Validation of the European Kidney Function Consortium Equation in Chinese Adult Population: An Equation Standing on the Shoulders of Predecessors

Recommendations for European laboratories based on the KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease

The 2024 Kidney Disease: Improving Global Outcomes (KDIGO) guidelines for chronic kidney disease (CKD) evaluation and management bring important updates, particularly for European laboratories. These guidelines emphasize the need for harmonization in CKD testing, promoting the use of regional equations. In Europe, the European Kidney Function Consortium (EKFC) equation is particularly suited for European populations, particularly compared to the CKD-EPI 2021 race-free equation. A significant focus is placed on the combined use of creatinine and cystatin C to estimate glomerular filtration rate (eGFRcr-cys), improving diagnostic accuracy. In situations where eGFR may be inaccurate or clinically insufficient, the guidelines encourage the use of measured GFR (mGFR) through exogenous markers like iohexol. These guidelines emphasize the need to standardize creatinine and cystatin C measurements, ensure traceability to international reference materials, and adopt harmonized reporting practices. The recommendations also highlight the importance of incorporating risk prediction models, such as the Kidney Failure Risk Equation (KFRE), into routine clinical practice to better tailor patient care. This article provides a European perspective on how these KDIGO updates should be implemented in clinical laboratories to enhance CKD diagnosis and management, ensuring consistency across the continent.

Which is the best glomerular filtration marker: Creatinine, cystatin C or both?

BACKGROUND

The glomerular filtration rate (GFR) is estimated by the serum or plasma concentration of creatinine and/or cystatin C using equations that include demographic data. The equations worldwide most widely used are those of the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) consortium and updated in 2021 to remove the Afro-American racial correction factor. In 2021 and then in 2023, the European Kidney Function Consortium also developed equations based on creatinine and cystatin C, usable across the full age spectrum, and constructed by including the Q value (i.e. the median creatinine or cystatin C in healthy men and women, which is customizable for specific populations).

METHODS

The aim of this narrative review is to examine the strengths and weaknesses of each biomarker.

RESULTS

Both biomarkers have non-GFR determinants, namely muscle mass, protein intake and tubular secretion for creatinine; dysthyroidism and systemic corticosteroids for cystatin C, as well as other more debated determinants (diabetes, obesity, proteinuria, inflammatory syndrome). These non-GFR determinants are the reason why no equation based on a single endogenous biomarker has an accuracy within 30% greater than 90% over the entire age spectrum (in at least one patient in 10, estimated GFR is at least 30% higher or at least 30% lower than the measured GFR).

CONCLUSION

Equations combining the two biomarkers provide a better estimate of GFR, particularly in the subgroup of patients whose estimates based on each of the biomarkers are highly discordant. These patients must also be identified as being at increased risk of morbidity, particularly cardiovascular, and mortality.

Estimating glomerular filtration in young people

Background

Creatinine-based equations are the most used to estimate glomerular filtration rate (eGFR). The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), the re-expressed Lund-Malmö Revised (r-LMR) and the European Kidney Function Consortium (EKFC) equations are the most validated. The EKFC and r-LMR equations have been suggested to have better performances in young adults, but this is debated.

Methods

We collected data (GFR) measured by clearance of an exogenous marker (reference method), serum creatinine, age and sex from 2366 young adults (aged between 18 and 25 years) both from Europe and the USA.

Results

In the European cohorts (n = 1892), the bias (in mL/min/1.73 m²) was systematically better for the EKFC and r-LMR equations compared with the CKD-EPI equation [2.28, 95% confidence interval (1.59; 2.91), -2.50 (-3.85; -1.76), 17.41 (16.49; 18.47), respectively]. The percentage of estimated GFR within 30% of measured GFR (P30) was also better for EKFC and r-LMR equations compared with the CKD-EPI equation [84.4% (82.8; 86.0), 87.2% (85.7; 88.7) and 65.4% (63.3; 67.6), respectively]. In the US cohorts (n = 474), the bias for the EKFC and r-LMR equations was better than for the CKD-EPI equation in the non-Black population [0.97 (-1.69; 3.06), -2.62 (-5.14; -1.43) and 7.74 (5.97; 9.63), respectively], whereas the bias was similar in Black US individuals. P30 results were not different between the three equations in US cohorts. Analyses in sub-populations confirmed these results, except in individuals with high GFR levels (GFR ≥120 mL/min/1.73 m²) for whom the CKD-EPI equation might have a lower bias.

Conclusions

We demonstrated that both the EKFC and r-LMR creatinine-based equations have a better performance than the CKD-EPI equation in a young population. The only exception might be in patients with hyperfiltration.

Data mining of reference intervals for serum creatinine: an improvement in glomerular filtration rate estimating equations based on Q-values

OBJECTIVES

Glomerular filtration rate (GFR) estimating equations based on rescaled serum creatinine (SCr/Q) have shown better performance, where Q represents the median SCr for age- and sex-specific healthy populations. However, there remains a scarcity of investigations in China to determine this value. We aimed to develop Chinese age- and sex-specific reference intervals (RIs) and Q-values for SCr and to validate the equations incorporating new Q-values.

METHODS

We included 117,345 adults from five centers for establishing RIs and Q-values, and 3,692 participants with reference GFR (rGFR, 99mTc-DTPA renal dynamic imaging measurement) for validation. Appropriate age partitioning was determined using the decision tree method. Lower and upper reference limits and medians were calculated using the refineR algorithm, and Q-values were determined accordingly. We evaluated the full age spectrum (FAS) and European Kidney Function Consortium (EKFC) equations incorporating different Q-values considering bias, precision (interquartile range, IQR), and accuracy (percentage of estimates within ±20 % [P20] and ±30 % [P30] of rGFR).

RESULTS

RIs for males were: 18-79 years, 55.53-92.50 μmol/L; ≥80 years, 54.41-96.43 μmol/L. RIs for females were: 18-59 years, 40.42-69.73 μmol/L; 60-79 years, 41.16-73.69 μmol/L; ≥80 years, 46.50-73.20 μmol/L. Q-values were set at 73.82 μmol/L (0.84 mg/dL) for males and 53.80 μmol/L (0.61 mg/dL) for females. After validation, we found that the adjusted equations exhibit less bias, improved precision and accuracy, and increased agreement of GFR categories.

CONCLUSIONS

We determined Chinese age- and sex-specific RIs and Q-values for SCr. The adjustable Q-values provide an effective alternative to obtain valid equations for estimating GFR.

Editorial: The evolution of glomerular filtration rate estimation: Embracing the new EKFC equation

Addressing the accurate staging of chronic kidney disease (CKD) represents a formidable challenge worldwide, resonating deeply within Ukraine's healthcare landscape. The intricacies of estimating CKD stages often result in the dilemma of its under- or overestimation, magnifying the urgency for precise assessment methods [1, 2]. The estimation of glomerular filtration rate (GFR) is a cornerstone in the assessment of kidney function, guiding diagnosis, management, and therapeutic decisions in patients with kidney disease. Traditional approaches, such as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, though widely utilized, grapple with inherent limitations, potentially compromising diagnostic accuracy [3]. To confront this critical issue, the European Kidney Function Consortium (EKFC) has introduced new equations for GFR estimation using both creatinine and cystatin C, which promise to enhance the precision and applicability of kidney function assessment. Moreover, a recent KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease has recommended using the combination of creatinine and cystatin C (creatinine and cystatin C–based estimated glomerular filtration rate [eGFRcr-cys]) for assessment GFR category (1B) [4]. However, the successful integration of the EKFC equation into clinical practice necessitates widespread awareness and adoption among healthcare professionals. This editorial aims to introduce the Ukrainian nephrology community, as well as a broad readership of the journal, to this innovative approach to kidney health assessment. The limitations of creatinine-based GFR estimation. Serum creatinine, a byproduct of muscle metabolism, has been the backbone of GFR estimation for decades. However, its utility is marred by significant limitations. Creatinine production varies with muscle mass, diet, and other physiological processes, leading to inaccuracies in GFR estimation, especially in individuals with atypical body compositions or dietary habits [5, 6]. Creatinine-based GFR estimation can also be less accurate in specific populations, such as the elderly, individuals with extreme body compositions, and those with acute kidney injury [3]. In these cases, the equations may not accurately reflect kidney function, leading to potential misclassification of CKD stages. Moreover, creatinine clearance exceeds GFR because creatinine is not only filtered by the glomeruli but also secreted by the proximal tubule. This can lead to an overestimation of GFR. Conversely, in conditions where muscle mass is reduced, such as in elderly or malnourished individuals, GFR may be underestimated [3, 5]. Lastly, some medications can inhibit tubular secretion of creatinine, thereby increasing serum creatinine levels without a corresponding decrease in GFR, leading to an overestimation of CKD severity [7]. The role of cystatin C in enhancing GFR estimation. Cystatin C, a low molecular weight protein produced at a constant rate by all nucleated cells, offers a promising alternative or complementary biomarker for GFR estimation [8]. One of the key advantages of cystatin C is its independence from muscle mass, making it particularly valuable in populations where muscle mass may vary significantly, such as the elderly, individuals with malnutrition, or those with muscle wasting conditions [9]. This independence from muscle mass confers cystatin C with greater accuracy in estimating GFR, especially in patients with early-stage kidney disease where changes in muscle mass may not yet significantly affect creatinine levels [8, 9].  Studies have demonstrated that cystatin C has a higher diagnostic sensitivity for abnormal GFR compared to creatinine [10]. This means that cystatin C is more likely to identify patients with reduced kidney function. Introducing the EKFC equation. The EKFC has developed a new equation that incorporates both serum creatinine and cystatin C levels, aiming to overcome the limitations of previous formulas. This innovative approach is grounded in extensive research and validation across various populations [11–13]. The EKFC equation is race- and sex-free, addressing ethical concerns associated with previous equations and promoting a more inclusive and equitable assessment of kidney function. By combining both markers, the EKFC equation offers a more reliable estimate of GFR, particularly in individuals where discrepancies between creatinine-based and cystatin C-based estimates exist. This approach aligns with the principles of personalized medicine, emphasizing tailored healthcare interventions based on individual patient characteristics. However, the successful integration of the EKFC equation into clinical practice requires widespread awareness and adoption among healthcare professionals. To facilitate this, the EKFC has made a calculator available online at ekfccalculator.pages.dev, and there are ongoing efforts to develop additional tools to improve the implementation of these equations. These tools are crucial for enabling healthcare providers to easily apply the EKFC equation in their clinical practice, ensuring that patients benefit from the most accurate and equitable assessment of kidney function available. In conclusion, the development of the EKFC equation marks a pivotal moment in the evolution of GFR estimation. As the medical community continues to embrace the EKFC equation, it is crucial to familiarize healthcare providers with its application and implications. We sincerely believe that the evidence, as outlined above, and this editorial support the widespread adoption of the EKFC eGFR equations in Ukraine.

Performance of the European Kidney Function Consortium (EKFC) creatinine-based equation in United States cohorts

Estimating glomerular filtration rate (GFR) is important in daily practice to assess kidney function and adapting the best clinical care of patients with and without chronic kidney disease. The new creatinine-based European Kidney Function Consortium (EKFC) equation is used to estimate GFR. This equation was developed and validated mainly in European individuals and based on a rescaled creatinine, with the rescaling factor (Q-value) defined as the median normal value of serum creatinine in a given population. The validation was limited in Non-Black Americans and absent in Black Americans. Here, our cross-sectional analysis included 12,854 participants from nine studies encompassing large numbers of both non-Black and Black Americans with measured GFR by clearance of an exogenous marker (reference method), serum creatinine, age, sex, and self-reported race available. Two strategies were considered with population-specific Q-values in Black and non-Black men and women (EKFCPS) or a race-free Q-value (EKFCRF). In the whole population, only the EKFCPS equation showed no statistical median bias (0.14, 95% confidence interval [-0.07; 0.35] mL/min/1.73m2), and the bias for the EKFCRF (0.74, [0.51; 0.94] mL/min/1.73m2) was closer to zero than that for the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI2021) equation (1.22, [0.99; 1.47]) mL/min/1.73m2]. The percentage of estimated GFR within 30% of measured GFR was similar for CKD-EPI2021 (79.2% [78.5%; 79.9%]) and EKFCRF (80.1% [79.4%; 80.7%]), but improved for the EKFCPS equation (81.1% [80.5%; 81.8%]). Thus, our EKFC equations can be used to estimate GFR in the United States incorporating either self-reported race or unknown race at the patient's discretion per hospital registration records.

Glomerular Filtration Rate Estimation in Adults: Myths and Promises

BACKGROUND

In daily practice, glomerular filtration rate (GFR) is estimated with equations including renal biomarkers. Among these biomarkers, serum creatinine remains the most used. However, there are many limitations with serum creatinine, which we will discuss in the current review. We will also discuss how creatinine-based equations have been developed and what we can expect from them in terms of performance to estimate GFR.

SUMMARY

Different creatinine-based equations have been proposed. We will show the advantages of the recent European Kidney Function Consortium equation. This equation can be used in children and adults. This equation can also be used with some flexibility in different populations.

KEY MESSAGES

GFR is estimated by creatinine-based equations, but the most important for nephrologists is probably to know the limitations of these equations.