Comparing Renal Function Measures

Enrolled patients underwent ^99 mTc-DTPA renal scan to measure the isotopic GFR. Serum and urine samples were collected on the same day of the renal scan. Serum samples were used to measure serum creatinine, CysC, and blood urea nitrogen (BUN). The Quantikine Human Cystatin C Immunoassay (R&D Systems Inc., Minneapolis, MN, USA), an enzyme-linked immunosorbent assay (ELISA), was used to measure serum CysC on a CX7 analyzer (Beckman, Brea, CA, USA). Creatinine clearance (CCr) was calculated through the following equation: CCr = (urinary creatinine × 24 hours of urine volume)/(serum creatinine × 1440) [24 (link)]. GFR was furthermore calculated using six estimating formulae, Cockcroft-Gault formula [25 (link)], the modification of diet in renal disease (MDRD) equations using 4 and 6 variables [26 (link), 27 (link)], and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations using creatinine alone (CKD-EPI Cr) [28 (link)], CysC alone (CKD-EPI CysC), or both CKD-EPI (Cr-CysC) [29 (link)] (Table 1). Isotopic GFR was used as the reference to which all other measures and estimating formulae of the renal function were compared. In this study, the upper limit of normal for serum creatinine was 1.2 mg/dL and early renal impairment was defined as a GFR of 60–89 mL/min.