Interestingly, IDO activity (i.e., KYN/TRP ratio) positively correlated with anhedonia severity in medication-free adolescents with depression alone (r = 0.42, p = 0.05) and in combination with controls (r = 0.44, p = 0.004), but this correlation was reduced when the combined sample included medicated depressed subjects (r = 0.30, p = 0.02) and did not reach significance in the depressed cohort alone when medicated subjects were included (r = 0.31, p = 0.06). Here, IDO1 is linked to depressive symptom measurement.