Circulating Methylated <i>SEPT9</i> for Detection of Hepatocellular Carcinoma in Cirrhosis

Importance: Hepatocellular carcinoma (HCC) surveillance in patients with cirrhosis remains suboptimal, with inadequate early-stage detection. α-Fetoprotein (AFP) demonstrates insufficient sensitivity. Circulating methylated septin 9 (SEPT9) has shown diagnostic promise. Objective: To determine whether methylated SEPT9 improves detection of HCC when combined with AFP in patients with cirrhosis undergoing surveillance. Design, Setting, and Participants: In this prospective, cross-sectional, diagnostic accuracy study, patients with cirrhosis undergoing routine HCC surveillance with ultrasonography and AFP were enrolled at 2 French academic centers from February 2018 through October 2024. HCC was diagnosed per international guidelines with centralized radiologic review, blinded to methylated SEPT9 results. Data were analyzed from October 2025 to January 2026. Exposures: Plasma methylated SEPT9 was analyzed from 3 independent plasma aliquots and classified by number of positive replicates (single-positive, double-positive, or triple-positive). Serum AFP was evaluated at a threshold of 20 ng/mL. Biomarkers were evaluated individually and in combination using disjunction logic (tier 1; maximizing sensitivity) or conjunction logic (tier 2; maximizing specificity). Main Outcomes and Measures: The primary outcome was the presence of HCC at enrollment. The primary end point was comparison of the area under the receiver operating characteristic curve (AUROC) between methylated SEPT9 and AFP. Secondary end points included diagnostic performance stratified by Barcelona Clinic Liver Cancer (BCLC) stage. Results: Among 574 participants, 414 (72.1%) were male, and the median (IQR) age was 63 (57-70) years. A total of 118 had HCC, including 51 (43.2%) with BCLC stage 0-A. Methylated SEPT9 outperformed AFP (AUROC: 0.79 [95% CI, 0.74-0.84] vs 0.71 [95% CI, 0.66-0.76], respectively; P = .002; posterior probability of superiority >99.8%). Tier 1a (at least single-positive methylated SEPT9 or AFP >20 ng/mL) achieved 87.8% (95% CI, 81.6-93.5) sensitivity and a negative likelihood ratio of 0.2 (95% CI, 0.1-0.3). Among 64 HCC cases missed by AFP, tier 1a recovered 50 (78%). For BCLC 0-A disease, tier 1a sensitivity was 74.5% (95% CI, 62.2-86.5) vs 23.5% (95% CI, 12.5-35.8) for AFP, 3.2-fold increase. Tier 2 (triple-positive methylated SEPT9 and AFP >20 ng/mL) achieved 99.6% (95% CI, 98.9-100) specificity, a positive likelihood ratio of 76.0 (95% CI, 26.9-181.0), and a diagnostic odds ratio of 112.1 (95% CI, 37.8-294.7). Conclusions and Relevance: In this diagnostic study, combining methylated SEPT9 with AFP substantially improved HCC detection in patients with cirrhosis, particularly for early-stage disease amenable to curative treatment. Prospective studies are needed to determine whether improved detection translates into survival benefit.