Marijuana (THC) Testing

Marijuana testing is used to detect the presence of the plant’s active chemicals or their breakdown products (metabolites), most commonly in urine or saliva. Marijuana testing may be used to screen for and confirm the presence of THC or THC metabolites that indicate marijuana or prescription cannabinoid use. Most marijuana tests look for an inactive metabolite of THC (THC-COOH), which stays in the body longer than THC itself.

Marijuana testing may be used for:

Medical Screening
Marijuana may be included in a panel of drug tests used for medical screening. This testing is used to determine what drugs or combination of drugs a person may have taken so that the person can receive proper medical treatment. With marijuana legalization in some areas, some clinics, emergency rooms and hospitals are no longer including marijuana testing in their drug testing panels.

A healthcare practitioner may order drug testing for medical reasons if an individual:

• Is being treated in an addiction recovery or chemical dependency program
• Is being treated for chronic pain, to ensure that the person is taking mediation as prescribed
• Experiences symptoms that may be drug-related or shows signs of intoxication, such as unconsciousness, nausea, delirium, panic, paranoia, increased temperature, chest pain, respiratory failure, seizures, and/or headaches
• Is involved in an accident that an emergency room physician suspects may be due to drug intoxication
• Is suspected of drug abuse or is known to use illegal drugs
• Is a candidate to receive an organ transplant
• Is being treated for cancer
• Is pregnant or has recently given birth, especially if she is thought to be at risk for drug abuse or has a newborn baby exhibiting certain characteristic behaviors
• Is an applicant for a life insurance policy, as part of an overall health evaluation

For a more in-depth discussion of medical drug screening, see the article Emergency and Overdose Drug Testing.

Legal, Forensic and Employment Testing
Drug testing for legal, forensic, or employment purposes differs from medical testing in sample collection procedures. Sample collection for forensic testing is strictly controlled, with the custody of a sample continuously documented to maintain a legal "chain-of-custody." After a sample is provided, it is closed and secured with a tamperproof seal. A custody and control form accompanies the sample throughout all handling, testing, and storage. This creates a continuous legally binding record for the sample. Test results are usually reviewed by a specially trained and licensed physician called a medical review officer prior to reporting.

Examples of this type of testing include:

• Court-mandated testing for someone who has been convicted of illegal drug use
• Testing or monitoring of a parent in a child custody case
• Forensic toxicology testing using a variety of body fluids and tissues that may be tested during a crime investigation; the goal may be to determine whether marijuana was a contributing factor to an accident or crime, such as DUI (driving under the influence).
• Some athletic programs may test for marijuana as part of a general toxicology screen to detect performance-enhancing drug abuse in athletes.

Marijuana testing may also be required by an employer. This is also referred to as "workplace drug testing," a specific type of forensic testing. It may be done prior to employment, on a random basis, following an accident, or if an employer has a reasonable suspicion that an employee is using illegal drugs.

Employment drug testing is common. It is required for employees of the U.S. Department of Transportation, military personnel, and other federal employees, and is an accepted practice in many other industries.

For additional information on this type of testing, see the article on The World of Forensic Laboratory Testing.

Medical Screening
Marijuana testing may be ordered by a healthcare practitioner, usually as part of a drug testing panel, during a medical evaluation or addiction treatment when a person is known or suspected of substance abuse or when a person has signs and symptoms that suggest drug use, including confusion or otherwise abnormal or dangerous behavior. Testing may also be ordered as part of general health screenings prior to the start of some insurance policies.

Legal, Forensic or Employment Testing
Forensic THC testing is performed when an employer or legal entity needs to determine if a person may have used marijuana. It may be ordered randomly or routinely as part of employment screening, during the hiring process, or part of broader drug testing in legal or forensic scenarios, such as after an accident. Testing may be ordered at random to satisfy court-ordered monitoring or probation requirements.

Just as for other types of lab tests, both screening and confirmatory tests are interpreted based on a defined cutoff level (e.g., 50 nanograms per milliliter (ng/mL) for urine samples). If a drug is not present, or if the drug is present at a concentration below the test cutoff, the test will be reported as "undetected," "absent" or "negative." If a drug is present at a concentration equal to or above the cutoff, the test will be reported as "detected," "present", "presumptive positive" or "positive."

A positive or detected initial THC screening result means that the person tested may have used marijuana. Since screening tests are not definitive, confirmatory testing is typically done to verify the results. A positive confirmatory test means the person had THC or THC-COOH in their body when the sample was collected, but it does not confirm when THC was used or ingested.

Testing urine for THC cannot be used to determine the source of THC or its metabolites, time of exposure, amount, or the person's level of impairment.

A negative result does not necessarily mean that the person did not use marijuana. The person's THC levels may be below an established cutoff or the THC may have been already metabolized and eliminated from the body.

Detection times for marijuana vary widely depending on an individual's metabolism, the dose, and frequency of use. Testing can indicate use anywhere from 3 days to more than 30 days prior to testing. Chronic, heavy users of marijuana may test positive for even longer than 30 days after last use.

As states legalize marijuana, roadside testing for driver impairment due to marijuana use has become an urgent issue. This is a challenging testing problem because THC concentrations in the blood drop off quickly and the degree of impairment THC causes varies widely from person to person. That makes it very difficult to set a single legal level of THC in the blood, as is set with alcohol. While THC is currently the compound tested most often in blood to determine impairment, other cannabinoids are under investigation.

Some states, like Colorado, currently use blood testing and a cutoff concentration for THC in the absence of a better option. They are also piloting roadside oral fluid testing as a solution for determining impairment. However, there are still many obstacles to overcome before these tests are reliable.

Some laboratories offer marijuana testing on samples of blood and hair.

• A blood sample can be collected by inserting a needle into a vein and drawing blood into a tube.
• Hair may be tested and is usually collected from the crown of the head, although hair may also be collected from other parts of the body.

Newborns may undergo testing when the mother is known or suspected to have used drugs, such as marijuana, during the last trimester of pregnancy. In these cases, samples of meconium, a newborn's first bowel movement are typically collected by healthcare practitioners soon after babies are born. In recent years, umbilical cord tissue has gained more attention in drug testing after delivery instead of meconium as it can be collected immediately following birth.