From the Guidelines
False positive cannabis screens can occur due to several factors, including certain medications, dietary supplements, foods, and laboratory errors, and confirmation testing using more specific methods like gas chromatography-mass spectrometry (GC-MS) is recommended to verify results. The most recent and highest quality study on this topic is from 2019, which highlights the importance of understanding the type of drug test used and its limitations 1. Some substances that can cause false-positive results include:
- Poppy seeds
- Pseudoephedrine
- Dextromethorphan
- Certain medications like nonsteroidal anti-inflammatory drugs (NSAIDs) and proton pump inhibitors
- Dietary supplements containing hemp seed oil or CBD products
- Foods with hemp ingredients Additionally, passive exposure to marijuana smoke in poorly ventilated areas and laboratory errors during sample collection, handling, or testing can also contribute to inaccurate results. It is essential to consider the individual's complete medical history, including prescribed medications, and the limitations of the selected matrix and potential cross-reactivity when interpreting drug test results 2, 3. If a positive screen occurs unexpectedly, confirmation testing using more specific methods like GC-MS is recommended to verify results, as these methods can distinguish between THC metabolites and other substances. In real-life clinical practice, it is crucial to prioritize caution and consider the potential consequences of false-positive results, including unnecessary stigma, treatment, and legal repercussions. Therefore, a thorough evaluation of the individual's medical history, the drug test results, and the potential for false positives is necessary to ensure accurate interpretation and appropriate management.
From the Research
Causes of False Positive Cannabis Screens
- Certain medications and substances can cause false positive results in cannabis screens, including ibuprofen 4 and proton-pump inhibitors (PPIs) such as pantoprazole, although one study found that PPIs did not cause false-positive THC results using the THC One Step Marijuana Test Strip® 5
- Other substances that may cause false positive results include: + Non-steroidal anti-inflammatory drugs (NSAIDs) + Certain antidepressants + Some over-the-counter (OTC) medications
- A review of the literature found that many cross-reactivities exist with other analytes, potentially causing a false-positive result in an initial drug screen, including false-positive results for cannabinoids 6
- Efavirenz, a non-nucleoside reverse transcriptase inhibitor, was found not to cause false positive urine cannabis test results using the AxSYM Cannabinoids Assay® 7
- The type of assay used can also affect the results, with some assays being more prone to false positives than others 8, 4, 5, 6