SP271GFR MEASUREMENT SAMPLING STRATEGY: A CRITICAL ELEMENT IN GFR DETERMINATION AT ALL LEVELS OF GFR

INTRODUCTION: Urinary inulin clearance is considered the gold standard of GFR measurement but plasma clearance protocols using more accessible and inexpensive iohexol and 99mTc-DTPA are far more commonly performed. Many different plasma sampling protocols exist (number of and timing of samples) but very little is known about the accuracy of these in general and in patients with specific characteristics. The aim of this study is to compare GFR determined by plasma clearance of iohexol and 99mTc-DTPA with varying sampling strategies against gold standard urinary inulin GFR and to identify sampling protocols with the greatest accuracy in eGFR subgroups and in patients with significant edema. METHODS: GFR was measured using urinary inulin clearance, plasma iohexol clearance, and plasma 99mTc DTPA clearance simultaneously in 77 subjects with known chronic kidney disease. Blood samples were collected at several timepoints to allow for evaluation of different sampling protocols. For each method, mean bias (alternate GFR - inulin GFR), precision (standard deviation of mean bias), and accuracy (the percent of alternate measures within 10% (P10) and 30% (P30) of inulin GFR were calculated for the entire cohort and for eGFR –EPI Cr subgroups (<30, 30-59 and ≥ 60 ml/min/1.73m2) and edema subgroups (1&2, 3&4). RESULTS: Mean age 62.3 ± 12.6 yrs, BSA 2.0 ± 0.3 m2, 95% white race, 40% female sex. Mean inulin GFR 33 ± 19 ml/min/1.73m2. For eGFR less than 30 ml/min/1.73m2, the most accurate protocol included samples between 4-10 hours (P30= 76%). The 2-10 hour protocol was less accurate (P30=60%). For eGFR greater than 60 ml/min/1.73 m2, the most accurate protocol included samples between 2-4 hours (P10=70%). More prolonged sampling (2-10 hours) lead to reduced accuracy (P10=50%). For all iohexol protocols, inulin GFR was overestimated to a greater degree in those with significant edema (stages 3&4) as compared to those without significant edema (stages 1&2). For those with signficant edema, the most accurate protocol (P30=64) had delayed intial and final samples (4-10 hours). CONCLUSIONS: Delay of both initial and final samples in iohexol plasma clearance protocols significantly increases GFR measurement accuracy in those with low GFR and significant edema. Early samples yield the best accuracy in those with higher GFRs. These results reveal the need for individualized GFR measurement protocols based on patient characteristics including GFR and edema status.

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