e15120 Background: Assays detecting methylated circulating tumor DNA (ctDNA) hold promise for colorectal cancer (CRC) screening, but there is a need to develop more accurate assays for early stages. In this study, we examined modifications to COLVERA (an IKZF1 and BCAT1 methylation marker qPCR assay) that added a third biomarker (IRF4) as well as introducing double stranded detection. Methods: Bisulfite converted DNA extracted from plasma collected from subjects undergoing diagnosis for CRC was assayed using either a double-stranded (ds) specific multiplexed qPCR for BCAT1 and IKZF1 detection (Assay 1) and/or a single-stranded (ss) specific BCAT1/IRF4, ds-specific IKZF1 multiplexed qPCR (Assay 2). The unmodified COLVERA test (Clinical Genomics) was used for comparison. Detection of any marker was considered positive. Results: Assay 1 performance evaluated against COLVERA on bisulfite converted DNA from 1453 samples showed an improved sensitivity for CRC detection (93/134 (69.4%) vs 79/134 (59.0%) COLVERA, p = 0.02), but specificity was suboptimal (no neoplasia: 861/1023 (84.2%) vs 946/1023 (92.5%) COLVERA, p < 0.0001). The decrease in specificity resulted primarily from modifications in the BCAT1 assay component (1023 no neoplasia: 60 positive with ss- BCAT1 vs 146 positive with ds- BCAT1, p < 0.0001). Thus, a novel configuration, Assay 2 (ss- BCAT1, ss- IRF4 and ds- IKZF1) was compared to Assay 1 using 189 previously untested specimens. Assay 2 showed a notable improvement in specificity (99/104 (95.2%) vs 93/104 (89.4%) Assay 1; p = 0.0351). Both assays exhibited comparable sensitivities for CRC (41/49 (83.7%) vs 39/49 (79.6%), Assay 1, p = 0.3633). Assay 2 was positive in: adenomas, 9/36 (25.0%); Stage I, 3/7 (42.9%); Stage II, 17/19 (89.5%); Stage III, 6/6 (100%); Stage IV, 7/7 (100%) and 8/10 (80%) unstaged CRCs. Prior COLVERA sensitivity estimates were 9% adenoma, 41% Stage I and 76% Stage II. Conclusions: Targeting both of the non-complementary bisulfite converted strands of selective regions of interest markedly improve ctDNA assay sensitivity for cancer including early lesions while achieving good specificity. Prospective evaluation in a true CRC screening population is now underway. Clinical trial information: 12611000318987.