Plot of eGFRcys vs. eGFRcr values at first measurement, kernel density of difference of eGFRcys-eGFRcr at first measurement, percent change in eGFRcys vs. eGFRcr over up to 5 calendar years, and the sensitivity and specificity of a decline of 30% or more in eGFRcr. Included the population with any cystatin C measurement by 2014 to allow for up to 5 years of follow-up for retesting. eGFRcr, estimated glomerular filtration rate based on creatinine; eGFRcys, estimated glomerular filtration rate based on cystatin C.