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Drug Testing

The following information has been compiled to help you better understand the drug screening process and options you may have if you feel your screen has been erroneous.


Urine Drug Screens

Screening of the blood and urine is useful when searching for drugs of abuse. Most toxic screenings search the urine for 5 abused drugs. These 5 substances include opiates, PCP, cocaine, marijuana, and amphetamines.

Although most urine drug screens are very unlikely to miss the presence of a drug being assayed, the possibility of a false positive result does exist.  This means that the test could return a positive result when the tested individual has not had any contact with an illicit substance. False positive tests are rare with an occurrence of approximately 1 – 2.5%.

For this reason, it is recommended that any and all positive screenings be followed up by a second, more selective test to confirm the result and rule out any false positives. This follow up test should be performed using a methodology that is different from that used on the initial screening test. This way the possibility of two simultaneous false positive results using two completely different methods should be quite low.

The current standard confirmatory test is gas chromatography/mass spectrometry (GC/MS). If the confirmation is positive, then it is a true positive test. This test increases the cost of testing, but essentially eliminates the possibility of false positive results.

The screens are reviewed by a doctor knowledgeable in drug screening, known as a Medical Review Officer (MRO). The professional MRO is a licensed physician (MD or DO) who has undergone extensive training in applying federal regulations to drug and alcohol testing. The role of an MRO is one of prevention, detection, and control of drug abuse in the workplace.

Positive test results on urine drug screens must be verified with a confirmatory test before they can be used for legal purposes. Any company that does drug screening may have to defend its action in a court of law if they are wrong or act only on a screening drug test report. This does not mean that everyone conducting urine screens for drugs of abuse confirms their results. If an individual loses their job or suffers some other sort of harm as a result of a positive urine drug screen that was not confirmed, the company or individual that caused the harm may be legally liable and subject to lawsuits for damages.

The potential for misuse of urine drug screens is significant. Most large companies have written policies and procedures to prevent this misuse. If you have reason to believe that you have been harmed by an inappropriate urine drug screen, it would be reasonable to seek the advice of a lawyer.


What to Do to Avoid a False Positive Drug Screen

False-positive results are possible, but highly unlikely. A false-positive result occurs when another substance in the body has a structure similar to that of one of the drugs being assayed in the screen. A few over-the-counter (OTC) medications that may sometimes lead to erroneous results include:

  • Dextromethorphan
  • Naproxen
  • Pseudoephedrine
  • Vicks inhaler
  • Weight loss supplements

Some of the medicines used in the hospital for surgical patients can also cause false positives on urine drug screens for amphetamines. Additionally, Amoxicillin has been associated with false positive urine screens for cocaine.

For these reasons, if you are scheduled for a drug test, you should report to the testing agent doing the drug screen all medications you have used recently. This includes herbal and OTC products as well as prescription medications. When in doubt, list it. If you do not list a medication or medical condition at the time of collection, it may be difficult or impossible to add this information at a later date, trying to explain your results.

Please visit Toxicology Screen (Medline Plus) for additional information.


What to Do if You Think Your Drug Screen Is Erroneous

You can challenge a positive result you feel is false, especially if there are dire consequences such as job loss or jail associated with the finding. Get a more specific test, such as gas chromatography/mass spectrometry (GC-MS), on the same sample. This test will either rule out the false-positive or confirm the original result. Only the true substance can show a positive on a confirmation test.

You should also consult with the pathologist or other senior clinician at the lab where your screen was actually done. They have the most up-to-date information and should be able to tell you which medications can give false positive results on their tests.

The following resources will provide you with additional information about drug testing:

Drug Testing (US Dept. of Labor)

Drug Testing & Workplace Issues (NIDA)

FAQ: Drug Screening 

NCAA Drug-Testing Program


Laboratories have ways of testing for adulterants. If adulterants are discovered, the screen will be thrown out and another urine sample will be required. Avoiding the use of all illegal drugs is the safest guarantee to passing a screen.


For more information:

Go to the Addiction and Substance Abuse health topic.