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Norclozapine Ratio Interpretation — MRCPsych Paper B MCQ

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HardSchizophrenia & PsychosisNorclozapine Ratio InterpretationMRCPsych Paper B

A 30-year-old man with schizophrenia has been stable on an unchanged dose of clozapine. Previous correctly timed 12-hour trough concentrations were clozapine 420 micrograms/L and norclozapine 230 micrograms/L. A repeat trough sample shows clozapine 110 micrograms/L and norclozapine 145 micrograms/L, giving a norclozapine-to-clozapine ratio of 1.32. There has been no change in smoking, physical health or interacting medication. What is the most likely explanation for this pattern?

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Correct answer: E — Recent partial non-concordance with clozapine

The answer is E. The ratio is inverted: norclozapine exceeds clozapine, while both concentrations—particularly clozapine—have fallen substantially from the patient’s previous results on the same dose. This pattern supports recent missed doses because the parent drug concentration falls before its metabolite becomes undetectable. The unchanged sampling time, smoking status, health and medication reduce important alternative explanations. A ratio alone is not a validated direct measure of CYP1A2 activity, so B cannot be inferred and dose escalation would be unsafe without investigating adherence. Reduced metabolism or toxicity would generally increase clozapine exposure, making A and C inconsistent. Clozapine is predominantly hepatically metabolised, so renal impairment does not adequately explain this abrupt pattern.

Reference: Ruan C-J et al. Exploring low clozapine C/D ratios, inverted clozapine-norclozapine ratios and undetectable concentrations as measures of non-adherence in clozapine patients. Schizophrenia Research. 2024;268:293-301. https://pubmed.ncbi.nlm.nih.gov/37487869/