How common are false positives in drug tests?
Initial Immunoassay screens can produce false positives — exactly why every non-negative screen is followed by a confirmation test using mass spectrometry. A defensible workplace drug test result has cleared both the laboratory confirmation step and the MRO review. The two-step structure means a true false positive — a verified positive that does not reflect drug use — is rare in well-run programs.
What is a false positive?
A "false positive" in drug testing is a result that reports the presence of a drug or metabolite when the donor has not used the substance. False positives are most common at the screening stage and are typically caught by the confirmation step. The two-step structure exists precisely to handle this — screening is fast and sensitive, confirmation is precise and specific.
How immunoassay can be fooled
Immunoassays use antibodies that bind to structural features of a target molecule. Different drugs and metabolites can share structural features, and an antibody designed for one molecule may bind to a structurally similar one with lower affinity — producing a positive signal at the screening cutoff. This is "cross-reactivity."
Common cross-reactants by drug class
Amphetamines
- Pseudoephedrine and other OTC decongestants — structurally similar to amphetamine; common cross-reactant.
- Bupropion (Wellbutrin) — can produce positive amphetamine immunoassay.
- Selegiline — metabolized to methamphetamine; not a "false" positive, but explainable.
- Ranitidine — historical concern, less so with newer immunoassays.
- Trazodone, labetalol, propranolol — rare reports.
Opiates
- Poppy seeds — genuine morphine/codeine ingestion, not cross-reactivity. Federal cutoffs were raised in 1998 to mitigate this.
- Quinolone antibiotics (ciprofloxacin, levofloxacin) — occasional reports.
- Dextromethorphan (cough syrup) — historical concern.
- Rifampin — rare reports.
Cannabinoids (THC)
- NSAIDs (especially older naproxen) — rare with modern immunoassays.
- Hemp / CBD products containing trace THC — genuine ingestion, not cross-reactivity.
- Dronabinol (prescription synthetic THC).
- Pantoprazole — rare historical reports.
Benzodiazepines
- Oxaprozin (Daypro, NSAID) — well-documented cross-reactant.
- Sertraline (Zoloft) — rare.
PCP
- Dextromethorphan at high doses.
- Tramadol, Venlafaxine — rare reports.
- Ketamine — low cross-reactivity.
How confirmation testing solves it
Confirmation testing uses GC-MS (gas chromatography–mass spectrometry) or LC-MS/MS (liquid chromatography–tandem mass spectrometry) , which separate molecules by mass and identify them with very high specificity. The confirmation step can tell the difference between, say, amphetamine and pseudoephedrine — molecules that share structural features but have distinct mass spectra. A confirmation step reports either the specific target analyte above the confirmation cutoff or a clean negative.
The MRO's role
The Medical Review Officer adds the clinical layer the laboratory cannot. When a confirmation reports a laboratory-positive, the MRO contacts the donor and asks about prescriptions, OTC medications, dietary factors, and any other potential explanations. If the donor has a verified legitimate medical explanation — say, a prescription for amphetamine for ADHD — the MRO reports the result to the employer as negative.
What to do if you suspect a false positive
- Don't panic; the process exists. Confirmation testing and MRO review are designed for this.
- Tell the MRO everything — prescriptions, OTC meds, recent dietary exposures, supplements.
- Have prescriptions ready to verify, including pharmacy contact information.
- Request a split-specimen retest if the result is verified positive and you believe it is in error. Under 49 CFR Part 40, donors have 72 hours after notification to request a retest of the Bottle B split specimen.
- Engage qualified counsel if the result has employment consequences and you believe it is in error.
Sources & references
drugtest.co content is sourced from primary regulatory and clinical references. We do not cite gray-market or "how to pass" sources.