In research studies, a commonly used cut point for nonadherence is 80% or less adherence of required dose. Missing two days per week means that she is 71% adherent to her prescribed sertraline. By this metric, it is easy to understand how 50+% of patients on antidepressants may meet criteria for nonadherence.

Publication reference : ten Doesschate, M.C.; Bockting, C.L.H.; Schene, A.H. Adherence to Continuation and Maintenance Antidepressant Use in Recurrent Depression. J. Affect. Disord. 2009, 115, 167–170
Reference URL: https://pubmed.ncbi.nlm.nih.gov/18760488/