Why are cutoff concentrations used? Are values below the cutoff valid?
Cutoff concentrations are generally accepted in the industry and are chosen to maximize the number of true positive samples, while minimizing the number of false positives. In this way, the maximal number of drug users are detected, while reducing as much as possible false positive screens which require needless confirmation. It is accepted laboratory procedure to report values less than the cutoff as negative. While values less than the cutoff may be real, the convention is to consider them negative. Calibrations, quality control and confidence are established for the cutoff concentrations.
