Comparing Cystatin C Estimated GFR With Creatinine Estimated GFR in Acute Kidney Injury Recovery

Introduction Current guidelines recommend creatinine-based estimated glomerular filtration rate (eGFRcr) to assess kidney recovery after AKI, but this may be inaccurate because of loss of muscle mass. Cystatin C eGFR (eGFRcys) is an alternative that is not similarly affected. Additionally, simple calculations (e.g. creatinine muscle index, CMI) incorporating the difference between eGFRcr and eGFRcys may indicate prognosis. We sought to determine whether eGFRcr differs from eGFRcys after AKI and whether CMI is associated with mortality. Methods The ARID (AKI Risk in Derby) Study is a prospective parallel-group cohort study. Hospitalised participants with and without exposure to AKI were matched 1:1 for age, baseline kidney function and diabetes. eGFRcr and eGFRcys at three months after admission were compared in 849 participants. Associations between CMI and outcomes including mortality, heart failure and hospitalisation were assessed at five years. Results: eGFRcys was lower than eGFRcr (53.4 [IQR 34.3 to 85.5] ml/min/1.73m 2 versus 68.4 [IQR 52.5 to 84.7] ml/min/1.73m 2 , P <0.001), with more pronounced differences in those with AKI. eGFRcys categorised more participants with CKD (in AKI group: eGFRcr <60ml/min/1.73m 2 in 44.9%; eGFRcys <60ml/min/1.73m 2 in 69.6%, P <0.001). In the AKI group, higher CMI was independently associated with lower mortality at five years (adjusted hazard ratio 0.931 [0.874-0.992] mg/d/1.73m 2 p=0.03). Conclusion There are significant differences at three months after AKI in eGFR derived from creatinine versus cystatin C. The magnitude of difference between these estimates is associated with subsequent mortality. Further research is required to determine the optimal approach to patient assessment after AKI.