A 55-year-old female with bipolar II disorder has been prescribed lamotrigine 200 mg daily and attending monthly supportive psychotherapy sessions. Over the past three months, her PHQ-9 scores have fluctuated between 12 and 16 after previously being stable at 6 to 8 for over a year. She reports feeling more depressed and hopeless. When the PMHNP reviews the pharmacy refill records, they reveal that the patient has only filled her lamotrigine prescription twice in the past four months, with a 45-day gap between fills. When asked about her medication, the patient states she has been taking it every day as prescribed. Her lamotrigine level is subtherapeutic at 1.8 mcg/mL. The PMHNP is evaluating the discrepancy between the patient's self-report and objective data. Which approach best reflects an appropriate evaluation of treatment adherence?