BMI Did Not Predict the Last THC-Positive Blood Sample in a Small Abstinence Study
Body mass index did not significantly predict time to the last THC-positive whole-blood specimen in a small study of frequent users. That null finding limits a simple body-size prediction, while leaving unanswered questions about body composition and individual differences.
The reported association was not significant
Karschner and colleagues followed 25 frequent cannabis users during monitored abstinence. For participants with positive THC specimens, the reported BMI correlation with time to the last positive specimen was −0.2, with p = 0.445.
This was not evidence for a reliable BMI-based detection clock. Nor did the nonsignificant result prove that adipose tissue has no role in cannabinoid disposition. BMI is a body-size index, not a direct measure of fat percentage or tissue THC stores.
The sex observation was exploratory
Five participants remained THC-positive throughout the seven sampling days, and all five were female. The authors discussed possible explanations, including body burden and physiological differences. They did not measure body fat percentage.
The participant with the highest THC concentrations had the lowest BMI among the women. Together, these observations resist a one-variable explanation. A small observed subgroup is not a validated sex-specific prediction rule.
Do not convert a hypothesis into a personal estimate
The sample was small and selected for extensive cannabis-use histories. The last positive also depended on the sampling schedule and quantification limit. These constraints belong in any application.
The supported conclusion is that this study did not establish BMI as a useful individual predictor of the reported endpoint. It supplied hypotheses for further research, rather than a formula for determining recency or impairment from sex and body size.
Related: Separating cannabis exposure from impairment.
Research cited
Karschner EL; Schwilke EW; Lowe RH; et al.. Addiction (2009). Do delta-9-tetrahydrocannabinol concentrations indicate recent use in chronic cannabis users?.
