1 Department of Mathematics, Université de Sherbrooke, 2500 Université Boulevard, Sherbrooke, Québec, J1K 2R1, Canada.
2 School of Mathematics and Statistics, The University of Melbourne, Parkville, Victoria, 3010, Australia.
3 Centre de recherche, Centre hospitalier universitaire de Sherbrooke, 12th Avenue North, Sherbrooke, Québec, J1H 5N4, Canada.
4 Department of Toxicology, Laboratoire de sciences judiciaires et de médecine légale, 1701 Parthenais Street, Montréal, Québec, H2K 3S7, Canada.
5 Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montréal, Québec, H4B 1R6, Canada.
6 Department of Criminalistics, Laboratoire de sciences judiciaires et de médecine légale, 1701 Parthenais Street, Montréal, Québec, H2K 3S7, Canada.

Qualitative methods hold an important place in drug testing, filling central needs in screening and analyses, among others, linked to per se legislation. Nevertheless, the bioanalytical method validation guidelines do not discuss this type of method or describe method validation procedures ill-adapted to qualitative methods. The output of qualitative methods are typically categorical, binary results, such as presence/absence or above cut-off/below cut-off. As the goal of any method validation is to demonstrate fitness for use under production conditions, qualitative validation guidelines should evaluate performance based on discrete, binary results instead of the continuous measurements obtained from the instrument (e.g. area). A tentative validation guideline for threshold qualitative methods was developed by in silico modelling of measurements and derived binary results. This preliminary guideline was applied to a liquid chromatography-tandem mass spectrometry method for 40 analytes, each with a defined threshold concentration. Validation parameters calculated from the analysis of 30 samples spiked above and below the threshold concentration (false negative rate, false positive rate, selectivity rate, sensitivity rate and reliability rate) showed a surprisingly high failure rate. Overall, 13 out of the 40 analytes were not considered validated. A subsequent examination found that this was attributable to an appreciable shift in the standard deviation of the area ratio on a day-to-day basis, a previously undescribed and unaccounted-for behaviour in the qualitative threshold method validation literature. Consequently, the developed guideline was modified and used to validate a qualitative threshold method, based on the binary results for performance evaluation and incorporating measurement uncertainty.