This is a single-center prospective observational study designed to evaluate the effectiveness of multimodal muscle ultrasound for the assessment of sarcopenia in kidney transplant recipients. Adult patients undergoing kidney transplantation will undergo both muscle ultrasound and bioelectrical impedance analysis (BIA) at predefined time points before and after transplantation.

The primary objective is to evaluate the diagnostic performance of muscle ultrasound for sarcopenia assessment, including its correlation and agreement with BIA-derived skeletal muscle index (BIA-SMI), as well as its diagnostic accuracy. Secondary objectives include describing the longitudinal changes in sarcopenia prevalence and muscle-related parameters from the pre-transplant period to 1 year after transplantation.

The study aims to provide evidence for a convenient, noninvasive, radiation-free, and reliable method for sarcopenia assessment in kidney transplant recipients.

Sarcopenia is highly prevalent among patients with end-stage renal disease and is associated with poor clinical outcomes. Although kidney transplantation may improve overall health status, perioperative stress, immunosuppressive therapy, and postoperative metabolic changes may influence muscle mass and function. Reliable assessment of sarcopenia in kidney transplant recipients remains clinically important.

Current methods for sarcopenia assessment include computed tomography (CT), dual-energy X-ray absorptiometry (DXA), and bioelectrical impedance analysis (BIA). However, CT and DXA are limited by radiation exposure, cost, and accessibility, while BIA may be affected by changes in hydration status, especially in transplant recipients. Muscle ultrasound is a portable, radiation-free, and bedside-available imaging modality that may provide a practical alternative for muscle assessment.

This prospective observational study will enroll adult kidney transplant recipients at Sichuan Provincial People's Hospital. Participants will undergo multimodal muscle ultrasound and BIA examinations at predefined perioperative time points. Ultrasound parameters will include muscle thickness, cross-sectional area, and echogenicity.

The primary study objective is to evaluate the diagnostic performance of muscle ultrasound for sarcopenia assessment using BIA-derived skeletal muscle index (BIA-SMI) as the reference standard. Correlation analysis, intraclass correlation coefficients, Bland-Altman analysis, and receiver operating characteristic (ROC) curve analysis will be performed to evaluate agreement and diagnostic accuracy.

Secondary objectives include evaluating longitudinal changes in muscle-related parameters and sarcopenia prevalence from the pre-transplant period to 1 year after kidney transplantation. Linear mixed-effects models or generalized estimating equations will be used to analyze temporal trends.

This study aims to establish muscle ultrasound as a practical and reliable tool for sarcopenia assessment in kidney transplant recipients and to improve understanding of perioperative muscle changes after transplantation.

Inclusion Criteria:

• Adult patients aged 18 years or older
• Patients scheduled to undergo kidney transplantation
• Ability to undergo muscle ultrasound and bioelectrical impedance analysis (BIA)
• Ability to provide written informed consent

Exclusion Criteria:

• Patients with severe limb deformity or conditions preventing muscle ultrasound assessment
• Patients with implanted electronic devices contraindicating BIA assessment
• Inability to complete study assessments

Adult kidney transplant recipients undergoing perioperative sarcopenia assessment at Sichuan Provincial People's Hospital.

Individual participant data will not be publicly available due to institutional policy and participant privacy considerations.