Comparison of GFR Measurements Assessed From Single Versus Multiple Samples

Background: Many previous studies have evaluated single-sample glomerular filtration rate (GFR) against multisample GFR, of which the single sample was a member, but none have compared single and multisample GFRs against an independent reference method. We therefore performed this comparison by using simultaneous independent multisample GFR measured with a different indicator. Setting & Participants: University hospital: patients and healthy volunteers (95 studies in 60 patients and 20 healthy participants). Healthy volunteers were studied fasting and after food; 10 of them had a repeated fasting study. Study Design: Diagnostic test study. Index Test: Single-sample GFR. Reference Test: Multisample GFR with a different indicator. Measurements: GFR was measured by using chromium-51 (Cr-51)-EDTA and iohexol, injected into opposite arms and scaled to 1.73 m(2). Blood samples, obtained bilaterally 20, 40, 60, 120, 180, and 240 minutes after injection, were assayed for indicator injected contralaterally. Single-sample GFR (Jacobsson method) was calculated from indicator concentrations at 3 and 4 hours. Single-sample GFR from 1 indicator was compared with multisample GFR from the other and vice versa, as well as from the same indicator. Differences were expressed as limits of agreement between paired measurements in Bland-Altman plots. Precision was expressed as the SD of the mean difference between paired measurements. Results: Limits of agreement between multisample GFRs measured by using Cr-51-EDTA and iohexol (-12 to 20 mL/min) were similar to the corresponding limits for single-sample GFR at 3 (-16 to 17 mL/min) and 4 hours (-11 to 17 mL/min). The precision of single-sample GFR at 4 hours by using Cr-51-EDTA for predicting iohexol multisample GFR (6.9 mL/min) was better than that of multisample GFR with Cr-51-EDTA (7.9 mL/min). When analysis was limited to patients with GFR less than 60 mL/min, single-sample GFR was slightly inferior to multisample GFR. In healthy participants, single-sample GFR with Cr-51-EDTA at 3 and 4 hours showed repeatability (SD of change, 9.4 and 9.3 mL/min) similar to multisample GFR with Cr-51-EDTA (10.7 mL/min). Single-sample GFR at 4 hours by using Cr-51-EDTA detected a food-induced increase in GFR (4.4 +/- 5.9 mL/min; P < 0.001) with more confidence than multisample GFR by using Cr-51-EDTA (4.6 +/- 7.5 mL/min; P < 0.01). Limitations: No separate gold standard (eg, inulin) to facilitate interpretation of observed differences between 2 markers. Conclusions: Single-sample GFR is as reliable as multisample GFR for measuring GFR, especially when GFR is greater than 60 mL/min. Am J Kidney Dis 54:278-288. (C) 2009 by the National Kidney Foundation, Inc.