the models to results from all published studies at the time that reported THC and/or THCCOOH concentrations measured by either RIA or gas chromatography-mass spectrometry with either internal or external standardiza-tion.Go To Passage

Materials And Methods Participants And Study Design

At 0900 on the day of testing, participants received a single oral dose of placebo (n ϭ 10) or up to 90 mg of rimonabant. Two hours later, they smoked a cannabis cigarette containing 2.64% THC by weight, estimated to contain ϳ20 mg of THC. The number of puffs and time between puffs were standardized.Go To Passage

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Analytical Method

Model I determined time estimates from plasma THC concentrations and model II from the plasma THCCOOH/THC concentration ratios. The formulas are reproduced below, with t representing the elapsed time in hours between the beginning of cannabis smoking and blood collection, and CI representing the 95% confidence interval for the estimate of t. The subscripts 1 and 2 refer to models I and II, respectively, and brackets indicate the concentrations of THC or THC-COOH in g/L: 123.420 ͮ These equations were developed from cannabinoid concentrations in plasma samples collected for up to 168 h after controlled smoking of a 1.75% and a 3.55% THC cigarette by each of 6 cannabis users residing continuously in a secure clinical research unit (19 ). Drug administration was not initiated until their urine cannabinoid concentrations were Ͻ20 g/L.Go To Passage

Results

The results for predicted elapsed times between the beginning of cannabis smoking and blood collection obtained with model I, model II, and a combination of the models are summarized in Table 2. The conditions for the single cigarette study were similar to those in the study used to produce the models, except that cigarettes contained 2.64% THC instead of either 1.75% or 3.55% THC, and length of inhalation and the time smoke was held in the lungs were not controlled. For model I, Table 2 reflects that, for 392 of 427 samples collected from 38 individuals after they began smoking the first cigarette, the observed time fell within the predicted range of elapsed time.Go To Passage

Discussion

Other studies also have shown that cannabis users tend to titrate their dose of drug to maintain the level of intoxication they Table 3. Predicted elapsed time between cannabis smoking and blood collection with plasma THC between 0.5 and 2.0 g/L and THCCOOH >2.5 g/L. prefer (20 ).Go To Passage