Measured GFR Implemented in a US Clinical Practice: Accuracy of eGFR Cannot Be Assumed without Measured GFR

Background: In clinical practice, GFR estimated from serum creatinine (eGFR-CR) is often assumed to be accurate for individual patients and eGFR estimated from either serum cystatin C (eGFR-Cys) or both (eGFR-Cr-Cys) is considered the “confirmatory” test. However, without clinical availability of measured GFR (mGFR) these assumptions cannot be verified. We have implemented mGFR (4-hour plasma iohexol clearance; non-radiolabeled) as part of routine clinical care at a large urban academic medical center. The test is orderable in Epic and implemented in our outpatient infusion center. We present case studies to highlight that mGFR is essential for clinical practice and eGFR-Cys is not a universal confirmatory test for GFR. Methods: We describe three patients that had simultaneous measurement of creatinine, cystatin C, and mGFR by 4-hour plasma iohexol clearance as part of routine clinical care. We defined eGFR to be accurate in assessing mGFR if it was within ± 5 mL/min/1.73m2 of the mGFR. Results: See Table 1 Conclusion: Availability of mGFR in clinical practice is essential for interpretation of eGFR, when an accurate assessment is needed. Cystatin C eGFR’s should not automatically considered to be an mGFR replacement.Table 1: The clinical and laboratory characteristics of three patients with simultaneous eGFR and mGFR