Elevation of the slope of inverse creatinine to define progressive chronic kidney disease in cats and associated risk factors

Feline chronic kidney disease (CKD) has highly variable patterns of progression. Objective methods to differentiate cats with stable and progressive CKD are lacking. This retrospective study aimed to determine a slope cutoff for reciprocal of creatinine (creatinine^-1) vs time to objectively differentiate stable and progressive feline CKD, and identify risk factors for CKD progression within 3 months of CKD diagnosis. Euthyroid client-owned cats with azotaemic CKD (n = 321) were included in this study. CKD cats that demonstrated stable (<25% increase) plasma creatinine concentration for >365 days (n = 194) were identified and the reciprocal plot of plasma creatinine vs time (creatinine^-1 plot) from these cats was used to determine a cutoff to differentiate stable from progressive CKD. Risk factors for progressive CKD, including response to 3 months of standard treatment, were explored by binary logistic regression. Variables associated with mortality were evaluated using Cox regression. A slope cutoff of -9.5×10^-5L/μmol/month (-8.4×10^-3dL/mg/month) for creatinine^-1 plot distinguished stable (n = 231) vs progressive (n = 90) CKD cats. Baseline plasma log-transformed fibroblast growth factor-23 (ln[FGF-23]), albumin concentrations, advancing age, and slope of creatinine^-1, phosphate and body weight within the first 84 days of CKD diagnosis were independent risk factors for progression. Progressive cats had shorter median survival time [95% CI] than non-progressive cats (287 [246-354] vs 894 [812-1052] days; P < 0.001). Overall, cats with stable and progressive CKD can be distinguished using slope of creatinine^-1 plot. Risk factors identified may facilitate early detection of cats with progressive CKD.