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 (⁵¹Cr)-EDTA and iohexol, injected into opposite arms and scaled to 1.73 m². 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 ⁵¹Cr-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 ⁵¹Cr-EDTA for predicting iohexol multisample GFR (6.9 mL/min) was better than that of multisample GFR with ⁵¹Cr-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 ⁵¹Cr-EDTA at 3 and 4 hours showed repeatability (SD of change, 9.4 and 9.3 mL/min) similar to multisample GFR with ⁵¹Cr-EDTA (10.7 mL/min). Single-sample GFR at 4 hours by using ⁵¹Cr-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 ⁵¹Cr-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.

Index Words: Glomerular filtration rate, chromium-51 (⁵¹Cr)-EDTA, iohexol, single sample, validation