Nephropathia epidemica (NE) is a hemorrhagic fever with renal syndrome caused by Puumala hantavirus. The severity of NE varies greatly. The aim of the present study was to evaluate whether serum indoleamine 2,3-dioxygenase (IDO) activity is associated with the severity of NE. A prospectively collected cohort of 102 consecutive patients with acute serologically confirmed NE was examined. Serum kynurenine, tryptophan, creatinine, CRP, and blood cell count were measured for up to 5 consecutive days after admission. The kynurenine to tryptophan (kyn/trp) ratio reflecting IDO activity was calculated. A maximum kyn/trp ratio >202 µmol/mmol had a sensitivity of 85% and a specificity of 75% for detecting maximum serum creatinine values >250 µmol/L by receiver operating characteristic (ROC) analysis. A maximum kyn/trp ratio >202 µmol/mmol (high IDO level) was also associated with other parameters reflecting the severity of the disease and renal impairment. Patients with high IDO levels had higher maximum serum creatinine (379 vs. 102 µmol/L, P<0.001), plasma C-reactive protein (104.1 vs. 72.1 mg/L, P=0.029), and blood leukocyte values (11.9 vs. 9.0 × 10(9) /L, P<0.001) compared to patients with kyn/trp ratio ≤ 202 µmol/mmol. They also had lower minimum urinary output (1,100 vs. 1,900 ml/day, P<0.001) and longer hospital stays (8 vs. 5 days, P<0.001). In conclusion, high serum IDO activity was associated with increased disease severity and renal impairment in NE.
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