Evaluation of non-polynomial equations for one-compartment correction of slope-intercept GFR: theoretical prediction and experimental measurement.

BACKGROUND: Polynomial equations for one-compartment correction of slope-intercept glomerular filtration rate (GFR) will underestimate values at high clearance rates. Non-polynomial correction equations that are independent of patient size and renal function would be advantageous and may have cross-species use. MATERIALS AND METHODS: The study explored the theoretical basis of firstly the Jodal and Brochner-Mortensen one-compartment correction equation, replacing plasma volume with extracellular fluid volume, and secondly an equation described by Peters. One-compartment correction factors (a which is related to plasma volume and v which is related to extracellular fluid volume) which avoided the need for scaling to body size were developed. Both factors were determined from the biexponential clearance curve of the markers iohexol and (51)Cr-EDTA in humans and iohexol in cats and dogs. Relationships between a and v and filtration function and body size were then determined using data from humans, cats and dogs to assess their validity and compare this with theoretical predictions. RESULTS: In all species, v was higher than a, as theoretically predicted. Both were significantly higher in humans than cats and dogs, ruling out cross-species use. Significant relationships were present between v and measures of filtration function in humans, but were weak with respect to a. Neither a nor v showed significant relationships with filtration function in animals or with body size in any species. CONCLUSIONS: a and v (which are factors independent of body size) can be used interchangeably for correcting slope-intercept clearance. However values of both for humans are higher compared to cats and dogs. Therefore a single cross-species factor cannot be used.