Abstract:Objective To explore the correlation between the systemic immune inflammatory index（SII） and hypersensitive C-reactive protein（hs-CRP）with the severity of acute appendicitis. Methods The clinical data of 335patients who underwent surgery for acute appendicitis at Nanjing Jiangning Hospital from January 1，2023 to December31，2024 were retrospectively analyzed. According to clinical manifestations and pathological findings，the patients were divided into the simple appendicitis group（n=181）and the complicated appendicitis group（n=154）. Additionally，100 healthy individuals who underwent physical examinations during the same period were selected as the control group.Preoperative total white blood cell count（WBC），lymphocyte count（Lym），neutrophil count（Neu），platelet count（PLT），and hs-CRP were collected in each group. The SII was calculated using the formula（PLT × Neu/Lym）.Differences in various indicators among the groups were compared. Spearman analysis was used to assess the correlation between SII and hs-CRP in patients with acute appendicitis，and receiver operating characteristic（ROC）curves were used to evaluate the predictive value of SII and hs-CRP for the severity of acute appendicitis. Results hs-CRP，WBC，and SII showed certain differences among different severity levels of acute appendicitis. Both hs -CRP and WBC were correlated with SII（r=0.524，0.491，P<0.05）. The combined detection of SII and hs-CRP showed higher diagnostic value for complicated appendicitis compared to either complete blood count or hs -CRP alone，with an area under the curve of 0.796（95%CI：0.705-0.835），a sensitivity of 73.55% ，and a specificity of 73.48%（P<0.01）.Conclusion Both SII and hs -CRP have a good predictive value for the severity of acute appendicitis，and combined detection has positive significance for the diagnosis of complicated appendicitis.