Simultaneous Glomerular Filtration Rate Determination Using Inulin, Iohexol and ^99m Tc-DTPA Demonstrates the Need for Customized Measurement Protocols

Urinary inulin clearance is considered the gold standard of glomerular filtration rate (GFR) measurement but plasma clearance of less expensive and more accessible tracers is more commonly performed. Many plasma sampling protocols exist but little is known about their accuracy. Here, the study objectives were to compare plasma iohexol and ⁹⁹mTc-DTPA GFR with varying sampling strategies to the GFR measured by urinary inulin and to identify protocols with the greatest accuracy according to clinical characteristics. GFR was measured simultaneously using urinary inulin, plasma iohexol, and plasma ⁹⁹mTc DTPA clearance. Blood was sampled between two and ten hours. For each method, bias, precision, and accuracy (P30 and mean absolute error) were calculated for the entire cohort and for eGFR-EPI creatinine subgroups (under 30, 30-59 and 60 or more ml/min/1.73m²) and the edema stage using urinary inulin clearance as the gold standard. The mean inulin GFR of the 77 participants was 33 ml/min/1.73m². Delay of both the initial and the final samples in plasma iohexol protocols yielded the highest accuracy in the setting of low GFR (under 30 ml/min/1.73m²). Early initial and final samples yielded the highest accuracy in the setting of high GFRs (over 60 ml/min/1.73m²). No sampling strategy was accurate in edematous patients. Thus, our study demonstrates that customization of GFR protocols according to the anticipated level of GFR are required to optimize protocol accuracy.

Keywords: Chronic Kidney Disease; Glomerular Filtration Rate; Inulin; Iohexol.