Objective      To measure the plasma level of amino acids in first-episode drug-naive depressive patients and analyze its difference with that in controls and its possible clinical value for assessing the severity of depression.       Methods      Plasma level of amino acids was measured in 26 first-episode drug-naive depressive patients and 25 healthy controls using isotope internal standard in combination with mass spectrometry. Severity of depression was assessed according to the Hamilton Depression Scale-17 (HAMD-17) and analyzed with the SPSS16.0 statistical package.       Results      The plasma levels of tryptophan

phenylalanine

lysine

methionine

γ-aminobutyricacid

α-aminoadipic acid

glutamine

ethanolamine and leucine were significantly higher in depressive patients than in healthy controls (P0.05). The plasma levels of aspartate and β-aminoisobutyric acid were negatively correlated with the scores of HAMD-17 in depressive patients (P=0.045

0.006; r=-0.397

-0.521). Regression analysis showed that the prediction model consisted of tryptophan

cysteine

and glutamine had a rather good prediction efficacy for the plasma amino acid level.       Conclusion      Metabolic disorder of amino acids occurs in early depressive patients. Circulating amino acids can be used in assessing the severity of depression and in its preliminary diagnosis.