Age- and sex-specific reference values for CT-based low skeletal muscle quantity and quality in healthy living kidney...

Computed tomography (CT) imaging is a useful tool for assessing skeletal muscle mass and quality. The present study aimed to determine age- and sex-specific reference values for CT-based skeletal muscle markers in a healthy population, and to correlate them with serum creatinine and 24-h urinary creatinine excretion (24h-UCE).

Skeletal muscle index (SMI) - a marker of muscle mass/quantity - and skeletal muscle radiation attenuation (SMRA) and intermuscular adipose tissue index (IMATI) - markers of muscle quality/myosteatosis - were determined using a deep-learning-based method from axial CT images at the level of the 3^rd lumbar vertebra in living kidney donors assessed between 01/2005 and 05/2023. Age- and sex-specific reference values were determined by the 5^th percentile, and correlation was tested with the Pearson correlation coefficient.

CT scans of 394 healthy individuals were included. The mean age was 53 years (SD 12), mean BMI was 25.2 kg/m² (SD 3.9), and 130 patients (33%) were male. The reference values for low skeletal muscle mass (SMI) in males were 43.7 cm²/m² (20-39 years), 44.9 cm²/m² (40-59 years), and 39.7 cm²/m² (≥60 years). In females, the corresponding values were 33.8 cm²/m² (20-39 years), 34.8 cm²/m² (40-59 years), and 31.2 cm²/m² (≥60 years). SMI showed a moderate correlation with serum creatinine (r = 0.452, p < 0.001) but a weak correlation with 24h-UCE (r = 0.188, p = 0.003). Correlations were all weak for SMRA (creatinine: r = 0.220, p < 0.001; 24h-UCE: r = 0.177, p = 0.006) and IMATI (creatinine: r = -0.101, p = 0.054; 24h-UCE, r = -0.108, p = 0.093).

The age- and sex-specific reference values reported here could be used in clinical practice and future studies to identify patients at risk of muscle decline.