Background and Aims: Severe acute pancreatitis (SAP) is a dangerous acute abdominal disorder, characterized by rapid progression, numerous complications and high mortality. Early detection of SAP may helpful for guiding the clinical treatment and improving the prognosis of the patients. Immature granulocyte percentage (IG%) is an emerging inflammatory indicator, which may display an obvious change in the early stage of diseases. Considering the inflammatory indicators play crucial roles in the development of SAP, this study was to conducted to evaluate the clinical value of IG% in the early detection of SAP.  
Methods: The clinical data of 521 patients diagnosed as acute pancreatitis (AP) according to the guidelines from January 2010 to October 2019 were retrospectively analyzed. Of the patients, 63 cases were SAP (SAP group) and 458 cases were mild or moderately severe AP (non-SAP group). The clinical features and the variables that included IG%, white blood cell (WBC) count, neutrophil to lymphocyte ratio (NLR), C-reactive protein (CRP) and amylase level were compared between the two groups of patients. The diagnostic efficiency of each inflammatory indicator for SAP was determined by receiver operating characteristic curve (ROC) analysis. 
Results: The mortality rate in SAP group was 2.2% (18/63) and in non-SAP group was 28.6% (10/458); the patients was older, and the proportions of cases requiring ICU admission and surgical intervention were higher and the length of hospital stay was longer in SAP group than those in non-SAP group (all P<0.05). The levels of IG%, WBC, NLR and CRP in SAP group were significantly higher than those in non-SAP group (all P<0.005), but there was no significant difference in serum the amylase levels between the two groups (P=0.163). The results of ROC analysis showed that the diagnostic powers of IG%, WBC, NLR and CRP for SAP were all had significance (all P<0.005); the area under the curve (AUC) of IG% in predicting SAP was 0.973, with a sensitivity of 100% and specificity of 93.8% at the optimal cut-off value of 0.9, which was superior to WBC (AUC: 0.665, sensitivity: 66.6%, specificity: 87.6%), NLR (AUC: 0.752, sensitivity: 73.3%, specificity: 76.5%) and CRP (AUC: 0.802, sensitivity: 100%, specificity: 54.9%). 
Conclusion: The IG% value in the blood routine greater than 0.9 may be an early indicator for pancreatic necrosis. Compare to traditional inflammatory indicators, IG% is more effective and reliable predictor of SAP.