Platelet monoamine oxidase response to lithium treatment in psychiatric patients.

Platelet monoamine oxidase (MAO) activity was studied in bipolar and schizophrenic patients treated with lithium and was found to be increased as a nonspecific drug effect. Greater MAO increase in manic patients was correlated with lesser clinical improvement. There was no correlation of MAO activity with short-term outcome in schizophrenic patients. Change in MAO was not correlated with lithium dosage or plasma levels. Patients with baseline MAO values below the median had the largest activity increases. Platelet MAO might thus be characterized as a state variable, increased by lithium as a nonspecific pharmacologic effect, with the increase associated with poor clinical outcome in manic patients.