Sex-Specific Differences in Metabolic Syndrome Classification Using IDF Versus NCEP-ATP III Criteria: A Systematic Review and Head-to-Head Meta-Analysis

Background/Objectives: Metabolic syndrome (MetS) classification varies between the International Diabetes Federation (IDF) and National Cholesterol Education Program Adult Treatment Panel III (NCEP-ATP III) definitions. We compared their same-sample classification yield by sex. Methods: PubMed/MEDLINE and Scopus were searched from database inception through 30 August 2026, with reference-list screening and citation tracking. Eligible observational studies assessed both definitions in the same adult sample; intervention-effect reports without an eligible observational comparison were outside the scope. One reviewer performed selection, extraction, and JBI-based appraisal. Random-effects models pooled sex-specific prevalence and prevalence ratios; revised ATP III was examined in sensitivity analysis. The review was not registered. Results: Seven studies (95,887 participants) were included; five (74,399 participants; 36,033 men and 38,366 women) contributed to the primary meta-analysis. The pooled IDF/NCEP-ATP III prevalence ratio was 0.95 (95% CI 0.65-1.37) in men and 1.15 (1.01-1.31) in women. Sensitivity estimates were 0.91 (0.70-1.19) and 1.08 (0.92-1.27), respectively. Heterogeneity was extreme (I² 92.3-99.9%). Three primary studies were conducted in Asian countries and represented 89.1% of primary-analysis participants. Conclusions: No clear pooled difference was observed in men; the modest female excess under IDF attenuated in sensitivity analysis. Five independent studies, extreme heterogeneity, and geographically concentrated samples limit robustness and generalizability. The findings provide a hypothesis-generating evidence summary, not proof of diagnostic equivalence, biological mechanisms, prognostic superiority, or treatment benefit. Definition-specific and sex-stratified reporting is warranted. No external funding was received.