What Causes False Positives On Drug Tests?
In our most recent article, the pharmacist discusses why you may be testing positive for illicit drugs when you don't take them and what you can do.
Hello! Can you run me through why drug screens come up false sometimes?
Jun 09, 2022- Standard drug screening tests use antibodies that can react with medications or foods you're not being tested for, causing false positives through a process called cross-reactivity.
- Common medications like Sudafed, Benadryl, ibuprofen, and certain antibiotics, as well as poppy seeds, are documented to trigger false positive results on routine urine tests.
- If you get a positive result, inform the testing agency of all medications you're taking, and you may be able to request a more accurate confirmation test like gas chromatography-mass spectrometry to clear your name.
Overview
Thanks for reaching out! A common prerequisite to being hired at a new company is to take a 'routine' drug test, most commonly a urinalysis. For most, this is no big deal...as you don't take drugs, you think nothing of it.
Can you imagine, however, how shocked and dismayed you'd be if the results are returned to you with an indication that you have failed? Not only can this put your new job in jeopardy, but you have also now been labeled a drug user.
Surprisingly, this happens more often than you would think and is often due to prescription and over-the-counter medications that millions of individuals take. Even some common foods can cause positive tests!
Why Do False Positives Happen?
The main issue is the tests themselves.
Most employers use simple, urine-based immunoassay tests known as an EIA (Enzyme-Linked Immunoassay) test or a CEDIA (Closed Enzyme Donor Immunoassay) test.
They both use similar principles. These tests work by utilizing antibodies to react to specific drug compounds. Basically, when a specified drug is present in a test urine sample, an antibody binds to it, which produces a reaction. When a reaction occurs, it is recorded as a 'positive' result.
With these urine tests, there can be a significant problem with 'cross-reactivity'.
Drugs that are NOT being tested for can occasionally react with the wrong antibody (i.e. 'cross react'), resulting in a 'false positive'. A common example we hear about is poppy seeds causing positive opioid tests. This is not a myth and many studies have shown this is possible.
Common Culprits
Below is a list of some common drugs that can cause false positives:
False Positive Opioid Tests:
- Poppy Seeds
- Levaquin (Levofloxacin)
- Rifampin
- Delsym (Dextromethorphan)
- Benadryl (Diphenhydramine)
False Positive Amphetamine Tests:
- Sudafed (Pseudoephedrine)
- Zantac (Ranitidine)
- Wellbutrin (Bupropion)
False Positive Marijuana Tests:
- Certain NSAIDs (e.g. ibuprofen, naproxen)
- Certain PPIs (e.g. Prilosec, Omeprazole, Prevacid)
What To Do Next
If you get a false positive on a drug test, what can you do? If your employer allows, you could retake the test. While false positives do happen, they are relatively uncommon and ideally, a repeat test will clear you.
In addition, there are tests that are much more accurate than the urine-based immunoassay tests.
There are two tests that can detect drug compounds in a specific manner, gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC).
These tests do not use antibodies and can accurately determine the exact molecule in question. If you received a 'false positive', these tests will be able to confirm that the offending drug compound is not present. You can inquire as to whether or not these tests can be utilized to clear you if you receive a 'false positive'.
Unfortunately, tests that are not immunoassay-based are more expensive and rarely used in a common drug screening. You may only have an option to request the more accurate tests if you are already employed at a company and need to prove your innocence.
On the plus side, we have a deeper understanding of how common 'false positives' really are. Most labs nowadays will run all positive screening results through more accurate and reliable confirmation tests.
Final Words
When taking a drug test, it is extremely important to make the testing agency aware of all the medications you are taking. They know what drugs are documented to cause 'false positives' and may further examine a positive result based on your medications.
References & sources
- Effect of poppy seed consummation on the positive results of opiates screening in biological samples — ScienceDirect
- Urine Drug Screening: Practical Guide for Clinicians — Mayo Clinic Proceedings
Glossary — Terms in This Answer
- Urinalysis
- A medical test that analyzes a urine sample to detect substances or medical conditions.
- Immunoassay
- A laboratory test that uses antibodies to detect the presence of specific substances in a sample.
- EIA
- An Enzyme-Linked Immunoassay test that uses enzymes and antibodies to identify drug compounds in urine.
- CEDIA
- A Closed Enzyme Donor Immunoassay test that works similarly to an EIA to detect drugs in urine samples.
- Cross-reactivity
- When a substance reacts with the wrong antibody in a test, causing an incorrect positive result for a drug not actually present.
- Gas chromatography-mass spectrometry
- A highly accurate laboratory technique that separates and identifies specific drug molecules without using antibodies.
- High-performance liquid chromatography
- An advanced laboratory test that precisely identifies and measures specific drug compounds in a sample.
- NSAIDs
- Non-Steroidal Anti-Inflammatory Drugs, a class of pain relievers such as ibuprofen and naproxen.
Dr. Brian Staiger, PharmD
Doctor of Pharmacy, board certified clinical pharmacist (BCPS), American Herbalist Guild, Medication Therapy Management certified
Brian Staiger, PharmD, BCPS, is a licensed pharmacist with more than 13 years of experience across retail pharmacy, clinical pharmacy, medication safety, pharmacy program development, and healthcare administration. He is the founder of HelloPharmacist.com, where he helps patients...
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