How positive drug screens can come back as negative tests

 Quick, on-the-spot drug tests, often called point-of-use or rapid tests, are designed to give a fast indication of possible drug use. However, it's surprisingly common for these initial tests to show a positive result, only for a more thorough lab test to come back negative. This discrepancy can be confusing, but it boils down to how these tests work.

The rapid tests are typically immunoassays. They're like a first alert, designed to catch groups of drugs, not specific ones. Think of them as a broad search net. While efficient for large-scale screening, this broad approach means they aren't always perfectly accurate.

They can be prone to "cross-reactivity," reacting to substances chemically similar to the target drug, even if that specific drug isn't present.

Imagine trying to identify a specific breed of dog just by its size. A small dog might trigger your "small dog" alert, but it could be a Chihuahua, a terrier, or a miniature poodle. The initial drug test is like that size-based alert – it flags something as potentially being in a certain category, but it doesn't pinpoint the exact substance.

Many everyday substances can cause this cross-reactivity. Common decongestants containing pseudoephedrine can mimic amphetamines on an immunoassay. Certain pain relievers, herbal supplements, and even foods like poppy seeds (which can cause a positive opiate result) can also trigger false positives. This is why a Medical Review Officer (MRO) is so crucial.

The MRO is a licensed physician who reviews all positive drug test results. They're a safeguard against false positives. They contact the person who took the test to discuss any medications, supplements, or other potential explanations for the initial positive. This conversation is key to uncovering legitimate reasons for the cross-reactivity.

The confirmatory test, done in a certified lab, uses more sophisticated methods like Gas Chromatography-Mass Spectrometry (GC-MS) or Liquid Chromatography-Mass Spectrometry (LC-MS). These tests are far more specific than immunoassays. They can identify individual substances with high accuracy, like DNA testing for flowers – pinpointing the exact species.

Sometimes, a substance might be present at a level detectable by the sensitive immunoassay, but below the detection limit of the GC-MS or LC-MS. This is especially true for drugs the body processes quickly (drugs with short half-lives). By the time the sample reaches the lab, the drug's concentration might have dropped too low for the confirmatory test to detect.

Another factor is the "cutoff level" of each test. Tests detect substances above a certain concentration. The immunoassay might have a lower cutoff than the GC-MS or LC-MS. So, the immunoassay might pick up trace amounts, but the lab test, with its higher cutoff, might not register it as positive.

Though rare, errors can occur in testing, like sample mix-ups or contamination. Labs follow strict protocols to minimize these risks, but human error and equipment issues can't be entirely eliminated.

Metabolism and time since ingestion are also important. The body processes drugs at different rates, and the time between ingestion and testing greatly affects the drug's concentration. A drug might be detectable soon after it's taken, but its concentration could fall below detectable levels by the time the confirmatory test is done.

Passive exposure, like inhaling secondhand marijuana smoke, can sometimes cause a presumptive positive. While the immunoassay might detect drug metabolites, the levels are often too low to be confirmed by GC-MS or LC-MS.

In short, a positive quick test followed by a negative lab test often happens because the initial test isn't very specific, or because the body processes the substance below the lab's detection level. The MRO's review is vital, ensuring accurate interpretation and minimizing false positives.

Published: March 11, 2025