Defining Reference Values for Skeletal Muscle Metrics on Abdominal CT Using Data From Healthy Young Adult Populations: A Systematic Review and Meta-Analysis

BACKGROUND. CT muscle metrics hold promise for opportunistic sarcopenia screening and individualized clinical risk stratification, but reference values applicable across broad populations are lacking. OBJECTIVE. To estimate reference cutoff values for CT skeletal muscle metrics using data from populations of healthy young adults. EVIDENCE ACQUISITION. The PubMed, Embase, Cochrane Library, Web of Science, and Scopus databases were searched through January 1, 2025 for studies reporting skeletal muscle index (SMI) and/or skeletal muscle density (SMD) on CT at the L3 vertebral level in healthy young adults (age range, 18-45 years). For SMI and SMD in both men and women, a random effects meta-analysis was used to estimate interstudy SD (as a measure of variance among studies) and mean values for a theoretic global population of healthy young adults. Presence of significant heterogeneity among individual study means was assessed using the Q statistic. Cutoff values for the theoretic global population corresponding with a T-score of -2 (i.e., values ≥ 2 SDs below the population's mean value) were calculated, incorporating the meta-analysis results and pooled intrastudy variance. EVIDENCE SYNTHESIS. The meta-analysis included 14 studies (16,958 individuals; 11,819 men, 5139 women) reporting SMI, of which seven studies (11,175 individuals; 8372 men, 2803 women) also reported SMD. The estimated global mean value for SMI was 54.6 in men and 42.4 in women and for SMD was 47.4 HU in men and 43.6 HU in women. The interstudy SD for SMI was 5.4 in men and 4.3 in women and for SMD was 1.9 in men versus 3.2 in women; significant heterogeneity was present among individual study means for both SMI and SMD in both men and women (all p<.001). The cutoff value corresponding with a T-score of -2 for SMI was 36.3 in men and 27.5 in women and for SMD was 36.4 HU in men and 28.1 HU in women. CONCLUSION. This meta-analysis of studies performed in healthy young adults provides reference mean values and standardized cutoffs analogous to a T-score of -2 for SMI and SMD at the L3 level on abdominal CT. CLINICAL IMPACT. These results can aid opportunistic screening for sarcopenia.