A 34-year-old female with major depressive disorder was started on sertraline 50 mg daily six weeks ago. At follow-up, she reports her PHQ-9 score has decreased from 22 to 15, she is sleeping somewhat better, and her appetite has improved. However, she continues to endorse persistent low mood, anhedonia, and difficulty concentrating at work. She denies side effects and reports taking the medication consistently. The PMHNP is evaluating treatment response at this visit. Which of the following best represents the appropriate clinical evaluation and next-step rationale for this patient?