A Negative THC Blood Sample Was Followed by Positives During Monitored Abstinence
A negative whole-blood THC sample did not necessarily mark the end of detectable THC in a monitored-abstinence study. Some participants produced later positive specimens while residing under continuous surveillance, so a negative-to-positive sequence alone could not establish new use.
The collection history was controlled
Karschner and colleagues studied 25 frequent, long-term cannabis users on a secure research unit. Whole blood was collected at admission and generally each morning over seven days. The THC quantification limit was 0.25 ng/mL.
Among the 16 participants with at least one positive specimen, two were below the quantification limit at admission but had a later positive. One participant had negative results at admission and on day three, with positives on several intervening and subsequent days.
A reporting boundary is not complete elimination
Below-quantification and positive results describe measurements relative to a method’s reporting boundary. They do not establish that all THC disappeared and then re-entered the body. The authors discussed variable release from tissue stores as a possible contributor, rather than experimentally isolating one mechanism.
This finding does not mean every negative-to-positive sequence is residual exposure. It supplies a documented alternative under the study conditions that must be considered before attributing a sequence to new consumption.
Keep the scope narrow
The study involved whole blood and selected heavy users. Its pattern should not be transferred automatically to urine, occasional use or another laboratory threshold. It also did not measure driving performance alongside each specimen.
A sequence interpretation needs collection times, assay limits and exposure history. The laboratory pattern and the inference about conduct remain separate questions.
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?.
