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Assessment of Iothalamate Plasma Clearance: Duration of Study Affects Quality of GFR

Indiana University School of Medicine and Veterans Administration Medical Center, Indianapolis, Indiana

Correspondence: Dr. Rajiv Agarwal, Professor of Medicine, Indiana University and Veterans Administration Medical Center, 1481 West 10th Street, Indianapolis, IN 46202. Phone 317-988-2241; Fax: 317-988-2171; E-mail: ragarwal{at}iupui.edu

Background and objectives: Measurement of GFR is important for the management of chronic kidney disease (CKD). Although bolus administration of radiocontrast agents is commonly used to measure GFR, the optimal duration of sampling to assess their plasma clearance is unknown. The purpose of this study was to evaluate whether the duration of plasma sampling influences precision and estimation of GFR.

Design, setting, participants, & measurements: GFR was measured by sampling plasma 12 times over 5 h in 56 patients with CKD (mean age 64 yr, 98% men, 79% Caucasian, 34% diabetics, estimated GFR 31.8 ± 14.2 ml/min/1.73 m²). In a subset of 12 patients we measured GFR by sampling plasma 17 times over 10 h.

Results: Short sampling intervals considerably overestimated GFR measured using total plasma iothalamate clearance, especially in larger patients. In the higher estimated GFR group (>30 ml/min/1.73m²), the 5-h GFR was 17% higher and 2-h GFR 54% higher compared with the 10-h GFR, which averaged 40.3 ml/min/1.73 m². In the lower estimated GFR group (<30 ml/min/1.73m²), the 5-h GFR was 36% higher and 2-h GFR 126% higher compared with the 10-h GFR, which averaged 22.2 ml/min/1.73 m². Short sampling duration also reduced the precision of the estimated GFR from 1.67% for 10-h GFR, to 3.48% for 5-h GFR, and to 7.07% for 2-h GFR.

Conclusions: GFR measured over a longer duration with multiple plasma samples spanning the distribution and elimination phases may improve precision and provide a better measure of renal function.

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