Clozapine levels may be reported by measuring the metabolite norclozapine, which is not correlated with efficacy, or as a total level (combination of clozapine and norclozapine). While norclozapine is not associated with efficacy, the ratio of clozapine to norclozapine may indicate adherence to the medication, or any enzymatic modulation (genetic or drug–drug interaction) that may increase or decrease total exposure. A ratio of 1.5 to 2.0 (clozapine to norclozapine) is optimal; a ratio <0.5 may indicate nonadherence; and a ratio >2.0 may indicate inhibited drug clearance. A 12-hour serum clozapine level of ≥350 ng/mL is more likely to predict treatment response.7
CASE CONTINUED
Mr. Q is carefully tapered from haloperidol while initiating clozapine at 25 mg/d. As he is titrated on clozapine, Mr. Q’s serum levels are periodically checked and compared with expected levels and levels associated with efficacy. Eventually, Mr. Q is titrated to a clozapine dose of 400 mg/d at bedtime.
While receiving clozapine for 4 weeks, Mr. Q’s psychotic symptoms resolve, and he is scheduled for follow-up in the outpatient clozapine clinic.
Related Resources
- De Leon J. A critical commentary on the 2017 AGNP consensus guidelines for therapeutic drug monitoring in neuropsychopharmacology. Pharmacopsychiatry. 2018; 51(1-02):63-68.
- Meyer JM. Is monitoring of plasma antipsychotic levels useful? Current Psychiatry. 2015;14(11):16,19-20.
Drug Brand Names
Aripiprazole • Abilify
Clozapine • Clozaril
Fluphenazine • Prolixin
Haloperidol • Haldol
Lithium • Eskalith, Lithobid
Lurasidone • Latuda
Olanzapine • Zyprexa
Perphenazine • Trilafon
Quetiapine • Seroquel
Risperidone • Risperdal
Valproic acid • Depakene, Depakote
Ziprasidone • Geodon