Many tests for some a particular substance/medical condition have false positives. A false positive is a case where outcome of a test is positive yet the substance or medical condition does not exist. One prominent example is the PSA test for Prostrate Cancer. One estimate I read stated that the false positive rate is almost 20 times the actual rate of detection. That means if you test positive for PSA you have 1 one 20 chance of actually having Prostrate Cancer. Many doctors are unaware of how to present the results of various tests. In all cases a second and maybe a third test needs to be run to find the correct results. People put too much stock into the results of even a single test.