Tyramine sulfate excretion may be a better predictor of antidepressant response than monoamine oxidase activity.

The tyramine sulfate excretion test was performed on 62 nonmelancholic depressed outpatients who then took part in a 6-week double-blind trial comparing imipramine, phenelzine, and placebo. In a double-blind design, nonresponders were switched to one of the active medications. Tyramine sulfate excretion failed to differentiate response from nonresponse to placebo. By contrast, phenelzine responders excreted significantly less tyramine sulfate than did phenelzine nonresponders, while there was a trend in the same direction for imipramine-treated patients. The presence of only eight phenelzine nonresponders dictates caution in interpreting these results. Baseline monoamine oxidase (MAO) activity did not distinguish responders from nonresponders or correlate with tyramine sulfate excretion. Although males had significantly lower MAO activity than females, controlling for sex did not alter these negative findings. These results fail to confirm a previous report of a significant correlation between MAO activity and treatment response in older, mainly melancholic patients.