When Validation Reveals the Limits　― The Next Step in Sudden Death Prevention for Hypertrophic Cardiomyopathy ―

years (area under the curve 0.75, 95% confidence interval 0.71-0.80)and, crucially, separated Class IIa from Class IIb (5-year cumulative incidence 6.7% vs. 4.9%, P=0.006).On the other hand, when the Class IIa threshold was used to simulate implantable cardioverter defibrillator (ICD) decisions, net reclassification improvement over contemporary American College of Cardiology (ACC)/American Heart Association (AHA) 2024 and European Society of Cardiology (ESC) 2023 frameworks was not significant. Sudden cardiac death (SCD) remains the most consequential event in hypertrophic cardiomyopathy (HCM).The 2018 Japanese Circulation Society/ Japanese Heart Failure Society (JCS/JHFS) guideline sought to organize everyday decision-making into a stepwise pathway tailored to Japanese practice and phenotypes, 1 and in this issue of the Journal, a new multicenter validation by Amano et al. tests how this framework performs in a large, real-world Japanese cohort and clarifies both its strengths and faults. 2 In 3,547 patients followed for a median of 5.8 years, SCD or an equivalent event occurred in 7.0%.The guideline achieved fair discrimination at 5