Glomerular filtration rate estimation by use of a correction formula for slope-intercept plasma iohexol clearance in cats

Objective-To develop a formula for correcting slope-intercept plasma iohexol clearance in cats and to compare clearance of total iohexol (Tlox), endo-iohexol (Enlox), and exo-iohexol (Exlox). Animals-20 client-owned, healthy adult and geriatric cats. Procedures-Plasma clearance of Tlox was determined via multisample and slope-intercept methods. A multisample method was used to determine clearance for Enlox and Exlox. A second-order polynomial correction factor was derived by performing regression analysis of the multisample data with the slope-intercept data and forcing the regression line though the origin. Clearance corrected by use of the derived formula was compared with clearance corrected by use of Brochner-Mortensen human and Heiene canine formulae. Statistical testing was applied, and Bland-Altman plots were created to assess the degree of agreement between Tlox, Enlox, and Exlox clearance. Results-Mean +/- SD iohexol clearance estimated via multisample and corrected slope-intercept methods was 2.16 +/- 0.35 mL/min/kg and 2.14 +/- 0.34 mL/min/kg, respectively. The derived feline correction formula was Cl(corrected) = (1.036 X Cl(uncorrected)) (0.062 X Cl(uncorrected)(2)), in which Cl represents clearance. Results obtained by use of the 2 methods were in excellent agreement. Clearance corrected by use of the Heiene formula had a linear relationship with clearance corrected by use of the feline formula; however, the relationship of the feline formula with the Brochner-Mortensen formula was nonlinear. Agreement between Tlox, Enlox, and Exlox clearance was excellent. Conclusions and Clinical Relevance-The derived feline correction formula applied to slope-intercept plasma iohexol clearance accurately predicted multisample clearance in cats. Use of this technique offers an important advantage by reducing stress to cats associated with repeated blood sample collection and decreasing the costs of analysis. (Am J Vet Res 2011;72:1652-1659)