Objective:To investigate the diagnostic and diagnostic values of plasma microRNA-21-3p (miR-21-3p) and miR-551-5p expression in patients with acute pancreatitis (AP).Methods:A prospective observational study was conducted. AP patients admitted to the Third People's Hospital of Hainan Province from January 1st 2017 to December 31st 2019 were enrolled. The patients were divided into mild acute pancreatitis (MAP) group, moderate severe acute pancreatitis (MSAP) group and severe acute pancreatitis (SAP) group according to their severity. Fasting venous blood was collected from all subjects the day after admission, and real-time quantitative polymerase chain reaction (PCR) as used to detect the expression levels of plasma miR-21-3p and miR-551-5p. Rehabilitation, discharge or death were end points of study. In addition, 50 healthy people in the same period were selected as the control group. The receiver operating characteristic (ROC) curve was used to analyze the value of the expression levels of plasma miR-21-3p and miR-551-5p for the diagnosis and prognosis of SAP. Pearson correlation was used to analyze the relationship between the expressions of miR-21-3p and miR-551-5p in SAP patients.Results:A total of 164 AP patients were enrolled, including 72 MAP patients, 47 MSAP patients and 45 SAP patients. Among the SAP patients, 27 cases survived and 18 cases died. There were no deaths in MAP group and MSAP group. The levels of plasma miR-21-3p and miR-551-5p in AP group were significantly higher than those in control group [miR-21-3p (2 -ΔΔCt): 2.17±0.90 vs. 0.65±0.12, miR-551-5p (2 -ΔΔCt): 1.80±0.73 vs. 0.42±0.08, both P < 0.01]. The expression levels of plasma miR-21-3p and miR-551-5p in AP patients increased gradually with the aggravation of the disease ( F values were 11.635, 10.204 respectively, both P < 0.01), and the expression levels of plasma miR-21-3p and miR-551-5p in SAP group were significantly higher than those in MSAP group and MAP group [miR-21-3p (2 -ΔΔCt): 3.16±1.08 vs. 1.85±0.71, 1.70±0.64; miR-551-5p (2 -ΔΔCt): 2.63±0.95 vs. 1.52±0.46, 1.36±0.40; all P < 0.01]. ROC curve analysis showed that the area under the ROC curve (AUC) and 95% confidence interval (95% CI) of the joint diagnosis of SAP with miR-21-3p and miR-551-5p were significantly higher than that of miR-21-3p or miR-551-5p alone [0.898 (0.841-0.960) vs. 0.820 (0.763-0.882), 0.806 (0.748-0.867), Z1 = 4.480, Z2 = 4.916, both P < 0.05], and the sensitivity was 90.7% and the specificity was 85.0%. The expression levels of plasma miR-21-3p and miR-551-5p in the death group of SAP patients were significantly higher than those in the survival group [miR-21-3p (2 -ΔΔCt): 3.75±1.17 vs. 2.66±0.87, miR-551-5p (2 -ΔΔCt): 3.17±1.04 vs. 2.24±0.83, both P < 0.01]. ROC curve analysis showed that the AUC and 95% CI of the combined prediction of death in SAP patients with miR-21-3p and miR-551-5p were significantly higher than that of miR-21-3p or miR-551-5p alone [0.933 (0.875-0.996) vs. 0.856 (0.794-0.917), 0.816 (0.759-0.874), Z1 = 4.395, Z2 = 5.520, both P < 0.05], and the sensitivity was 95.2% and the specificity was 87.5%. Correlation analysis showed a positive correlation between the expression level of plasma miR-21-3p and miR-551-5p in SAP patients ( r = 0.827, P < 0.001). Conclusions:The increased expression of plasma miR-21-3p and miR-551-5p are positively correlated with the severity of AP patients. The combination of the two items has a better value in the diagnosis and prognosis evaluation of SAP.