Comparison of Plasma Clearance With Early-Compartment Correction Equations and Urinary Clearance in High GFR Ranges.

Introduction: Glomerular filtration rate (GFR) is measured from the late plasma disappearance curve of an exogenous tracer, after correction for the early decay-corresponding to the distribution of the tracer-using various equations. These equations display the highest discrepancies in the GFR range above 90 ml/min per 1.73 m, and their respective performances against a reference, urinary GFR measurement are unclear.

Methods: In patients with mGFR >90 ml/min per 1.73 m from 6 different cohorts, we compared GFR obtained from the plasma clearance of iohexol or Cr-ethylenediamine tetraacetic acid (EDTA), after correction using Chantler (C), Bröchner-Mortensen (BM), Fleming (F), Jodal-Bröchner-Mortensen (JBM), and Ng (N) equations, with urinary clearance of the same tracers or inulin.

Results: In 438 participants (median age 41 [39-42] years, 43% women), the median urinary clearance was 100.8 (94.7-112.6) ml/min per 1.73 m. Plasma clearances using the correction equations were 105.7 (96.8-119.2), 102.4 (95.2-112.9), 100.7 (93.6-111.1), 102.6 (95.2-113.4), and 106.0 (98.2-117.6) ml/min per 1.73 m for C, BM, F, JBM, and N, respectively. Concordance correlation coefficients between plasma and urinary clearances were poor for all equations. Compared with urinary clearances, BM, F, and JBM displayed the best accuracy within 10% (73%, 72%, and 71%, respectively, vs. 63% and 66% for C and N), whereas BM and JBM had the lowest median biases. Accuracy of all equations was especially low in the hyperfiltration range (urinary clearance >130 ml/min per 1.73 m).

Conclusion: The BM and JBM equations displayed the best overall performances to correct for the early disappearance curve. Results of these equations should be interpreted with caution, especially in the highest GFR range.