Objective:To explore the value of serum activin A (ACT-A) level in early identification of moderate and severe acute pancreatitis (AP).Methods:A prospective case control study was conducted. A total of 120 patients with AP admitted to department of hepatobiliary surgery of Affiliated Nanhua Hospital of Hengyang Medical College of University of South China between October 2020 and April 2022 were recruited. According to the revised Atlanta classification, all patients were classified into mild AP group and moderate-to-severe AP group. The blood samples within 24 hours of onset were drawn, and the serum ACT-A and C-reactive protein (CRP) levels were detected by enzyme-linked immunosorbent assay (ELISA). The Ranson score and the modified CT severity index (MCTSI) were performed. Pearson correlation method was used to analyze the correlation of various parameters. The receiver operator characteristic curve (ROC curve) was plotted to analyze the predictive value of ACT-A and CRP for moderate-to-severe AP.Results:A total of 120 patients with AP were enrolled, including 83 patients with mild AP and 37 patients with moderate-to-severe AP. Serum ACT-A and CRP levels within 24 hours of onset in the moderate-to-severe AP group were significantly higher than those in the mild AP group [ACT-A (ng/L): 140.4±37.7 vs. 53.9±30.5, lg CRP: 1.42±0.91 vs. 0.77±0.70, both P < 0.01], and the Ranson score and MCTSI score were also significantly higher than those in the mild AP group (Ranson score: 5.3±1.3 vs. 1.8±1.6, MCTSI score: 5.5±1.0 vs. 2.7±1.2, both P < 0.01). Correlation analysis showed that the serum ACT-A level was positively correlated with serum CRP level, Ranson score and MCTSI score ( R2 value was 0.272, 0.841, 0.616, respectively, all P < 0.05). ROC curve analysis showed that the serum ACT-A, CRP and Ranson score had predictive value for moderate-to-severe AP. The area under the ROC curve (AUC) was 0.948 [95% confidence interval (95% CI) was 0.909-0.986], 0.711 (95% CI was 0.606-0.815), 0.946 (95% CI was 0.910-0.982), respectively. When serum ACT-A > 112.6 ng/L, the sensitivity and specificity of predicting moderate-to-severe AP were 78.38% and 96.39%, respectively, which was better than serum CRP with sensitivity and specificity of 72.92% and 66.27%, respectively, and the specificity was better than Ranson score (71.08%). Conclusion:ACT-A can be detected in the early stage of AP, and it is positively correlated with the disease severity, which can early identify moderate-to-severe AP.