An overview of glomerular filtration rate testing in dogs and cats

Abstract

Determination of glomerular filtration rate (GFR) is a valuable, yet underused, diagnostic tool for evaluating renal function in dogs and cats. This article first reviews the hormonal and hemodynamic factors which contribute to GFR, followed by a description of considerations when selecting a pharmacokinetic model and methods of animal-to-animal standardization. The best-characterized existing GFR markers, including creatinine, radiolabeled markers, and iohexol, are reviewed in depth, as well as alternative but lesser used techniques. A weighted means analysis of reported GFR measurements in healthy dogs and cats and a review of selected studies that have examined GFR alterations in animals with naturally occurring and experimental diseases provide the reader with preliminary guidelines on expected GFR results in these species and disease conditions.

Fig. 1

Calculation of GFR is affected by the pharmacokinetic model chosen. In all three graphs, true elimination pharmacokinetics of a hypothetical marker are indicated by the solid black line; measured concentrations of the marker in serum or plasma at various time points are indicated by black dots overlying the black line. A. In a one-compartment model, all time points are used to calculate a single ‘best fit’ line, the slope of which is used to calculate GFR (dashed line). B. In a two-compartment model, initial redistribution of the marker is determined from early time points (dotted line), whereas later time points are used to calculate GFR (dashed line) as for the one-compartment model. C. In non-compartmental models, time points are used to construct trapezoids for estimating the area-under-the-curve.

Fig. 2

A nephron demonstrating the characteristics of an ideal GFR marker. A marker (hexagons) used for measurement of GFR should freely enter the glomerulus via the afferent arteriole (1), and be filtered across the glomerular capillary endothelium, basement membrane, and podocytes (2) into Bowman’s space (3). The marker should then pass through the tubules without reabsorption or secretion (4), and pass into the urine (5). Not all marker molecules which enter the glomerulus will be filtered into the urine during each pass through the capillary bed, and instead will exit via the efferent arteriole (6) and be filtered during subsequent passes through the kidney. GFR by plasma clearance is determined by rate of disappearance of marker from the blood (6), whereas GFR by renal clearance is determined by rate of appearance of marker in the urine (5).