The tldr; answer is that it depends on the dosage, amount of trace THC in the CBD oil, type of test performed and the cutoff of the particular test. False positives with an immunoassay based test can occur, but a second confirmatory test using LC-MS/MS can unequivocally identify the THC metabolite in urine. It might even be possible to surpass the LC-MS/MS cutoff level if the amount of THC is near the maximum 0.3% allowed and the cutoff is sufficiently low as it is with a typical SAMHSA (Substance Abuse and Mental Health Services Administration) urine drug tests for safety sensitive jobs. If you are drug tested regularly, it might be best to choose a product made from CBD isolate and request to see the lab results instead of full spectrum CBD that includes low amounts of THC. Read a more in-depth explanation below.
A question that often comes up is if CBD will cause you to fail a drug test. CBD is an isomer of THC, meaning that it has the same chemical formula of C21H20O2, but the atoms are arranged in a different configuration. Urine drug tests can be categorized into 3 categories: point of care (POC) cups, clinical chemistry analyzer, and LC-MS/MS. GC-MS isn’t really widely used and has been mostly replaced by tandem mass spec with less sample preperation.
POC cups and clinical chemistry analyzers both function based on an immunoassay based technique. Clinical chemistry analyzers seem to have slightly better specificity than POC cups, but both are plagued with issues regarding false positives and even false negatives. Clinical chemistry analyzer drug tests are based on an enzyme multiplied immunoassay technique (EMIT). EMIT is based on the competition between drug-labeled glucose-6-phosphate deyhydrogenase (G6PDH) and free drug in the sample for a fixed amount of antibody binding sites. When there is the drug of interest is in the urine sample, those drugs bind to the antibody and the drug labeled enzyme converts NAD to NADH. This change in NAD causes an light absorbance change at 340 nm in the sample cuvette and can be quantified using spectroscopy. A diagram of is this is displayed in the figure below.
One main problem with this process is that there are various cross reactive chemical species that could have slight binding capability to the antibody and cause a false positive if the concentration is high enough. A prime example would be that of amphetamines where a lot of OTC drugs such as pseudoephedrine have a slight cross reactivity and have been known to cause false positives. Certain drugs classes have more specificity than others. Through observation, amphetamines have the most false positives, while cocaine, heroin, oxycodone, and to a lesser extent THC have the highest specificity. There are even false negatives that can result of the binding dynamics of the assay, such as the metabolite for clonazepam, 7-aminoclonazopam, which has a negative charge at physiological pH and only exhibits around a 0.2% reactivity with the assay.
Based on experience from performing these tests in the past, there is some cross reactivity apparent with people taking CBD oil where a low result that barely passed the cutoff was observed with the immunoassay based test and nothing was observed with the confirmatory LC-MS/MS test. There is also a very slight cross reactivity observed when negative urine is spiked with analytical grade CBD standards. However, much more cross reactivity is observed when spiking samples with CBN and THCV. This is not unsurprising because the package insert that come with these assays list CBN as having a large 50% cross reactivity with the THC metabolite being tested. If someone does test positive for THC, or really any other drug that they are not supposed to be taken the sample goes through a more time consuming confirmation process using LC-MS/MS. LC-MS/MS will unequivocally determine if the person was consuming THC.
However, in the real world this is often more complicated. CBD oil manufacturers are not regulated by the FDA and there is no guarantee that you’re getting what the label says that you’re getting. Both marijuana and CBD oil come from a cannabis plant. The difference is that CBD oil comes from CBD rich hemp that has a lot less THC in the form of THCa and THC. The federal definition that qualifies cannabis as hemp is an arbitrary quantity of 0.3% THC. This arbitrary definition of 0.3% THC faces the question of whether this includes total THC, which includes THCa, or if it is only delta-9 THC. The DEA and USDA says that it is total THC, but the Farm Bill allowing for the legalization of hemp only specifies THC. The issue is that THCa can naturally degrade into THC when stored for long periods, which then degrades into CBN. THCa is also the most prevalent form of THCa found in cannabis and gets decarboxylated into psychoactive THC when smoked.
There are a variety of 3rd party reports online and in the news media of how someone was taking CBD and failed for the confirmatory LC-MS/MS for the THC metabolite causing them to lose their safety sensitive job. Some reports even had the CBD product they were using tested by a separate lab and found that the THC content in the full spectrum CBD oil was below the 0.3% maximum. As a result, it is always best to choose a product made from CBD isolate from a company willing to provide 3rd party test results to ensure that there is no way for THC to slowly build up and pass the low SAMHSA cutoff of 15 ng/mL.
Users of CBD might also want to choose a broad spectrum CBD oil which has all of the cannabinoids within the plant, but has THC selectivity removed. In this case, it is more important to review the test results because sometimes “full spectrum” and “broad spectrum” are mistakenly used interchangeably. Broad spectrum CBD oil might have the added benefit of the entourage effect.