What causes positive drug tests? It's not just drugs… or poppy seeds.

You may have heard that poppy seeds in your salad dressing or on your bagel can mess up a drug test and trigger a positive result for opiates in your body. While there are some caveats to this (washing and proper seed processing can remove impurities and cooking tends to significantly reduce the opiate content of poppy seeds), it is true that poppy seeds can be a problem. Poppy seeds themselves do not actually contain any opiate compounds, but are often adulterated with opiate-containing flower juice during processing.

But it's not just poppy seeds that can falsify a rapid drug test. “There are a variety of things that can cause false positives,” says Gwen McMillin, a toxicologist and professor of clinical pathology at the University of Utah. Poppy seeds don't actually cause anything INCORRECT Positives, notes McMillin. “[Contaminated] Poppy seeds actually contain opiates,” she says—but many medications, even common over-the-counter medications, can confound tests with entirely different compounds.

The most commonly used types of rapid drug tests are so-called immunoassays. These tests vary widely and are conducted by many companies. Different versions of it can screen for amphetamines, benzodiazepines, PCP, opiates, synthetic opioids, marijuana, and other regulated substances. The antibodies built into the tests react with compounds selected to represent likely drug use: metabolites that the body excretes in urine. However, the tests do not look for the exact target substances they may be looking for. McMillin compares it to a wooden game board with slots for marbles – marbles of similar size and shape fit into the same holes.

Another analogy is a fishing net, she says. “A net is designed to catch fish, but you don’t know exactly what kind of fish it catches,” she says Popular Science. “The immunoassay is really brilliant because it can detect all sorts of things that look like fish. If you just try to do a check, that’s really important… but the consequence is false positives.”

The antibody is the most important part of the design of an immunoassay, and it can be a difficult balancing act to aim for specificity while ensuring that an entire class of related controlled substances are included in the first line of detection. False positives are often the result of test takers choosing width. But they also happen because of fundamental chemical limitations: Certain groups of drugs, particularly amphetamines, are metabolized into small, rather inconspicuous molecules that resemble many things, says Algren.

Cold medicines, antihistamines and decongestants, antidepressants, blood pressure medications, and diabetes medications are among the long list of things that can cause false positive amphetamine tests. Even over-the-counter cold medications can be detected as PCP. Codeine, some antibiotics commonly used to treat urinary tract infections, high blood pressure medications, and anti-nausea medications are some examples of pills that can give false positive results for opioids and opiates. Quinine, the antimalarial drug used as a flavoring agent in tonic water, can also trigger some of these tests. CBD products and even certain types of baby detergent can show up as positive when tested for cannabis/THC. And researchers continue to discover previously unknown examples of cross-reactive compounds.

Sensitivity also plays a big role. Manufacturers don't want tests to miss existing drugs. However, if they design them with low enough thresholds, rapid tests can even detect erroneous, incidental or secondary exposures.

Although these weaknesses are widely known among toxicologists and immunoassay tests come with package inserts detailing possible cross-reactivity with nontarget drugs, clinicians performing these tests on patients in hospitals or maternity units may not be aware of all of the many regulatory and common pathways to a positive urine drug test. And that can cause problems if time is short and confirmatory tests are not ordered. In recent years, several lawsuits have been filed against hospitals after newborns were placed in foster care based on the results of parents' rapid drug tests. McMillin said patient care can also be compromised if rapid drug screens are used alone as the basis for approving surgeries and other treatments.

False-negative results are also not uncommon. Some tests are not sensitive enough to detect the concentration of a drug circulating in a person's body. And some newer substances may not actually show up in a test that would theoretically be designed to detect their presence. For example, synthetic and semi-synthetic opioids such as fentanyl and oxycodone are not detected in all opioid drug tests because their metabolites and chemical components are different. This, too, can cause problems for patients and even lead to people being unfairly excluded from chronic pain management programs, where a false negative test could lead providers to falsely believe that patients are not taking their medications as prescribed, says Adam Algren. a medical toxicologist and emergency physician at University Health Hospital and the University of Missouri-Kansas City.

Ideally, any surprising result from an immunoassay drug test would be confirmed by more specific and accurate follow-up laboratory tests, both Algren and McMillin say. However, these slower and more expensive confirmatory tests are not used in every hospital system or care facility. “I think many physicians are quick to view immunoassay results as the end result and then make decisions based on that without confirming them or considering whether there are other things at play,” Algren says.

“I personally don’t feel like enough attention is being paid to confirmatory testing,” agrees McMillin. Both experts say they would like to see this change.

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