Population-Specific CT-Derived Sarcopenia Cutoff Values and Inter-Vertebral Level Correlations in Healthy Thai Adults Using Automated Muscle Segmentation

OBJECTIVES: Establishing optimal sarcopenia cutoffs is essential, as L3 is not always included in CT imaging, and reference levels vary across populations. The absence of Southeast Asia-specific thresholds may lead to misclassification, highlighting the need for regionally appropriate diagnostic criteria. This study establishes population-specific sarcopenia cutoffs (T10-S1) in the Thai population and defines BMI-adjusted thresholds. It also assesses skeletal muscle index correlations across vertebral levels to refine L3-based sarcopenia evaluation. METHODS: This retrospective study analyzed 475 healthy Thai adults (aged 18-40 years) who underwent emergency department CT between January 2010 and June 2023. Skeletal muscle parameters were measured from T10 to S1 using custom software. Measurements included skeletal muscle area (SMA), skeletal muscle radiation attenuation (SMRA), skeletal muscle index (SMI), and height-adjusted skeletal muscle index (SMIH1). Sex-stratified analyses and correlations between vertebral levels were performed. RESULTS: At L3, sarcopenia cutoffs using SMI and SMIH1 were 23.3 cm²/m² and 37.1 cm²/m for females, and 31.3 cm²/m² and 53.9 cm²/m for males, respectively. Strong correlations were observed between L3 and adjacent vertebral levels (r > 0.8), with correlation strength decreasing with anatomical distance. Application of international cutoffs to the Thai population resulted in overestimation of sarcopenia cases. BMI-adjusted variable thresholds demonstrated superior diagnostic accuracy compared to fixed cutoffs. CONCLUSION: This study establishes the first population-specific sarcopenia reference values for Thai adults and validates the use of alternative vertebral levels when L3 imaging is unavailable. Our findings emphasize the importance of population-specific and BMI-adjusted cutoff values for accurate sarcopenia diagnosis in Thai adults. ADVANCES IN KNOWLEDGE: This study establishes the first population-specific sarcopenia cutoff values for healthy Thai adults, providing the first evidence-based reference standards for the Thai populations. At L3, the recommended cutoffs are SMI 23.3 cm²/m² and SMIH1 37.1 cm²/m for females, and SMI 31.3 cm²/m² and SMIH1 53.9 cm²/m for males. Strong correlations (r > 0.8) between L3 and adjacent vertebral levels validate the use of alternative assessment sites when L3 imaging is unavailable: T12-L1 for chest CT protocols, L3 for abdominal CT protocols, and L4-L5 for pelvic CT protocols. Application of Western cutoffs to the Thai populations results in significant overestimation of sarcopenia prevalence, emphasizing the critical importance of population-specific diagnostic criteria.