Questions like this are routinely directed to the DAR Hotline staff. Frustrated callers scratch their heads in puzzlement, people caught in the act of smoking a joint nonetheless test clean on a THC urine diagnostic screening device that’s run a short time later. The answer to the question is relatively simple. Be patient.

The most active psychogenic ingredient in marijuana is tetrahydrocannabinol, most commonly referred to as “THC.” This psychoactive substance and drug is concentrated in the leafy part of the marijuana plant, the greatest single accumulation of THC is found in flowering tops or “buds.” These parts of the plant are harvested, cleaned and dried for eventual smoking or eating, smoking being the most popular means of ingestion. Whether marijuana is smoked in a hand-rolled “joint” or via a water-filtered pipe called a “bong,” THC filled gas is absorbed in the lungs and instantly deposited into the bloodstream. Breeching the vitally protective blood-brain barrier, THC selectively targets a set of nervous system receptors and binds to them. When activated by THC, “cannabinoid” receptors prompt a number of pleasurable experiences that reduce feelings of anxiety and tension. These things happen in very quick order once THC enters the bloodstream. For those who choose to eat marijuana in impregnated brownies, bagels or cookies, THC absorption is much slower and its effects less euphoric.

Once THC is active in the bloodstream, the body goes about trying to find it and eliminate it. A class of proteins known as enzymes are given the task of eliminating THC from the bloodstream. Like a pack of attack dogs, enzymes take off after THC and begin snapping and snipping away at it. With bite after bite taken out of THC, the enzymes end up creating second order substances called metabolites; some of these breakdown products look quite similar to their THC parent, some even have their own unique psychoactive capabilities as the same receptors that bind THC. As the enzymes plow their way through the accumulation of THC, metabolites undergo a transformation that allows them to be filtered by the kidneys and sent to the bladder for final elimination. This process takes time, it can’t be made to go any faster or slower.

When an individual is caught smoking marijuana, the subject of this vignette was likely to have a bladder that was full with urine to some level or another. The urine in the bladder at that moment in time reflected biological activity for a number of hours that preceded the puffing of the joint. Acquiring a urine sample at this point in time from the smoker in question will only render results depicting a timeframe that preceded the incident. If this person hadn’t smoked marijuana before, a urinalysis for THC will yield negative results. The lag between ingestion of a drug and sufficient accumulation of metabolites in the bladder can take hours. Complicating this scenario further is the factthat all drug screens are created with cut-off levels, so-to-speak concentration thresholds that dictate at what point a suspicious sample is officially reported as being positive. For marijuana, the most common setting for THC screening is 50 ng/ml; a sample containing 51 ng/ml or more will be recorded as positive, 49 ng/ml or less will be recorded as negative. An explanation and discussion of cut-off levels is discussed elsewhere in the DARS Newsletter. Suffice to say, the existence of cut-off levels in drug testing creates and additional chemical speed bump that extends the amount of time it takes for THC and metabolites to reach reportable levels in urine.

Controlled experimentation and testing of marijuana smokers suggests that reliable, detectable concentrations of THC metabolites in urine occur 2-3 hours following ingestion. Detectable levels of THC metabolites may persist for up to 3-5 days following a single incident of marijuana smoking. But for planning purposes, drug testing of someone who was caught “red handed” smoking the drug should not be undertaken for at least 2 hours following discovery. If a situation allows an evaluator to hold off urinalysis screening for 4 hours or so, the odds of a positive drug test report are even greater. For someone who has orally consumed marijuana, it’s best to double the waiting period before acquiring a urine sample for testing.

Reproduced with permission from The MEDTOX® Journal