IDO1 and chronic obstructive pulmonary disease: In the clinics, increased IDO and TPH activities were strongly associated to 30-day death and or intensive care unit admission and/or 18 month mortality in patients with COPD exacerbations [61]; an increase in serum kynurenine and a decrease in tryptophan was observed in patients with pneumonia with correlations between IDO activity / kynurenine levels and severity or mortality [62], while kynurenine levels were associated with 28-day mortality in critically ill adult patients [63].