At face value, it would appear to be a straightforward matter as to whether or not a drug is present upon analysis of a material or object. This, however, is not always so, and especially when we are dealing with drug traces only.
Simon Bunter's article entitled "Cognitive Bias in Fingerprint Evidence: does the alleged 'matching' ridge detail even exist?" has been published in the Winter 2019/Spring 2020 edition of The Writ.
In the world of medicine, a good physician will generally use initial test results as a starting point for diagnosis.