Difference between kidney function by cystatin C versus creatinine and association with muscle mass and frailty.

BACKGROUND: A higher difference in estimated glomerular filtration rate by cystatin C versus creatinine (eGFRDiff = eGFRCys - eGFRCreat) is associated with decreased frailty risk. Since eGFRCreat is influenced by muscle more than eGFRCys, muscle mass may explain this association. Previous work could not account for this when considering regional muscle measures by imaging. Deuterated creatine (D3Cr) dilution measures whole body muscle mass (kilograms). We aimed to determine whether eGFRDiff is associated with D3Cr muscle mass and whether muscle mass explains the association between eGFRDiff and frailty. METHODS: Cross-sectional analysis within the multicenter MrOS Study at Year 14 (visit 4). 490 men of the original cohort of 5994 MrOS participants (aged ≥65 at enrollment) were included. Exposure was eGFRDiff (= eGFRCys - eGFRCreat), calculated using CKD-EPI equations 2012/2021. Primary outcome was D3Cr muscle mass. Secondary outcome was phenotypic pre-frailty (one or two criteria) and frailty (≥three criteria) including the following: weight loss, weakness, slow gait, physical activity, poor energy. The association of eGFRDiff with D3Cr muscle mass was examined by linear regression, that with prefrailty / frailty by multinomial logistic regression. RESULTS: Mean ± SD age was 84 ± 4 years, eGFRCreat 68 ± 16, eGFRCys 52 ± 16, eGFRDiff -15 ± 12 mL/min/1.73 m2 and D3Cr muscle mass 24 ± 4 kg. For each SD increment in eGFRDiff, D3Cr muscle mass was 1.4 kg higher on average, p < 0.0001 (fully adjusted). Higher eGFRDiff was associated with lower odds of frailty (OR = 0.63 95% CI [0.45;0.89]), but this was partially attenuated and insignificant after additionally adjusting for D3Cr muscle mass (OR = 0.85 95% CI [0.58; 1.24]). CONCLUSIONS: Higher eGFRDiff is associated with lower odds of frailty among late-life men. D3Cr muscle mass accounts for some of this association. This suggests that non-GFR determinants of creatinine and cystatin C, such as muscle mass, play a role in explaining the association of eGFRDiff with frailty. Future studies are needed to confirm.

Age-Related Differences in Oncological Outcomes in Renal Cell Carcinoma: Impact of Functional Conservation as Measured by Postoperative eGFR.

INTRODUCTION: We sought to determine whether loss of renal function increases risk of recurrence and metastases in renal cell carcinoma (RCC), and whether this impact was age-related. MATERIALS AND METHODS: We performed a retrospective analysis of the International Marker Consortium for Renal Cancer (INMARC) registry. Patients were separated into younger (<65 years old) and elder (≥65 years old) age groups, and rates of de novo estimated glomerular filtration rate (eGFR<45 mL/min/1.73m2 [eGFR<45]) were calculated. Multivariable analysis (MVA) was conducted for predictors of progression-free survival (PFS) and all-cause mortality (ACM). Kaplan-Meier Analysis (KMA) was conducted for PFS and overall survival (OS) in younger and elder age groups stratified by functional status. RESULTS: We analyzed 1805 patients (1113 age<65, 692 age≥65). On MVA in patients <65, de novo eGFR<45 was independently associated with greater risk for worsened progression (HR=1.61, P=.038) and ACM (HR=1.82, P=.018). For patients ≥65, de novo eGFR<45 was not independently associated with progression (P=.736), or ACM (P=.286). Comparing patients with de novo eGFR<45 vs. eGFR ≥45, KMA demonstrated worsened 5-year PFS and OS in patients <65 (PFS: 68% vs. 86%, P<.001; OS: 73% vs. 90%, P<.001), while in patients ≥65, only 5-year OS was worsened (77% vs. 81%, P<.021). CONCLUSION: Development of de novo eGFR<45 was associated with more profound impact on patients <65 compared to patients ≥65, being an independent risk factor for PFS and ACM. The mechanisms of this phenomenon are unclear but underscore desirability for nephron preservation when safe and feasible in younger patients.