What is a false positive for drug tests? A false positive is a positive test result when the person actually should have a negative result. It can happen when a medication cross-reacts with the immunoassay. This is due to a similar structure in common substances.
Over-the-counter medications, other prescribed medications, and certain foods can cause a false positive.
What are some common reasons for a false positive due to cross-reaction?
- Natural Herbal Supplements and Vitamin Supplements– Among the most notable are Vitamin B supplements; these can give a false positive for THC.
- Ibuprofen –this over-the-counter pain reliever could also give false positive results for benzodiazepines and barbiturates.
- Cold Medicines – Many over-the-counter cold remedies contain synthetic amphetamines, more commonly known as pseudoephedrine, which are also used to produce methamphetamines, a class of illegal drugs.
What is a false negative result? A false negative is a negative test result when a person actually should have a positive result.
Why do false negatives occur? Diluted urine can cause a false negative because the urine drug concentration is below the threshold or cutoff level. A person may also receive a false negative depending on the length of time between the substance use and testing for the substance, as well as the actual amount of the substance used. (Reworded a bit.)
Opiates can be a risk for false negatives because the metabolite may not be as easily detected. Morphine panels with a higher, more sensitive cutoff of 300 ng/mL may be a better option if you are at a higher risk for a positive for opiates.
For any pre-employment drug testing, it is in one’s best interest to provide a detailed list of items that could alter test results, including:
- over-the-counter medications and pain relievers
- cold remedies
- vitamin supplements
- herbal supplements
- nutritional supplements
- prescription medications
- foods and beverages consumed up to 72 hours prior to the test
The goal is to avoid false negatives and false positives. Being aware of what you ingest and what can cross-react is most important. Following the testing instructions closely can help avoid a false negative. Also make sure you are testing for the correct substance’s metabolite and that you are aware of the cutoff level when reading your results.