EMIT, ELISA or RIA), which gives a yes/no result as to whether cannabis has been detected. The test is set to be sensitive to a predetermined cutoff level. The standard cutoff is 50 nanograms per milliliter (ng/ml) of THC-COOH in urine for the Department of Transportation and other federal testing programs (a nanogram equals one billionth of a gram; in different units the limit is 50 micrograms per liter). Because immunoassay screens are vulnerable to urine sample tampering and interference by contaminants, they are not entirely reliable. Also, they don’t give precise results and may register positive above or below their supposed detection threshold.
It is therefore essential that positive samples be confirmed with the more accurate gas chromatograph mass spectrometer (GCMS), which provides an exact, numerical measurement of the amount of THC-COOH in the sample. The GCMS virtually eliminates the chance of false positives in exchange for a relatively high rate of “false negatives,” where levels are below the threshold. To further reduce the possibility of false positives for illicit use, a positive GCMS test still needs to be followed up with an interview by an MRO to determine if there is a legitimate medical reason for the positive test.
The U.S. Substance Abuse and Mental Health Services Administration (SAMHSA) has issued extensive mandatory guidelines for federal workplace drug testing programs.3 SAMHSA guidelines require that positive screens for marijuana be confirmed by GCMS at a level of at least 15 ng/ml, and they require MRO review. The guidelines include safeguards to assure sample security, chain of custody, split specimen backup testing, and prevention of dilution and tampering. However, private employers do not regularly follow the SAMHSA guidelines, and even those required to do so are often derelict in following them diligently. In particular, many employment screening programs neglect to perform the GCMS backup test or MRO review. Unfortunately, job applicants have no recourse for appealing defective pre-employment tests.
A white paper by the National Workrights Institute found that drug test results are not always reliable even when issued by federally certified labs. The NWI paper found that certified labs do not consistently follow federally mandated procedures; that federal regulations allow labs to make mistakes in 10% of the samples used in certification; that labs do not always maintain proper chain of custody; and that the accuracy of certified labs has never been tested.4
Confounding Factors
Cross Reactions
No commonly used substance is known to produce a false positive for marijuana. Unusual exceptions are the prescription anti-heartburn drug pantoprazole (Protonix®) and the antiretroviral drug efavirenz (Sustiva®), which have been reported to cause a false positive for marijuana on immunoassay screens. However, both can be distinguished from marijuana by a GCMS backup test. In the past, high doses of ibuprofen (Advil®) could cause positives on the EMIT screen, but this problem has been fixed. Other prescription and over-the-counter products can cause false positives for amphetamines, opiates and other illicit drugs on immunoassay tests. In all such cases, false positives can be eliminated by a backup GCMS test.
Medical Marijuana
A growing number of states and foreign countries have passed laws allowing for the medical use of marijuana. If marijuana were treated like other prescription drugs, workers would be excused from positive drug tests on pre-employment tests if they presented a valid prescription. However, due to marijuana’s federally illegal Schedule I status, it is not generally recognized as a valid medication. In particular, medical marijuana is not allowed in any work situation for which drug testing programs are mandated or regulated by federal law, such as those in the transportation and defense industries.
In other situations, some employers who aren’t subject to federal regulations will allow for medical marijuana in states where it is legal, but many others will not.
In general, most state medical marijuana laws do not protect workers from employment drug testing. One exception is Arizona, which forbids employers from discriminating against medical marijuana users unless compelled to do so by federal law. However, the supreme courts of both California and Oregon have ruled that workers aren’t protected from drug testing under state medical marijuana laws.
In the criminal justice system, some states, such as California, may excuse probationers and parolees from drug tests if they have a medical recommendation. Of course, this doesn’t apply to federal prisoners.
Marinol®
Marinol (generic name: dronabinol) is an FDA-approved Schedule III drug containing THC in pill form, which is often prescribed as a marijuana substitute. Marinol shows positive on drug tests just like marijuana because it contains THC. Because Marinol is a legal prescription drug, employers may excuse a THC-positive test if you have a valid prescription for it.
The normal procedure in employment situations is for positive drug tests to be reviewed by a medical review officer (MRO). If you can show that you have a legitimate prescription for a legal controlled substance such as Marinol, the MRO will normally excuse a positive test for it. Note however that some employers may disallow any controlled substances, legal or otherwise, in safety-critical jobs, sports, the criminal justice system, etc.
Because marijuana and Marinol are indistinguishable on normal drug tests, some users get a Marinol prescription as an excuse for testing positive. However, some employers are suspicious of Marinol for precisely this reason and may take extra measures to prevent abuse. This can be done by means of a special test available from certain labs that can distinguish Marinol from marijuana by detecting natural compounds that are unique to the plant and not in the pill. If you are using Marinol, beware that you might be asked to take such a test to prove that you aren’t also using marijuana.
THC Substitutes
There exist other marijuana-like drugs that don’t contain THC and are therefore not detectable on standard drug tests. One of these is the prescription drug nabilone (Cesamet®), a synthetic cannabinoid medicine that is sometimes prescribed as an anti-nauseant. Others include the various recreational marijuana substitutes known as “Spice,” which were peddled as legal (though less safe) alternatives to marijuana until being outlawed.
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