The relationship between some plasma clearance methods for estimation of glomerular filtration rate in dogs with pyometra

The purpose of the present study was to compare different pharmacokinetic m odels for estimation of glomerular filtration rate (GFR) in 50 dogs with py ometra. GFR was estimated by plasma clearance (CLplasma) of iohexol by four 1-compartment methods (CL1c), a 2-compartment method (CL2c), and the trape zoidal method (CLtr). Regression analysis was performed to establish correc tion formulas for prediction of CLtr, from the CL1c, values and to find opt imal times of sampling. Standardization of clearance values to body weight (kg), body surface area (m(2)) and extracellular fluid volume (ECFV) was co mpared by ranking of values. CLtr and CL2c values were similar, whereas CL1 c overestimated CLtr. CLtr could be predicted from 2 samples at 2 and 3 hou rs after injection, using the formula CLtr = 4.52 + 0.84CL(1c) - 0.00080(CL tc)(2) (R-2 =.97). Similar relationships were found when sampling at 2 and 4 hours or at 2, 3, and 4 hours after injection, whereas predictions from t he 3- and 4-hour estimates were not optimal (R-2 =.79). The 2-sample method s for calculating GFR/ECFV generally produced unreliable predictions of the complete curve GFR/ECFV values. For some dogs, the choice of standardizati on procedure substantially changed the apparent level of renal function rel ative to other dogs in the study. We conclude that by applying an appropria te correction formula, GFR may be estimated using 2 blood samples at 2 and 3, or 2 and 4 hours after injection of iohexol when renal function is norma l or moderately reduced. The method of standardizing the analysis with resp ect to body size may influence interpretation of the results substantially.