What is the target threshold plasma level for olanzapine response?

• A. Higher threshold: 50-100 nmol/L
• B. No monitoring needed
• C. Monitor threshold plasma level for response (9-27 nmol/L)
• D. Plasma levels of other antipsychotics

Answer: (C)

The concept being tested is therapeutic drug monitoring for olanzapine: there is a target plasma concentration range that correlates with a clinical response. For olanzapine, evidence supports a threshold around 9–27 nmol/L, below which response is less likely and above which the chance of improvement increases. This makes it the best choice because it directly links a measurable level to the likelihood of therapeutic effect, guiding dose adjustments, adherence checks, and consideration of pharmacokinetic factors (like metabolism or interactions) that can lower exposure. The other options don’t fit: a higher, undefined threshold isn’t supported as a universal target; claiming no monitoring is needed ignores the utility of TDM in cases of nonresponse or questionable adherence; and focusing on plasma levels of other antipsychotics misses the olanzapine-specific threshold that informs response.