To the Editor.—
In a study from our institute of unipolar depressed subjects, improvement in depressive symptoms was significantly correlated to increases in the ratio of the total plasma concentration of tryptophan to the sum of the concentrations of five of the competitors with tryptophan for entry into the CNS: valine, leucine, isoleucine, phenylalanine, and tyrosine.1 In that study, the ratio appeared to be a state indicator of depression, at least in subjects who demonstrated a rapid improvement in their symptoms. The study was carried out in hospitalized subjects over a five-day period, during which no treatment was provided. Following is a brief report of the results of a follow-up study on that ratio in a treated inpatient sample of depressed patients. We were interested in using the ratio to distinguish clinical subtypes of unipolar depressive disorders and as a state indicator of depression over a longer time period.