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 itsaccuracy has been questioned. We hypothesized that reportedinaccuracies reflect the use of inappropriate mathematical models. GFRwas measured in 14 intact and 5 unilaterally nephrectomized conscious male Sprague-Dawley rats (mean weight 368 ± 12 g) by bothsingle-bolus (25 mg/kg) and constant-infusion techniques (0.693 mg · kg¹ · min¹).The temporal decline in plasma inulin concentration was analyzed through biexponential curve fitting, which accounted for renal inulinloss before complete vascular and interstitial mixing. We compared ourmathematical model based on empirical rationale with those of otherinvestigators whose studies suggest inaccuracy of single-bolus methods.Our mathematical model yielded GFR values by single bolus that agreedwith those obtained by constant infusion [slope = 0.94 ± 0.16 (SE); y intercept = 0.23 ± 0.64; r = 0.82]. Incomparison to the data obtained by constant inulin infusion, thismethod 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 byusing 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 byconstant infusion of inulin. Attempts to use methods of analysis forsimplicity or expediency can result in unacceptable measurements relative to the clinical range of values seen.