BackgroundSeasonal influenza is an important cause of morbidity and mortality worldwide. Immune activation after stimulation with interferon‐gamma leads to increased production of neopterin but also results in increased tryptophan catabolism through indoleamine 2,3‐dioxygenase (IDO). Our pilot study determined neopterin serum levels and IDO activity in patients with influenza infection and investigated whether neopterin is linked to clinical outcome parameters (mortality ≤30 days, acute cardiac events (ACE) length of hospitalization, ICU admission).MethodsNeopterin concentrations were analyzed in serum samples of 40 patients with a confirmed diagnosis of influenza infection and in‐hospital treatment for >24 hours. Data were compared to values of 100 healthy blood donors and 48 age‐matched pneumonia patients. In a subgroup of 14 patients, tryptophan and kynurenine concentrations, as well as kynurenine‐to‐tryptophan ratio, were analyzed.ResultsIn all influenza patients, neopterin concentrations were increased and significantly higher compared to those determined in patients with pneumonia and healthy controls. Positive correlations between the duration of hospitalization and neopterin were found. Significantly higher levels of kynurenine, kynurenine‐to‐tryptophan ratio, and lower levels of tryptophan were seen in influenza patients compared to healthy controls.ConclusionsNeopterin seems to be related to the course of the disease and could be a valuable biomarker to identify patients at an elevated risk of a worsened outcome; however, further prospective validation studies are needed to support the here presented preliminary results.