Correlation of I-125 iothalamate and tc-99m DPTA measurements of GFR using the single injection method

Simultaneous renal imaging and determination of GFR can be obtained using Tc-99m DTPA. In order to assess the accuracy of the GFR value, a comparison was made between commercial sources of Tc-99m DTPA and I-125 iothalamate. The calculation assumed monoexponential loss of each radiopharmaceutical from the renal compartment. Plasma measurements were made at two sampling intervals, 90 to 120 minutes and 180 to 240 minutes. An apparent volume of distribution calculated for plasma concentrations at 2 hours and 3 hours was examined as a single sample indication of GFR. GFR values appear to be higher with DPTA than with iothalamate at the earlier interval. Choice of the later sampling times may be desirable for patients with known renal disease. The apparent volume of distribution at 3 hours may be a convenient single sample for monitoring or surveying patients for renal disease.