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Drug Tests: Don't Fall Victim to a "False-Positive"

IMAGE Here is what you need to know about drug testing and how to prevent false-positives caused by other substances.

Drug Testing in the Workplace

Random drug testing is becoming increasingly popular in the workplace. Airline, railroad, trucking, pipeline, mass transit, and shipping industries do drug tests under US government mandate.

Private companies conduct drug testing to keep health insurance costs down, improve employee productivity, decrease absenteeism, and ensure a safe work environment.

A positive drug test is cause for dismissal in many companies. Most companies that drug test do not hire job candidates who test positive.

Drug Testing in Schools

Drug testing is also becoming more common in schools that have students who participate in extracurricular activities.

What Are They Testing For?

The most common substances being tested for include the following:

Other substances that may be tested for include:

  • Heavy alcohol use
  • PCP (phencyclidine)
  • Hallucinogenic drugs (LSD, peyote, mescaline, psilocybin, and ecstasy)
  • Inhalants (fumes or gases that are inhaled to produce intoxication such as glues, aerosols, butane, ether, nitrous oxide, and solvents)

Types of Drug Tests


Urine testing is the most popular type of test because it is simple, noninvasive, and highly accurate. A component of marijuana called THC can be detected in the urine. Anabolic steroids can also be detected through urine testing. The ability of the test to detect 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. In the majority of situations, a screening test is used that utilizes an immunoassay, which detects 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. Thus, a substance may be present, 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 highly specific and is typically used when testing for the presence of a specific drug is needed.


Hair tests are less susceptible to adulteration than urine tests. Hair provides a semi-permanent record of drug use. Drugs circulating throughout the body cling to the hair as it grows. Therefore, 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.

Typically, drugs cannot be washed out of the hair. This is in contrast to drugs in smoke, which stick to the hair and can be removed with shampoo. However, products used on the hair might, at times, interfere with 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 placed on the upper arm, midriff, or lower back to collect sweat. Contaminants from the environment cannot penetrate the adhesive barrier from the outside, so the patch can be worn during normal activities. To prevent tampering, the adhesive plastic film cannot be reapplied once 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 determine 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 detect if someone has been using drugs within the past few days. This type of test can detect 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 tests for alcohol consumption. Devices are available for home use. There are also disposable devices.

False-Positive Results

In some cases, a drug test may report the presence of illicit drugs, although none were taken. While this is not common, no test is 100% accurate. Lab errors account for some of the mistakes, but most false-positives may be attributed to over-the-counter drugs and foods that can affect the test. Below are some examples.

If you take or eat: You could test positive for:
Ibuprofen Marijuana, barbiturates, or benzodiazepines
Cold remedies Amphetamine
Hay fever remedies Amphetamine
Nasal decongestants Amphetamine
Diet pills Amphetamine
Sleep aids Barbiturates
Poppy seeds* Opiates/morphine
Hemp food products** Marijuana

*Large amounts of poppy seeds—for example, a pastry filled with poppy seeds—would need to be consumed to cause a positive test. However, sophisticated testing can discern poppy seeds from opiates.

**Hemp oil is eaten for its supposed nutritional benefits.

If you are mistakenly found positive for drug use, evaluate whether you have taken any substances known to cause false-positives.


National Institute on Drug Abuse

US Food and Drug Administration

Canadian Resources

Healthy U

Public Health Agency of Canada


Detection and measurement of drugs. National Highway Traffic Safety Administration website. Available at: Accessed July 20, 2016.

Drug testing 101. Separated Parenting Access & Resource Center website. Available at: Accessed July 20, 2016.

Fact sheet: Drug testing in the criminal justice system. US Department of Justice website. Available at: Published March 1992. Accessed July 20, 2016.

Frequently asked questions about drug testing in schools. National Institute on Drug Abuse website. Available at: Updated September 2014. Accessed July 20, 2016.

Gourlay D, Heit H, Caplan Y. Urine Drug Testing in Primary Care: Dispelling the Myths and Designing Strategies. Monograph PharmaCom Group, Inc; 2002.

Hair drug tests. HealthStreet website. Available at: Accessed July 20, 2016.

Immunoassay urine drug test (UDT). Virginia Commonwealth University School of Medicine website. Available at: Published July 9, 2008. Accessed July 20, 2016.

Reisfield GM, Bertholf R, Barkin RL, Webb F, Wilson G. Urine drug test interpretation: what do physicians know? J Opioid Manag. 2007;3:80-86.

Substance check. Home Health Testing website. Available at: Accessed July 20, 2016.