A comparison of the plasma disappearance of iohexol and Tc-99m-DTPA for the measurement of glomerular filtration rate (GFR) in diabetes

Background: Changes in glomerular filtration rate (GFR) provide a valuable indicator of the progression of diabetic nephropathy. GFR is most commonly measured by the plasma clearance of radioisotopes, however, use of iohexol, a non-ionic radiocontrast medium, is a recently described alternative and has shown good agreement with inulin clearance. A one-compartment model is used for calculating GFR in most Australian centres but a two-compartment m odel is more accurate. Aims: To set up a non-radioisotopic method for assessment of GFR using iohe xol, and to compare this with the currently used Tc-99m-diethylene-triamine -penta-acetic acid (DTPA) method. Secondly, to compare GFR results using an unmodified one-compartment model with a one-compartment model subjected to the Brochner-Mortensen modification. Methods: Twenty-one patients with diabetes had assessment of GFR with simul taneous measurements of Tc-99m-DTPA and iohexol plasma clearance. Plasma cl earance was determined by the slope intercept method and then modified acco rding to the Brochner-Mortensen equation. Plasma iohexol concentrations wer e determined by capillary electrophoresis. Results: There was no significant difference between iohexol and Tc-99m-DTP A derived GFR values, difference 4.3+/-7.7 mL/minute (mean +/- SD). This wa s despite Tc-99m-DTPA protein binding demonstrated in the range of 5-10%. C omparison of GFR;results using an unmodified one-compartment model with a B rochner-Mortensen corrected one-compartment model showed higher GFR values with the former, in the range of 20-30% for GFR values >100 mL/minute. Conclusion: Iohexol provides an efficient alternative to radioisotopic meth ods for serial measurement of GFR in diabetic patients with hyperfiltration , incipient and overt nephropathy. A one-compartment model with its inheren t overestimation of GFR should be replaced by the Brochner-Mortensen modifi ed one-compartment model.