Comparison of simultaneous plasma clearance of ^99mTc-DTPA and ⁵¹Cr-EDTA: can one tracer replace the other?

Both ^99mTc-DTPA and ⁵¹Cr-EDTA are widely used to determine glomerular filtration rate (GFR), but few direct comparative studies exist. The shortage of ⁵¹Cr-EDTA makes a direct comparison highly relevant. The aim of the study was to investigate if there is any clinically relevant difference between plasma clearance of ^99mTc-DTPA and ⁵¹Cr-EDTA. Patients ≥18 years of age referred for routine GFR measurement by ⁵¹Cr-EDTA were prospectively enrolled. The two tracers (10 MBq ^99mTc-DTPA (CaNa ₃-DTPA) and 2.5 MBq ⁵¹Cr-EDTA) were intravenously injected at time zero. A standard 4-sample technique was applied with samples collected at 180, 200, 220 and 240 min, if the estimated GFR (eGFR) was ≥30 mL/min. A comparison of single-sample GFR based on the 200 min sample was also conducted. Fifty-six patients were enrolled in the study. All patients had an estimated GFR >30 mL/min/1.73 m ². No patients suffered from ascites or significant oedema. The mean ⁵¹Cr-EDTA plasma clearance was 82 mL/min (range 16–226). The plasma clearances determined by the two methods were highly correlated (r = 0.993). The plasma clearance was significantly higher when measured by ^99mTc-DTPA than by ⁵¹Cr-EDTA (p = 0.01), but the numerical difference was minimal (mean difference 1.4 mL/min; 95% limits of agreement (LOA) –6.6 to 9.4). The difference between the two methods was independent of the level of renal function. Similar results were found for one-sample GFR. No clinically relevant differences were found between the plasma clearance of ^99mTc-DTPA and that of ⁵¹Cr-EDTA. Therefore, ^99mTc-DTPA can replace ⁵¹Cr-EDTA when needed.