Citation

Schwilke EW, Gullberg RG, Darwin WD, Chiang CN, Cadet JL, Gorelick DA, Pope HG, Huestis MA. Addiction 2011; 106(3): 499-506.

Copyright

(Copyright © 2011, John Wiley and Sons)

DOI

10.1111/j.1360-0443.2010.03228.x

PMID

unavailable

Abstract

Aims To develop and validate empirically 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 A total of 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 creatinine‐normalized 11‐nor‐9‐carboxy‐Δ9‐tetrahydrocannabinol (CN‐THCCOOH) excretion in chronic, daily cannabis users. These models are valuable for clinicians, toxicologists and drug treatment staff and work‐place, military and criminal justice drug‐testing programs.