High serum creatinine nonlinearity: a renal vital sign?

Patients with chronic kidney disease (CKD) may have nonlinear serum creatinine concentration (SC) trajectories, especially as CKD progresses. Variability in SC is associated with renal failure and death. However, present methods for measuring SC variability are unsatisfactory because they blend information about SC slope and variance. We propose an improved method for defining and calculating a patient's SC slope and variance so that they are mathematically distinct, and we test these methods in a large sample of US veterans, examining the correlation of SC slope and SC nonlinearity (SCNL) and the association of SCNL with time to stage 4 CKD (CKD4) and death. We found a strong correlation between SCNL and rate of CKD progression, time to CKD4, and time to death, even in patients with normal renal function. We therefore argue that SCNL may be a measure of renal autoregulatory dysfunction that provides an early warning sign for CKD progression.