Serum myo-inositol and valine improve metabolomic-based estimated glomerular filtration rate among kidney transplant recipients

Background: Close monitoring of glomerular filtration rate (GFR) is essential for the management of patients post kidney transplantation. Measured GFR (mGFR), the gold standard, is not readily accessible in most centers. Furthermore, the performance of new estimated GFR (eGFR) equations based upon creatinine and/or cystatin C have not been validated in kidney transplant patients. Here we evaluate a recently published eGFR equation using cystatin C, creatinine, myo-inositol and valine as measured by nuclear magnetic resonance (eGFR_NMR). Methods: Residual sera was obtained from a cohort of patients with clinically ordered iothalamate renal clearance mGFR (n = 602). Kidney transplant recipients accounted for 220 (37%) of participants. Results: Compared to mGFR, there was no significant bias for eGFRcr or eGFR_NMR, while eGFRcr-cys significantly underestimated mGFR. P₃₀ values were similar for all eGFR. P₁₅ was significantly higher for eGFR_NMR compared to eGFRcr, while the P₁₅ for eGFRcr-cys only improved among patients without a kidney transplant. Agreement with mGFR CKD stages of <15, 30, 45, 60, and 90 ml/min/1.73 m² was identical for eGFRcr and eGFRcr-cys (61.8%, both cases) while eGFR_NMR was significantly higher (66.4%) among patients with a kidney transplant. Conclusion: The 2021 CKD-EPI eGFRcr and eGFRcr-cys have similar bias, P₁₅, and agreement while eGFR_NMR more closely matched mGFR with the strongest improvement among kidney transplant recipients.