Wang M, et al. - As indoleamine 2, 3-dioxygenase (IDO) is a key enzyme in the degradation of tryptophan (Trp) to kynurenine (Kyn), researchers examined if IDO activity, measured as the Kyn to Trp ratio, could have utility in assessing prognosis of acquired immune deficiency Sydrome (AIDS) patients with pneumocystis pneumonia (PCP). They measured the Kyn and Trp concentration by ultraperformance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) in plasma samples in a total of 49 AIDS-PCP patients in the analysis. After anti-PCP treatment, they observed significantly lower Kyn/Trp ratio in AIDS patients with PCP. Kyn/Trp ratio was identified as useful for predicting the mortality of AIDS-PCP. A higher mortality rate was noted in the high Kyn/Trp ratio group than the low Kyn/Trp group. This suggests an association of activation of the kynurenine pathway with the severity and fatal outcomes of AIDS patients with pneumocystis pneumonia.