Reference values and cut-points for trunk myopenia and myosteatosis in Chinese adults: a secondary analysis of the China Action on Spine and Hip study.

Background: Sarcopenia is a prevalent condition associated with adverse health outcomes, necessitating accurate assessment of skeletal muscle quantity and quality. Computed tomography (CT) is the gold-standard imaging modality for evaluating trunk skeletal muscle area (TrSMA) and density (TrSMD), with opportunistic CT offering a means to assess these metrics without additional radiation exposure. However, the clinical application of CT-based sarcopenia assessment in Chinese populations is hindered by the absence of large-scale, population-specific reference values and cut-off points, particularly for both myopenia (low muscle mass) and myosteatosis (fatty infiltration). This study aims to establish sex- and age-specific percentile reference values and diagnostic cut-off points for CT-derived TrSMA and TrSMD at the L1 and L3 vertebral levels with a large cohort of community-dwelling Chinese adults. Methods: We retrospectively analyzed abdominal CT scans from 4,016 community‑dwelling Chinese adults (21-80 years) across 12 regions. TrSMA and TrSMD at L1 and L3 were segmented using OsiriX. Sex- and age-specific percentile curves were modeled using the lambda-mu-sigma (LMS) method within the generalized additive models for location, scale, and shape (GAMLSS) framework. Estimated age-related differences in TrSMA and TrSMD from ages 25 to 75 years were calculated using LMS-derived medians. Sex‑specific cut‑off points for myopenia and myosteatosis were defined in the 21- to 40-year subgroup using the mean minus two standard deviations (M‑2SD) approach. Results: From ages 25 to 75 years, LMS-derived L1/L3-TrSMA values were lower by 0.45%/0.55% per year in men and 0.33%/0.41% per year in women; corresponding differences in L1/L3-TrSMD were 0.38%/0.35% and 0.55%/0.52% per year. More pronounced age-related decreases were observed in peri- and postmenopausal women and older men. M‑2SD cut‑offs at L1 were 96.0 cm Conclusions: We provide the first CT‑based, sex‑ and age‑specific reference values and cut‑points for trunk myopenia and myosteatosis in Chinese adults, facilitating standardized sarcopenia diagnosis and research.