The diagnostic performance of the L-tryptophan/competing amino acids ratio in major depression.

L-tryptophan (L-TRP), the competing amino acids (CAA) valine and leucine and the cortisol levels taken at 8 a.m. after administration of dexamethasone on the previous day, were determined in 160 patients suffering from depression. The ratio between the L-TRP values and the sum of the competing amino acids was calculated. The clinical relevance of the L-TRP/CAA ratio in the case of major depression (DSM-III) versus minor depression (dysthymic disorder, atypical depression and adaptation disorder with depressive mood) was studied in comparison with the DST. Patients suffering from major depression showed a significantly decreased (p = 0.0006) L-TRP/CAA ratio. The combination of a decreased L-TRP/CAA ratio (cut off point less than or equal to 0.130) or an abnormal DST (cut off point greater than or equal to 3.5 micrograms/dl) is the best criterion which allows 59.0% of the patients suffering from major depression to be identified correctly with a specificity of 90.9%; the above-mentioned criterion permits 70.0% of the patients to be classified correctly into their actual group.