Late Sheep Fat Biopsies Supplied Residue Evidence Beyond the Plasma THC Curve

The sheep study measured THC in subcutaneous fat as well as plasma. Positive biopsies at selected later dates and subsequent nondetections did not identify an exact clearance day for every animal.

The tissue sampling schedule differed between animals

The pilot animal had fat sampled at fourteen, twenty-eight and fifty-six days. The remaining seven animals were sampled at twenty-eight and ninety-one days. The pilot had detectable THC at fourteen days but not the later biopsies. Four of the other seven had detectable THC at twenty-eight days, with none detected at ninety-one days.

A gap between biopsies is not a measured disappearance time

These results show what was detectable at the sampled sites and dates under the analytical method. They do not establish the precise day THC became undetectable between assessments. A negative subcutaneous sample also cannot establish absence from every other tissue or prove that all drug-related effects had ended.

Residues and withholding decisions require their own evidence

The paper explicitly notes that pharmacokinetics alone cannot determine withholding periods. Tissue-depletion studies and appropriate analytical criteria are needed for that purpose. The useful contribution here is direct tissue evidence alongside a plasma model, with different observation schedules retained. Neither the last positive biopsy nor the later nondetection should be turned into a universal human detection window or an exact livestock clearance rule.

Related analysis: the study’s broader findings and limitations.

Research cited

Stevens and colleagues (2024). The Pharmacokinetics of Δ9-Tetrahydrocannabinol in Sheep.