THC window of detection

Post Reply

THC window of detection

Post by asolanomd » Tue Jun 08, 2010 11:23 pm

Along the lines of the previous post:

I had a non-DOT donor that on 03/22/2010 tested positive for marijuana metabolites at level of 118 ng/ml and was reported out by me as positive. He then had a follow-up test on 05/25/2010 at level of 21 ng/ml and again was reported as positive. He has hired a lawyer and is contesting the result. Is there a formula that one can use to estimate rate of excretion to see if this is residual excretion or if this level is consistent with use between tests 1 and 2?

Robert Swotinsky MD
Site Admin
Posts: 882
Joined: Sun May 23, 2010 3:42 pm
Location: Sudbury, MA

Re: THC window of detection

Post by Robert Swotinsky MD » Wed Jun 09, 2010 9:21 pm

This case presents a donor who tests positive for marijuana once, and 64 days later tests positive for marijuana again. The donor apparently claims that the second test represents the same use as caused the first test and thus should not be treated as a new positive.

An MRO cannot determine the donor's honesty in situations like this. It is difficult if not impossible to tell if residual excretion caused a positive after an interval of more than a few days. There are studies that examine drops in normalized urine THCA values after last use. (A normalized THCA value is the THCA concentration divided by the creatinine concentration.) But, those studies look at tests conducted every day or two, not tests separated by long intervals. The MRO is responsible for determining if there is a legitimate medical explanation for the result, i.e., if the donor used dronabinol that was prescribed for him/her. If no prescribed dronabinol, the result is positive. If the MRO is asked for his / her opinion about new v. residual excretion, the MRO can present interesting analyses and probabilities. However, I don't think the MRO can be definitive, unless the separation between the two tests is extreme.

The maximum length during which a heavy marijuana user can test positive after last use has not been firmly established. Popular belief, as a Google search will reveal, has it that heavy users can test positive for many months after last use. This belief is partly supported by a 1982 study by George Ellis and colleagues that monitored 52 heavy users of marijuana after last use by testing their urine with an immunoassay calibrated at 20 ng/mL. One subject in this study had a positive screen at 77 days after last use. That one subject was the outlier. The average time period for last positive among heavy users in the Ellis study was one month. This time frame cannot be applied to today’s workplace testing programs where marijuana is more potent but the tests are two-step (immunoassay and confirmation) and use higher cutoffs, typically including a 50 ng/mL immunoassay. There are no studies that test the maximum window of detection under today’s conditions. However, there are data that show the urinary elimination half-life of THCA in heavy smokers of marijuana averages about 3 days (range 0.8-9.8) . Because most drugs are essentially eliminated after five half-lives, this correlates to a window of detection of THCA in heavy smokers of marijuana of about 15 days (range 4 - 49 days).

If this donor sues and you're an expert, raise these issues to support your opinion that the second result was not due to residual excretion. A courtroom is the correct venue for weighing opinions based on the strength of the evidence. You can't prove the result wasn't caused by residual excretion. But, there's a lot of evidence that it wasn't.


Re: THC window of detection

Post by DrSteve » Thu Jun 10, 2010 5:46 pm

I, similar to Dr. Swotinsky, feel that about 6 weeks is the time from a positive test to clear. There is no creatinine from either of the 2 tests, but the first test is fairly low, and in my opinion, not likely to remain positive for 6 weeks, or in this case even longer.

Post Reply