What Causes a Positive THC Urine Test?

Heavy cannabis users who smoke on a daily basis can test positive for drugs in their urine for up to six weeks after they stop. False positives have been reported due to certain medications and antibiotics. Learn more about what causes false positives on THC urine

What Causes a Positive THC Urine Test?

Heavy cannabis users who smoke on a daily basis can test positive for drugs in their urine for up to six weeks after they stop. False positives have been reported in the use of cannabis with certain medications, such as dronabinol, efavirenz, NSAIDs, promethazine, riboflavin, ethacrynic acid and baby soaps. These medications usually won't interfere with drug testing. However, in rare cases, some antibiotics are known to cause inaccurate test results.

Rifampicin, which is used to treat tuberculosis, may appear as opiates in some rapid urine tests. In addition, a false positive result may be possible even more than 18 hours after taking a single dose of the antibiotic. Only blood sample measurements are likely to be correlated with a person's degree of exposure; attempts to correlate urine concentration with deterioration or time of administration of the dose are complicated by variations in individual metabolism, accumulation of metabolites in the chronic user, and changes in urine volume due to diet, exercise, and age. Therefore, a positive urine cannabinoid test result indicates only the likelihood of previous use. Smoking a single marijuana cigarette produces THC metabolites that can be detected over several days with the cannabinoid test.

THC can accumulate in body fat, creating higher excretion concentrations and longer detectability. If the main reason for the evaluation is an effect on performance, the result of the urine cannabinoid test alone cannot indicate a deterioration in performance or assess the degree of risk associated with continuing to perform tasks. If the main reason for the screening tests is a history of marijuana use, the urine test for cannabinoids should be able to detect previous use for up to two weeks in the casual user and possibly for a longer time in the chronic user. A false positive occurs if the marijuana test is done one day after exposure. In addition, there is a high chance of a false positive occurring if the person being tested is confined in a room similar to that of smokers.

The smoke you inhale after smokers exhale can cause traces of marijuana in your body. The decay of the wafer can cause a false positive for up to two days after ingesting the cookie. SYVA markets two different cannabinoid analysis kits with a difference of two to four times in the amount of THC metabolite needed to produce a positive test result. However, there's little chance that you'll have enough THC to test positive on screens used by the federal government and many private employers. Regardless of the test kit used, the test results must be interpreted by qualified personnel and the positive results must be verified, so that there is a very limited chance of obtaining a false positive result.

If you are sure that the result is false, take action immediately and discuss the false positive results. Confirmation techniques may be specific to a particular THC metabolite, while screening kits react with virtually all THC metabolites, making it an additional complication in confirming test results. If the manufacturer's instructions are followed correctly, urine samples containing at least 9-carboxy-THC will test positive at least 95% of the time. THC is the active ingredient in marijuana and can persist in the urine for several days or weeks depending on how often it was used. While a positive drug test indicates that some form of illicit drug is present in the body, simple tests such as immunoassay (AI) tests are often not enough to demonstrate sobriety or lack thereof.

Since many states are resistant to legalizing marijuana, it is very important to know about potential false positives (FP) in urine testing for THC as this could compromise patients' eligibility to work or compete in sporting events. If confirmatory advanced drug screening tests are positive and you get a 100% positive result, it's likely a false positive; you'll probably want to see an attorney. The urine of marijuana users contains amounts of 9-carboxy-THC in both free and conjugated forms as well as other cannabinoids (THC and its metabolites) detectable by the test.