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False Positive Drug Tests: How Labs and MROs Catch Them

A false positive is a test result that shows a drug when none is present. On urine drug tests, false positives happen mostly at the first screening step, when something other than the target drug reacts with the test. Lab tests are built to catch this. Any screen that isn't negative gets a second, more specific confirmation test, and then a physician, the medical review officer (MRO), reviews the result. This guide explains where false positives come from, how they are caught, and what to do if you think your result is wrong. It is general information, not medical advice.

Updated October 14, 2026 · Checked against the official sources listed at the end

Where false positives come from

Most lab urine tests start with an immunoassay screen. Quest describes it as a binding reaction between the drug and an antibody that signals whether the target compound might be present.

The word "might" matters. SAMHSA's guide for doctors explains that cross-reactivity happens when a test can't tell the substance it's looking for from chemically similar substances. It says substances other than the target drug can react with the antibody and produce a false-positive result. FDA's page on home drug tests makes the same point: foods, supplements, drinks and medicines can affect screening results.

Examples from federal guidance

SAMHSA's 2012 guide names several examples of substances that can cause a false-positive screen. Whether one affects a given test depends on the test kit used:

• Some over-the-counter decongestants, such as pseudoephedrine, can register as amphetamine.
• Phentermine, an appetite suppressant, commonly causes a false-positive first screen for amphetamines.
• Dextromethorphan can cause a false positive for PCP in some tests.
• Bupropion can cause false-positive amphetamine results on one type of screening test.
• Some proton-pump inhibitors have caused positive marijuana screens.

How confirmation testing catches them

Quest tests a non-negative screen again on a different portion of the specimen, using gas or liquid chromatography with mass spectrometry. Quest says these instruments can identify the specific drug and tell it apart from other compounds.

SAMHSA says that when a screening result is confirmed this way, a false positive is highly unlikely if the test is performed correctly. On Pact's 10-panel, every screen that isn't negative is confirmed at the lab before anything else happens.

Harder cases: amphetamines and opiates

SAMHSA calls amphetamine tests among the hardest to interpret. Some over-the-counter nasal inhalers contain l-methamphetamine. SAMHSA says a standard confirmation for methamphetamine can't tell illicit use from use of that kind of product, and that a separate stereospecific test can. Under DOT rules, an MRO may ask a lab for that type of testing (49 CFR 40.141(b)(2)).

Poppy seeds can contain morphine and codeine. SAMHSA says the federal workplace cutoff for morphine and codeine was raised from 300 to 2,000 ng/mL to avoid positives from poppy seeds. Pact's panel uses 2,000 ng/mL for opiates. Under DOT rules, morphine or codeine below 15,000 ng/mL is verified positive only if the MRO finds clinical evidence of unauthorized use, and poppy seeds are not accepted as an explanation at or above 15,000 ng/mL (40.139).

Real results that aren't misuse

Some results are true positives from a legal prescription. That isn't a false positive. The lab correctly found the drug. SAMHSA notes, for example, that Adderall is an amphetamine and will test positive for amphetamine.

This is what the MRO is for. On Pact's 10-panel, a lab result that isn't negative goes to the MRO, a licensed physician who contacts the person tested to ask about prescriptions before the result is final. Under DOT rules, if there is a legitimate medical explanation, the MRO verifies the test as negative (40.137(d)).

Some results aren't false positives

SAMHSA says cocaine screening tests are highly specific for the cocaine metabolite and don't cross-react with other substances. It also notes that coca tea contains cocaine and can cause a positive. Under DOT rules, an MRO must not accept coca tea, hemp products, or stories of unknowing use as explanations (40.151).

Instant tests and home tests

False positives matter most when there is no confirmation step. FDA's page on home drug tests says not to take serious action until the lab result comes back, because many factors can cause a false positive on the home test. FDA also says an amphetamine result should be considered carefully even when confirmed, because some over-the-counter medicines produce the same results. Pact's 10-panel is a lab test, so every non-negative screen is confirmed before the MRO reviews it.

If you think your result is wrong

• Answer the MRO's call, or call back quickly. Have every prescription, over-the-counter medicine and supplement you took ready, with your prescriber's and pharmacy's details.
• Tell the MRO the truth. The MRO is checking for a medical explanation.
• Ask Pact about retest or split-specimen options for a non-DOT test before you order anything else. Pact's sales are final.
• Don't stop or change a medicine because of a drug test. Talk to your prescriber.

Common questions

Sources

• Quest Diagnostics: Urine drug testing FAQ (PDF, March 2018)
• SAMHSA: Clinical Drug Testing in Primary Care, TAP 32 (PDF, 2012)
• FDA: Drugs of abuse home use test
• Quest Diagnostics: Commonly ordered urine unit codes (PDF, 2/2019)
• 49 CFR 40.137: Verifying results for marijuana, cocaine, amphetamines, opioids and PCP (eCFR)
• 49 CFR 40.139: Verifying opiate results (eCFR)
• 49 CFR 40.141: How the MRO gets information (eCFR)
• 49 CFR 40.151: What MROs are prohibited from doing (eCFR)

Related guides

• The MRO's Role in DOT Drug Testing
• What a 10-Panel Drug Test Screens For
• How Long Do Drug Test Results Take?
• 5-Panel vs 10-Panel Drug Test: Which Should You Use?
• Background Checks (and Drug Tests) for Volunteers
• DOT vs Non-DOT Drug Tests: Which One Do You Need?