Prediction of response to nortriptyline and phenelzine by platelet MAO activity.

Thirty-seven depressed patients over the age of 55 were treated for 5-7 weeks with either nortriptyline, a tricyclic antidepressant, or phenelzine, a monoamine oxidase (MAO) inhibitor. Patients' platelet MAO activity was measured following a drug washout period before treatment. Patients with higher MAO activity had a better response to treatment, regardless of which drug was used.