Background and aim: The accurate non-invasive diagnosis of spontaneous bacterial peritonitis (SBP) in patients with decompensated liver cirrhosis has not been achieved yet. The aim of the study was to obtain an unmistakable diagnosis of SBP using a new simple serum bioscore, made by combined measurement of procalcitonin (PCT), erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), which we called the PEC index. Methods: This Cross sectional analytic study comprised 178 cirrhotic patients with ascites (60 patients with SBP and 118 patients with sterile ascites), after excluding non-SBP infection, during the period from March 2019 until September 2019. In all participants, serum levels of PCT, ESR and CRP were measured, and PEC index was calculated [PEC index= PCT x (ESR+CRP)]. Results: Patients with SBP (n=60) had significantly higher serum PEC index than those with sterile ascites (n=118) (41.0 / 31.2-93.0 vs. 9.9 / 5.9-15.0, P < 0.001). PEC index distinguished culture positive cases significantly (P < 0.001). Using Receiver operating characteristic (ROC) statistics, the sensitivity and specificity of PCT, at a cutoff value of 0.590 ng/mL, for SBP diagnosis, were 81.67% & 93.33%, respectively (area under the curve [AUC]= 0.879; 95% confidence interval [CI]: 0.809-0.948). The sensitivity and specificity of ESR, at a cutoff value of 27.0 mm/hour, were 73.33% & 61.67%, respectively (AUC= 0.679; 95% CI: 0.581-0.776). The sensitivity and specificity of CRP, at a cutoff value of 21.0 mg/L, were 93.33% & 51.67%, respectively (AUC= 0.736; 95% CI: 0.639-0.833). While, the sensitivity and specificity of PEC index, at a cutoff value of 20, were highest (98.33% & 96.67%, respectively, AUC= 0.977; 95% CI: 0.940-0.996). Conclusion: Serum PEC index makes an accurate noninvasive diagnosis of SBP, after excluding other infections.