Performance-Based Frailty Tools Capture Distinct Risk Phenotypes in Kidney Transplant Candidates: A Comparative Analysis of FRAIL and SPPB.

Frailty predicts adverse outcomes in kidney transplant candidates, but it remains unclear whether different frailty tools identify the same or distinct at-risk patients. In this retrospective cohort study, 953 kidney transplant candidates listed between 2018 and 2022 underwent frailty assessment using both the FRAIL (Fatigue, Resistance, Ambulation, Illnesses, Weight Loss) scale and Short Physical Performance Battery (SPPB). In a subset with available CT imaging, body composition was analyzed using AI-based tools. Frailty prevalence differed by instrument: 6.2% by FRAIL versus 24% by SPPB. Waitlist mortality occurred in 9.5% (91/953) of candidates. Both tools independently predicted waitlist mortality (FRAIL:aHR 2.38, 95%CI 1.34-4.20,p=0.003; SPPB:aHR 1.66, 95%CI 1.07-2.60,p=0.025). SPPB-defined frailty showed stronger associations with post-transplant outcomes, including longer hospital stay, and 30-day and one-year readmissions. CT analysis revealed that SPPB-defined, but not FRAIL-defined, frailty was associated with lower muscle density and increased visceral adiposity, suggesting a mechanistic basis. The FRAIL scale and SPPB capture distinct dimensions of frailty with complementary strengths. FRAIL better reflects symptom burden and candidates at risk for waitlist mortality, while SPPB more closely associates with post-transplant complications. Results support systematic use of both instruments to optimize risk stratification and identify candidates who may benefit from targeted prehabilitation.