Patients were receiving: olanzapine, aripiprazole, amisulpride, risperidone, quetiapine, sulpiride and haloperidol. Just over one-third (35%) of the total sample had subtherapeutic antipsychotics plasma levels, including 12% of patients who had undetectable levels. Rates of hospitalization were higher in individuals with a subtherapeutic plasma level (31% vs 11%).
http://www.psychiatrictimes.com/depression/clinically-relevant-developments-psychiatry/page/0/1
So 35% of people who are treatment resistant may in fact not be treatment resistant.