Muscle Quality Assessed by the Phase Angle Can Predict Renal Prognosis in Patients with Chronic Kidney Disease

<b>Background/Objectives</b>: Muscle quality reflects functional characteristics beyond muscle mass and may offer prognostic insights in patients with chronic kidney disease (CKD). This study investigated whether muscle quality predicts sarcopenia and renal outcomes in non-dialysis CKD patients. <b>Methods</b>: We prospectively recruited 93 patients with CKD (stage G1-5). Muscle quality was evaluated using phase angle (PhA) derived from bioelectrical impedance analysis. Sarcopenia was diagnosed based on criteria from the Asian Working Group for Sarcopenia. The primary renal outcome was defined as a > 30% decline in eGFR or the initiation of dialysis. Logistic regression identified factors associated with sarcopenia. Kaplan-Meier and Cox regression analyses were used to assess predictors of renal outcomes. <b>Results</b>: Lower PhA (odds ratio [OR] 0.044, <i>p</i> = 0.002) and BMI (OR 0.648, <i>p</i> = 0.015) were independently associated with sarcopenia. Sex-specific PhA cutoffs of 4.50° for men and 4.00° for women yielded high diagnostic accuracy for sarcopenia (AUC: 0.878 and 0.969, respectively). Over a median observation period of 574 days, patients with lower PhA values showed a significantly higher risk of poor renal outcomes, independent of confounding factors (hazard ratio [HR] 4.540, <i>p</i> = 0.011), even after adjustment for age, sex, BMI, systolic blood pressure, diastolic blood pressure, eGFR, and albumin. However, muscle mass was not significantly associated with renal prognosis. <b>Conclusions</b>: PhA is a non-invasive marker that may reflect muscle quality and is associated with sarcopenia and renal outcomes in CKD. It may help inform risk stratification in clinical practice.