AIMS
To develop and empirically validate a mathematical model for identifying new cannabis use in chronic, daily cannabis smokers.
DESIGN
Models were based on urinary creatinine-normalized (CN) cannabinoid excretion in chronic cannabis smokers.
SETTING
For model development, participants resided on a secure research unit for 30 days. For model validation, participants were abstinent with daily observed urine specimens for 28 days.
PARTICIPANTS
48 (model development) and 67 (model validation) daily cannabis smokers were recruited.
MEASUREMENTS
All voided urine was collected and analyzed for 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THCCOOH) by gas chromatography-mass spectrometry (GCMS, limit of quantification 2.5 ng/mL) and creatinine (mg/mL). Urine THCCOOH was normalized to creatinine, yielding ng/mg CN-THCCOOH concentrations. Urine concentration ratios were determined from 123,513 specimen pairs collected 2–30 days apart.
FINDINGS
A mono-exponential model (with two parameters, initial urine specimen CN-THCCOOH concentration and time between specimens), based on the Marquardt-Levenberg algorithm, provided a reasonable data fit. Prediction intervals with varying probability levels (80, 90, 95, 99%) provide upper ratio limits for each urine specimen pair. Ratios above these limits suggest cannabis re-use. Disproportionate numbers of ratios were higher than expected for some participants, prompting development of two additional rules that avoid misidentification of re-use in participants with unusual CN-THCCOOH excretion patterns.
CONCLUSIONS
For the first time, a validated model is available to aid in the differentiation of new cannabis use from residual CN-THCCOOH excretion in chronic, daily cannabis users. These models are valuable for clinicians, toxicologists, drug treatment staff, and workplace, military and criminal justice drug testing programs.
Mouth alcohol, if present in high enough concentrations, can falsely bias the accurate measurement of end-expiratory breath alcohol. Mouth alcohol will be eliminated over time, however, and can be modeled with a single term decaying exponential of the form: B0e−kt + C. It is important, however, to determine the model and its parameters when alcohol is already present within the biologic system. Using three individuals as their own controls, mouth alcohol was administered both before and after alcohol consumption followed by breath alcohol analysis performed at approximately 0.5 min intervals. The results showed that both model parameters (B0 and k) are effected and that the asymptotic value (C) is reached much sooner when alcohol already exists in the end-expiratory breath. Considering only three individuals were involved, the forensic-science importance appears to be that, as the end-expiratory breath alcohol concentration increases, the time necessary for the mouth alcohol to decrease to unbiased levels is decreased. Fifteen min of observation time prior to breath alcohol analysis appears to be more than adequate at forensically relevant concentrations.
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