Accuracy of GFR Estimating Equations in Patients with Discordances Between Creatinine and Cystatin C-Based Estimations.

Background Cystatin C is recommended as a confirmatory test to eGFR when more precise estimates are needed for clinical decision making. Although eGFR on the basis of both creatinine and cystatin (eGFR(cr-cys)) is the most accurate estimate in research studies, it is uncertain whether this is true in real-world settings, particularly when there are large discordances between eGFR based on creatinine (eGFR(cr)) and that based on cystatin C (eGFR(cys)) Methods We included 6185 adults referred for measured GFR (mGFR) using plasma clearance of iohexol in Stockholm, Sweden, who had 9404 concurrent measurements of creatinine, cystatin C, and iohexol clearance. The performance of eGFR(cr), eGFR(cys), and eGFR(cr-cys) was assessed against mGFR with median bias, P-30, and correct classification of GFR categories. We stratified analyses within three categories: eGFR(cys) at least 20% lower than eGFR(cr) (eGFR(cys)eGFR(cr)). Results eGFR(cr) and eGFR(cys) were similar in 4226 (45%) samples, and among these samples all three estimating equations performed similarly. By contrast, eGFR(cr-cys) was much more accurate in cases of discordance. For example, when eGFR(cys)eGFR(cr) (8% of samples), the median biases were -4.5, 8.4, and 1.4 ml/min per 1.73m(2). The findings were consistent among individuals with cardiovascular disease, heart failure, diabetes mellitus, liver disease, and cancer. Conclusions When eGFR(cr) and eGFR(cys) are highly discordant in clinical practice, eGFR(cr-cys) is more accurate than either eGFR(cr) or eGFR(cys).