Objective To assess the value of procalcitonin (PCT) for the differential diagnosis between infectious and non-infectious systemic inflammatory response syndrome (SIRS) after cardiac operation.Methods Patients diagnosed with SIRS after cardiac surgery and admitted to Department of Cardiovascular Surgery of Tokyo Medical and Dental University Graduate School between April 1st,2011 and March 31st,2013 were retrospectively studied.A total of 142 patients with SIRS were included,and they were divided into infectious group (n =47) or non-infectious group (n =95) according to the diagnostic criteria of the Surviving Sepsis Campaign Guidelines for Management of Severe Sepsis and Septic Shock:2012 (SSCG2012).The patients with infectious SIRS were included,and there were 11 with sepsis,12 with severe sepsis without shock,and 24 with septic shock respectively.The clinical data of patients were compared,and the receiver operating characteristic curve (ROC curve) was plotted to assess the diagnostic value of infection and non-infectious diseases for PCT,C-reactive protein (CRP) and white blood cell count (WBC),as well as the diagnosis of the severity of sepsis.Results PCT,CRP,and WBC were significantly higher in the infectious SIRS group than those in the non-infectious SIRS group [PCT (μg/L):2.80 (1.24,10.20) vs.0.10 (0.06,0.21),Z=-9.020,P=0.001; CRP (mg/L):158.0 (120.0,199.0) vs.58.0 (25.0,89.0),Z=-7.264,P=0.001; WBC (× 109/L):15.5 (11.0,22.6) vs.9.3 (7.2,12.6),Z=-5.792,P=0.001].PCT had the highest sensitivity (91.5％) and specificity (93.7％) for differential diagnosis,with a cut-off value for infectious SIRS of 0.47 μg/L,and the cut-offvalue of CRP and WBC were 119.5 mg/L and 10.85 × 109/L,respectively.There was no significant difference in WBC among sepsis group,severe sepsis group,and septic shock group [× 109/L:12.40 (9.10,24.20),13.30 (9.93,16.93),20.40 (13.45,28.6),x2=5.638,P=0.060],while PCT,CRP had significant difference [PCT(μg/L):1.37 (0.72,1.85),3.16 (0.48,13.24),3.68 (1.67,20.96),x2=7.422,P=0.024; CRP (mg/L):120.0(74.0,180.0),135.7 (81.7,181.3),171.1 (151.5,306.0),x2=9.524,P=0.009].PCT was more reliable than CRP in diagnosing severe sepsis without shock,but it was ineffective for diagnosing septic shock.The cut-off value of PCT for diagnosing severe sepsis without shock was 2.28 μg/L,and the sensitivity was 66.7％,specificity was 90.9％.Cut-off value of CRP for the diagnosis of septic shock was 149.5 mg/L,with the sensitivity of 83.3％,and the specificity of 66.7％.Conclusions PCT was a useful marker for the diagnosis of infectious SIRS after cardiac operation as compared with WBC and CRP.The optimal PCT cut-off value for diagnosing infectious SIRS was 0.47 μg/L.