Determination of Ultrasound Reference Values for Diagnosing Low Muscle Mass in Older Chinese Adults

Ultrasound is a promising tool for diagnosing sarcopenia, yet standardized cut-off points and criteria are currently lacking. This study aimed to identify the optimal muscle sites and reference values for diagnosing low muscle mass in older Chinese adults using ultrasound, with dual-energy X-ray absorptiometry (DXA) as the reference standard, and to compare its diagnostic performance with bioelectrical impedance analysis (BIA).

We included 1011 participants aged over 60 years. Fat thickness (FT), muscle thickness (MT) and muscle cross-section area (CSA) at various sites, including biceps brachii, rectus abdominis, rectus femoris, vastus intermedius, vastus lateralis, vastus medialis and tibialis anterior, were assessed for participants. DXA measurements were used as the standard for defining low muscle mass. Receiver operating characteristic (ROC) curve analysis and 10-fold cross-validation were conducted to evaluate the prediction performance of ultrasound parameters for low muscle mass. The Youden index was employed to determine optimal cut-offs, while sensitivity, specificity and accuracy were calculated to assess diagnostic performance. Intra-class correlation coefficients (ICC) were used to evaluate inter-rater reliability between two examiners.

In males, the biceps brachii CSA (AUC = 0.832, 95% CI: 0.793-0.870) and in females, the tibialis anterior MT (0.833, 95% CI: 0.789-0.877) demonstrated superior predictive power for low muscle mass compared with other ultrasound parameters. A biceps brachii CSA < 7.1 cm² in males and tibialis anterior MT < 2.3 cm in females were identified as effective indicators for diagnosing low muscle mass, with good sensitivities (73.9% in males, 73.6% in females), specificities (78.6% in males, 76.8% in females) and accuracies (75.7% in males, 75.8% in females). These values were comparable with those obtained using BIA (sensitivity: 62.6%, specificity: 80.2% and accuracy: 71.5%). Low muscle mass as defined by ultrasound was significantly associated with poor performance on the Short Physical Performance Battery, Activities of Daily Living and frailty indices, with effect sizes higher than DXA or BIA defined low muscle mass. The inter-rater reliability was excellent for biceps brachii CSA (ICC = 0.869, 95% CI: 0.650-0.937) and good for tibialis anterior MT (ICC = 0.730, 95% CI: 0.622-0.810).

Muscle ultrasound demonstrated excellent inter-rater reliability and stronger associations with adverse health outcomes compared with DXA and BIA, making ultrasound a preferable tool for assessing low muscle mass. This study provides valuable reference values for ultrasound-based diagnosis of sarcopenia in older Chinese adults.