Treatment of Metabolic Syndrome Components and Prevalence Estimates Under Four Operational Definitions: Ten-Year Trends and Treatment–Residual Phenotypes in Korean Adults (KNHANES 2015–2024)

Background/Objectives: Metabolic syndrome is a cluster of cardiometabolic risk factors rather than a single disease, and several operational definitions accept current drug treatment for an individual component as an alternate indicator of that component. The definitions differ in which components carry this provision, so prevalence estimates may change with treatment uptake, as well as with metabolic abnormality. This study aimed to quantify how the treatment criteria of the component definitions contributed to the level and ten-year trend of metabolic syndrome prevalence among Korean adults and to describe treatment–residual phenotypes. Methods: We analyzed 54,455 adults from the Korea National Health and Nutrition Examination Survey, 2015–2024, under the harmonized definition (A), the Korean Fact Sheet definition (B) and two analytic indicators, a counterfactual indicator with published treatment effects added back (C) and measured values only (D). Trends were estimated as survey-weighted prevalence ratios (PRs) per year, crude and adjusted for age and sex, and the 2015–2024 change was decomposed into population aging, abdominal obesity and other measured abnormality and treatment criteria. Cases under definition A were classified into four phenotypes by treatment status and the number of measured abnormal components. Results: Crude prevalence under definition A rose from 29.3% in 2015 to 35.6% in 2024 (PR per year 1.021, 95% CI 1.015–1.027; age- and sex-adjusted 1.008, 1.003–1.013), whereas the adjusted PR was 0.990 (0.985–0.996) for definition B and 0.983 (0.977–0.989) for definition D. Over 2015–2021, definitions A and B and indicator C all increased (adjusted PR 1.010–1.019). Of the 6.3-percentage-point crude rise under definition A, aging accounted for 3.8 points, abdominal obesity for 2.6 and the treatment criteria for 4.9 (lipid criteria 4.2), offset by a 5.1-point decline in other measured abnormality. The gap between definitions A and B widened in both 2015–2021 and 2022–2024. Treated cases with ≥3 measured abnormal components remained about one-third of cases (3.66 million adults in 2015 and 5.15 million in 2024). Conclusions: In Korean adults, the treatment criteria, chiefly for the lipid components, accounted for a growing part of the harmonized-definition prevalence, together with population aging and abdominal obesity. Reporting treatment-inclusive and measured-only indicators side by side, with the applicable treatment criteria stated would make surveillance figures easier to interpret.