Drug Tests: Don't Fall Victim to a "False-Positive"
by Cynthia M. Johnson, MA
Here is what you need to know about drug tests and how to lower the risk of false-positives caused by other substances.
Drug Testing in the Workplace
Random drug testing is becoming more common in the workplace. Airline, railroad, trucking, pipeline, mass transit, and shipping industries do drug tests under US government law.
Private companies also do drug testing to keep health insurance costs down, improve productivity, lower absenteeism, and create a safe workplace.
Most companies do not hire people who test positive and also fire current employees who test positive.
Drug Testing in Schools
Drug testing is also becoming more common in schools that have students who do after school activities.
What They are Testing
The most common substances being tested for are:
Other substances that may be tested for are:
Types of Drug Tests
Urine testing is the most popular type of test. It is simple, noninvasive, and very accurate. A part of marijuana called THC can be picked up in urine. Anabolic steroids can also be found through urine testing. The ability of the test to find these substances depends on how much was used and for how long.
Urine drug testing is widely used for testing for opioids and illicit drugs. Most of the time, a screening test with an immunoassay is used. It can find the parent drug and/or metabolite. It usually tests for marijuana, cocaine, opiates, PCP, and amphetamines.
Typically, the screening immunoassay detects the amount of drug present in urine above a predetermined cutoff concentration. So a substance may be there, but if the concentration of that drug is below the cutoff, the result will be negative. For more precise results, the confirmatory urine drug test is done by gas chromatography/mass spectrometry or high-performance liquid chromatography. This test is very specific and is often used when testing for the presence of a specific drug is needed.
Hair tests can give a semi-permanent record of drug use. Drugs circulating inside the body cling to the hair as it grows. So, if the drug was taken recently, it will be found in hair near the scalp. If the drug was taken in the past, the drug will be found in hair that has grown out.
Drugs cannot be washed out of the hair. This is not the same as drugs in smoke, which stick to the hair and can be removed with shampoo. However, products used on the hair might, at times, get in the way of test accuracy.
Hair testing has an advantage because it can detect drug use over longer periods of time (usually around 90 days). This test is available for home use.
To test sweat, a bandage-like patch is put on the upper arm, midriff, or lower back to collect sweat. Contaminants from the environment cannot get through the patch from the outside, so the patch can be worn during normal activities. To prevent tampering, the adhesive plastic film cannot be reapplied after it is removed.
Residue tests are typically found in home testing kits that allow parents to test their children. The kit comes with a pad that picks up drug residue by swiping the pad on the child's belongings. The pad then is mailed to the manufacturer for testing. The major flaw with this test is that it cannot find out whether any drug residue found was left by the child under suspicion or by someone else.
Blood tests are the most accurate test, but are not used routinely in the workplace or school.
A saliva test is a way to find out if someone has been using drugs within the past few days. This type of test can find many illegal substances, such as heroin and cocaine. But, it may be less reliable in detecting marijuana.
This involves exhaling into a hand-held machine to test for alcohol. Devices are available for home use. There are also one time use devices.
Sometimes, a drug test may report the presence of drugs when a person did not take any. This is not common, but no test is 100% accurate. Lab errors account for some of the mistakes, but most false-positives may be due to over the counter drugs and foods that can affect the test, such as:
*Large amounts of poppy seeds would need to be eaten to cause a positive test. Good tests can tell poppy seeds apart from opiates.
**Some people eat hemp oil because they believe it has nutritional benefits.
If you are mistakenly found positive for drug use, think about whether you have taken any substances known to cause false-positives.
National Institute on Drug Abuse
US Food and Drug Administration
Alberta Health Services
Detection and measurement of drugs. National Highway Traffic Safety Administration website. Available at:
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Accessed June 23, 2021.
Drug testing. Substance Abuse and Mental Health Services Administration—US Department of Health and Human Services website. Available at: https://www.samhsa.gov/workplace/resources/drug-testing. Accessed June 23, 2021.
Fact sheet: Drug testing in the criminal justice system. US Department of Justice website. Available at: https://www.ojp.gov/sites/g/files/xyckuh241/files/archives/ncjrs/dtest.pdf. Accessed June 23, 2021.
Hair drug tests. HealthStreet website. Available at: https://www.health-street.net/drug-tests/hair. Accessed June 23, 2021.
Substance test. Home Health Testing website. Available at:
...(Click grey area to select URL)
Accessed June 23, 2021.
Last reviewed June 2021 by EBSCO Medical Review Board
Last Updated: 6/23/2021
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