Depth-Corrected Versus Non Depth-Corrected GFR Determination by Quantitative Renal Scintigraphy in the Dog

Abstract

Using established methodology, 99mTc-diethylenetriaminepentaacetic acid (99mTc-DTPA) quantitative renal scintigraphy was performed on 22 healthy sedated dogs to measure glomerular filtration rate (GFR). Using each dog as its own control, GFR values were calculated with and without correction for photon attenuation due to kidney depth. The renal depth measurement was made from a right lateral scintigraphic image. Simultaneously, plasma clearance (PC) of 99mTc-DTPA was determined using a two-sample, one-compartment pharmacologic model with samples at 20 and 180 minutes. One to three days later, the dogs were sedated and computed tomographic (CT) images were obtained of both kidneys with the dogs positioned as during scintigraphy. Measurements of renal depth were made from both the center of the kidney and the renal crest to the skin directly dorsal to the center of the kidney and to a line perpendicular to the skin at the dorsal midline. The dogs were manually lifted and positioned again and a second CT scan was performed to evaluate repeatability of these measurements. For all dogs, depth-corrected (DC) and non depth-corrected (nDC) GFR measurements were not different (p=0.3926). For small dogs, DC and nDC GFR measurements were not different (p=0.2332). Correlations between scintigraphy and CT depth measurements were poor for the left kidney and modest for the right kidney, making depth correction of doubtful value in dogs of this weight. For large dogs, DC and nDC GFR measurements were different (p=0.0263). Correlations between scintigraphy and CT depth measurements for the left kidney were modest, suggesting that using scintigraphy for depth correction may introduce error into depth corrected GFR measurement in large dogs. Before recommendations are made to abandon depth correction in large dogs it would be necessary to repeat the study with the gold standard of inulin clearance. Separate linear regression formulas for small and large dogs may improve the accuracy of GFR measurements by scintigraphy. DC and nDC GFR measurements were significantly different from PC when considering all dogs together (p<0.0001 and p=0.0003 respectively) and when considering small dogs alone (p<0.0001 for both). In large dogs, PC was also found to significantly differ from DC (p=0.0016). Only when comparing PC to nDC GFR in large dogs were the values significantly similar (p=0.5179). Because numerous other studies have indicated good agreement between scintigraphy and plasma clearance, the plasma clearance measurements obtained in this study are questionable.