Abstract: OBJECTIVE：To investigate diagnostic values of human serum cysteine protease inhibitor S (CST4)，squamous cell carcinoma antigen (SCC)，carbohydrate antigen 199 (CA19-9) and soluble cytokeratin component (CYFRA21-1) in oral and maxillofacial malignant tumors. METHODS：A total of 232 patients with oral and maxillofacial malignancies tumors who were admitted to our hospital from January 2022 to November 2022，and 40 healthy physical examination subjects were selected during the same period as the research objects. Serum levels of CST4，SCC，CA19-9 and CYFRA21-1 from them were analyzed. ROC curve was developed to evaluate diagnostic efficacy of the different indexes. RESULTS：The serum median levels of CST4，SCC and CYFRA21-1 in the malignant tumor group were significantly higher than those in the benign tumor group and the healthy control group (P<0.05). There was no difference in expression levels of CST4 in patients with malignant tumors at different sites and pathological types (P>0.05). ROC curve showed that serum CST4 detection had a high diagnostic value for the tumors (AUC=0.954，sensitivity 97.5%，specificity 78.45%，cut off value 74.68 ng/mL). However，the combined detections of CST4，SCC and CYFRA21-1 were more effective and specific in the clinical diagnosis of the malignant tumors. CONCLUSION：Serum CST4，SCC and CYFRA21-1 were highly expressed in oral and maxillofacial malignant tumors. Compared with the single detection of serum tumor markers，the combined detection of the three had higher reference value in the diagnosis of malignant tumors，and can be recommended as useful tumor markers.

Key words: oral and maxillofacial malignancies, cysteine protease inhibitor S, tumor marker, combined detection