What Does False Positive Mean?
False positive, within the context of drug testing, is a term used to refer to test results that indicate the existence of a condition or finding that is not actually present. The result is “untrue” or false. In comparison, a false negative would indicate that a condition was not present when in fact the condition did exist. A test that results in a high level of false positives is considered to be lacking in “sensitivity.” A test that results in a high number of false negatives lacks “specificity.”
Drug screening tests may sometimes show a false positive result because of the limitations in sensitivity or accuracy of the test used. For this reason, a positive result from a drug screening test is usually followed up with a more detailed confirmation drug test to verify the results.
WorkplaceTesting Explains False Positive
False positive test results may be caused by various reasons including flaws in the test design or testing procedure, or the presence cross-reactive substances in a test sample. Point of collection (POC), or drug screening tests, that rely on immunoassay technology are more likely to produce a false positive or false negative than more sophisticated laboratory tests, such as gas chromography mass spectrometry tests (GC/MS).
Legal substances such as poppy seeds, cold medications, and some prescription medications may trigger a positive result because their chemical composition mimics the targeted drugs. Because laboratory tests are more sensitive than rapid immunoassay tests, laboratory tests can better distinguish between the substance of interest, the drug, and a similar substance.
For employees subject to federal drug testing rules, a positive result indicated by a drug screening test must be followed up with a confirmation test to eliminate the possibility of a false positive.