# Combined Creatinine and Cystatin C Equations Improve Estimation of Glomerular Filtration Rate in Kidney Transplant Recipients

## TL;DR

Combining creatinine and cystatin C equations provides more accurate GFR estimates in kidney transplant patients compared to using either marker alone.

## Contribution

This study demonstrates that combined creatinine and cystatin C equations outperform individual methods in estimating GFR in kidney transplant recipients.

## Key findings

- Combined creatinine/cystatin C equations showed higher P30 values (85.5–87.6%) compared to creatinine (75.2–81.4%) or cystatin C (83.1–84.3%) equations.
- The CKD-EPI combined 2021 equation achieved the highest agreement (79.3%) in chronic kidney disease staging.
- All equations had low median bias and significant correlation with measured GFR (R² = 0.672–0.745).

## Abstract

Glomerular filtration rate (GFR) is a crucial parameter in post-transplant follow-up (PTF). CKD-EPI 2009 creatinine-based equation remains the most used estimated GFR (eGFR) and only few data are available on the other equations, based on creatinine, cystatin C or their combination. We evaluated 10 GFR estimation equations on 242 kidney-transplant recipient patients having measured GFR (mGFR) determination (urinary clearance of 99mTc-DTPA) with simultaneous plasma enzymatic creatinine and serum cystatin C (immunoturbidimetry or immunonephelemetry) assessments. Five creatinine (MDRD 2006, CKD-EPI 2009 and 2021, EKFC 2021, KRS 2023), two cystatin C (CKD-EPI 2012, EKFC 2023) and three combined eGFR (CKD-EPI 2012 and 2021, combined EKFC) were evaluated. All equations were significantly correlated with mGFR (R² = 0.672–0.745) with a low median bias (+4.2 to −1.1 mL/min/1.73 m²). Chronic kidney disease staging agreements were all above 68% (maximum: 79.3% for CKD-EPI comb 2021). Percentages of eGFR comprised in between 30% of the mGFR ranged from 85.5% to 87.6% (combined equations), from 83.1% to 84.3% (cystatin C equations) and from 75.2% to 81.4% (creatinine equations). Combined creatinine/cystatin C eGFR equations with a P30 value greater to 85% of transplant recipients appeared closer to mGFR than cystatin C or creatinine eGFR.

Comparison of methods for estimating glomerular filtration rate (GFR) in kidney transplant recipients, showcasing equations using creatinine, cystatin C, and a combination of both. The chart highlights the performance of these methods against measured GFR using urinary clearance of 99mTc-DTPA across 242 kidney transplant recipients. Median bias and concordance in CKD classification and mean P30 values are presented, demonstrating the combined method’s superiority. Various GFR equations from multiple years are listed.