False Positive Drug Tests

An initial positive drug test result often starts with a presumptive immunoassay screen. Those screens can react to a surprisingly wide range of substances. Confirmation testing, and in many cases the Medical Review Officer, is relied on to settle the result.

What Is a False Positive?

A false positive is when a drug test screens as positive but the donor does not actually have that substance in their system. This happens because the initial immunoassay screen is only presumptive: it is a broad preliminary test that deliberately checks for a wide range of related chemicals so as not to miss any true positives. The trade-off is that legitimate medications and even some foods can cause a similar reaction, even if the drug in question is not actually present.

This high-risk design makes sense in a screening context, but initial positive results do not confirm use: they only flag the specimen for follow-up testing. The key is a confirmatory test. Immunoassays may react to similarly structured chemicals, but to definitively identify the specific drug, a more selective confirmatory test such as gas chromatography/mass spectrometry (GC/MS) is used.

What Substances Can Trigger Screens?

The chemicals that trigger an immunoassay screen are not limited to illegal drugs. Some over-the-counter and prescription medications have been documented to cross-react with these assays. Popular cough and cold remedies, many antidepressants, and even some common antibiotics can all cause a positive immunoassay screen, even when taken as directed by a physician, if they contain a cross-reactive compound.

Specific foods such as poppy seeds are also potent cross-reactors, particularly with opioid panels. Herbal supplements and medications based on natural products also appear to trigger screens, although the exact compounds responsible may be unknown if the origin is not well documented.

Why Confirmatory Testing Is Necessary

Confirmatory testing is a more analytical process that can distinguish between the broad category of similarly structured chemicals that triggered the immunoassay and the specific drug the test looked for. Typically GC/MS is applied for this definitive identification, but in some programs, liquid chromatography with tandem mass spectrometry (LC-MS/MS) may also be used. This method further separates the chemicals in the specimen and checks their molecular weight to match the specific drug, or confirm no such molecule is present.

Steps to Take After a Positive

If you receive an unexpected positive result, your first step should be to list any medications, prescription or over-the-counter, as well as any herbal supplements you have taken. Notify your contact at the testing lab or your employer immediately, as many programs have stringent follow-up rules that reputable human resource departments have to enforce in good faith. Follow the exact process given in your program documentation, and do not try to work around it.

The MRO reviews the initial test results and the medication information you provide. They are responsible for verifying any legitimate medical explanation for a positive test result. Presenting the MRO with a copy of the prescription, a photo of the medication bottle showing the dosage and date, or sometimes a printout from the pharmacy, can help clarify legitimate use.

If the MRO is still not satisfied, they may consult with the prescribing physician, and in some cases, a retest of the original specimen may be conducted.

When to Seek Help

The appearance of an unexpected drug test result can be unsettling, even when the result is often due to cross-reactivity in the initial screen. However, by knowing which medications can cross-react, understanding the drug testing process, and seeking mediation review from MRO when appropriate, you can navigate a positive result with confidence.