Rapid Determination of Glomerular Filtration Rate by Single-bolus Inulin: a Comparison of Estimation Analyses

Rapid measurement of glomerular filtration rate (GFR) by an inulin single-bolus technique would be useful, but its accuracy has been questioned. We hypothesized that reported inaccuracies reflect the use of inappropriate mathematical models. GFR was measured in 14 intact and 5 unilaterally nephrectomized conscious male Sprague-Dawley rats (mean weight 368 +/- 12 g) by both single-bolus (25 mg/kg) and constant-infusion techniques (0.693 mg . kg-1 . min-1). The temporal decline in plasma inulin concentration was analyzed through biexponential curve fitting, which accounted for renal inulin loss before complete vascular and interstitial mixing. We compared our mathematical model based on empirical rationale with those of other investigators whose studies suggest inaccuracy of single-bolus methods. Our mathematical model yielded GFR values by single bolus that agreed with those obtained by constant infusion [slope = 0.94 +/- 0.16 (SE); y intercept = 0.23 +/- 0.64; r = 0.82]. In comparison to the data obtained by constant inulin infusion, this method yielded a very small bias of -0.0041 +/- 0.19 ml/min. Two previously reported models yielded unsatisfactory values (slope = 1. 46 +/- 0.34, y intercept = 0.47 +/- 1.5, r = 0.72; and slope = 0.17 +/- 1.26, y intercept = 17.15 +/- 5.14, r = 0.03). The biases obtained by using these methods were -2.21 +/- 0.42 and -13.90 +/- 1. 44 ml/min, respectively. The data indicate that when appropriate mathematical models are used, inulin clearance after single-bolus delivery can be used to measure GFR equivalent to that obtained by constant infusion of inulin. Attempts to use methods of analysis for simplicity or expediency can result in unacceptable measurements relative to the clinical range of values seen.