Abstract:Abstract: Objective To detect the methylation level of plasma cell-free DNA ( cfDNA) in patients with colorectal cancer ( CRC),and analyze its clinical significance. Methods The blood samples from 38 CRC patients ( Stage I -I ) and 38 healthy controls were collected,and serum carcinoembry onic antigen( CEA) levels were detected by a chemiluminescence assay. Plasma cfDNA was extracted from blood samples, and detected by high-throughput targeted methylation sequencing. Then, the methylation score of cfDNA was calculated and its relationship with the clinicopathological parameters of CRC patients was analyzed. The clinical values of cfDNA alone and combined with CEA in the screening of CRC were evaluated by the ROC curve. Results There was no significant difference in the concentration of cfDNA between CRC patients and healthy controls (34.41+5.00 vs 36.59+4.46, t=0.326, P=0.745), but the methy lation score of cfDNA in CRC patients was significantly higher than that of healthy controls (0.53+0.27 vs 0.26+0.18, t=6.134, P<0.001). The positive rate of cfDNA methylation in CRC patients with lymph node metastasis was significantly higher than that in patients without lymph node metastasis ( 88.9% vs 60%,2 =4.323, P= 0.038), but there was no significant diference between different gender, age and tumor location groups ( P>0.05). The ROC curve analysis showed that the areas under the ROC curve ( AUCR0C) of cfDNA methylation, serum CEA and the combination of them in the screening of CRC were 0.892 ( 95%CI: 0.814-0.970), 0.810 ( 95%CI: 0.707-0.909) and 0.930 (95%CI: 0.867 4-0.996 7), respectively. The sensitivity, speeificity and cut-off value of efDNA methylation in the screening of CRC were 84.21%, 86.84% and 0.280 5, respectively, and those of serum CEA alone were 42. 1 %,97.3% and 4.0, respectively. The sensitivity, specifity and accuracy of cfDNA methylation combined with serum CEA in the screening of CRC were 89.47%, 91.89% and 90.67%, respectively. Conclusion The detection of plasma cfDNA methylation has high sensitivity and specificity in the screening of CRC, and is related to lymph node metastasis, which has a good clinical application value.