Chronic kidney disease (CKD) is a major global public health problem [1] and mortality is increasing [2] . A study carried out in Burkina Faso in the town of Bobo-Dioulasso showed that acute renal failure resulted in a mortality rate of 10.1% [3] . Severe chronic renal failure resulted in a mortality rate of 47.7% during hospitalisation at the Souro Sanou University Hospital (CHUSS) [4] . In view of this high prevalence, early and accurate identification of the clinical stage by estimating glomerular filtration rate (GFR) is a prerequisite for better management of patients with kidney failure. Traditional methods use formulas for estimating GFR that include creatinine, which is a marker physiologically influenced by age, sex, muscle mass and race. In the face of these difficulties, cystatin C, a protein freely filtered by the glomerulus, has emerged as a promising biomarker of renal function.

Cystatin C is a small protein consisting of 120 residues forming a single polypeptide chain [5] . It belongs to the family of type 2 cysteine proteinase inhibitors [5] . Cysteine proteinases irreversibly hydrolyse a peptide bond in an amino acid sequence and play an important role in cell regulation, cell proliferation and adhesion, apoptosis, lipid metabolism and immune response [5] . It is widely distributed and found in most body fluids, including plasma [6] . Its interest in assessing renal function stems from its physical and chemical properties as an endogenous marker. In addition, due to its constant production, it is relatively freely filtered by glomeruli, is reabsorbed and catabolised by proximal renal tubular cells and is not affected by muscle mass [7] , unlike creatinine [8] . Despite its great value in diagnosing kidney damage, some authors have shown that laboratory tests for cystatin C cost around 10 times more than creatinine, which limits its use in routine practice [7] . Furthermore, cystatin C could also have other advantages in a resource-limited setting through the prediction of coronary artery disease independently of its role in estimating renal function [9] - [11] . In Burkina Faso, there are few studies on cystatin C in GFR estimation [12] . With this in mind, we conducted the present study to assess the contribution of cystatin C to GFR estimation in patients with end-stage chronic kidney disease undergoing haemodialysis at the Bobo-Dioulasso CHUSS in Burkina Faso.

In total, our study included 39 patients with CKD undergoing haemodialysis at the SS-TH, whose duration of dialysis ranged from 3 months to 49 months.

The primary objective of this study was to evaluate the contribution of cystatin C to estimating GFR in patients with end-stage renal disease (ESRD) undergoing hemodialysis at the SS-TH in Bobo-Dioulasso, Burkina Faso. The main limitation of the study is the sample size obtained, which does not allow extrapolation of the results to the entire population of ESRD patients undergoing hemodialysis. Similarly, our limited reagent resources and the cross-sectional nature of the study could lead to various biases (selection and confounding factors). Despite these limitations, one of the strengths of this study is that it enabled the study of useful biomarkers with predictive value for kidney damage and cardiovascular risk.

Cystatin C is a low molecular weight protein of approximately 13 kDa that is constantly produced by all nucleated cells, filtered freely in the glomerulus and metabolised in the proximal tubule [15] . Its metabolism in humans appears to be characterised by constant synthesis independent of nycthemeral variations, significant glomerular filtration and total renal elimination in the proximal tubules. These aspects explain the interest in measuring it in renal disorders. Traditionally, creatinine has been the most widely used marker in laboratories for assessing renal function, because it is easier to measure (Jaffé and enzymatic method) than cystatin C. A study conducted by the General Chemistry Survey 2019 of the College of American Pathologists indicated that only 7% of US clinical laboratories offered cystatin C tests and that more than 90% of them referred the tests to external commercial laboratories [16] . In Burkina Faso, cystatin C does not appear to be widely tested in medical laboratories. A single study by Da et al. on the prevalence of chronic kidney disease in retired people in the town of Bobo-Dioulasso used a GFR calculation based on cystatin C [12] .

Given the limitations of creatinine measurement, particularly because of its influence on muscle mass, cystatin C measurement offers several advantages. Our study showed no significant correlation between serum cystatin C and anthropometric parameters (weight, height and BMI), patient age, number of years on dialysis and traditional markers of renal function (urea, enzymatic creatinine and Jaffet) in end-stage CKD patients on dialysis. In fact, several studies have shown that cystatin C is little influenced by factors such as muscle mass, age, sex and diet, making it a much more advantageous biomarker than creatinine for investigating renal function [17] [18] .

In our study, a positive and significant correlation was observed between cystatin C and markers such as serum albumin (r = 0.574, p = 0.0001) and transthyretin (r = 0.572, p = 0.0001). These results suggest that serum albumin and transthyretin concentrations increased at the same time as serum cystatin C concentrations increased. This could be explained by the possible complications of end-stage renal disease under dialysis, where renal clearance is reduced, leading to retention of serum proteins such as albumin, transthyretin and cystatin C.

In our study, a negative and significant correlation was observed between cystatin C and DFG-CKD-EPI-CystC values obtained using cystatin C (r = −0.829, p-value = 0.000) and that coupling cystatin C and serum creatinine (r = −0.470, p-value = 0.002). These data suggest that a fall in GFR-CKD-EPI-CystC and GFR-CKD-EPI-Creat-CystC values is associated with an increase in serum cystatin C concentrations. Thus, the measurement of cystatin C and its inclusion in the DFG-CKD-EPI formulae alone or combined with creatinine are consistent with the fact that cystatin C is an indicator of the deterioration of renal function. Indeed, several authors have shown that patients with advanced CKD (G3 - G5) have higher serum cystatin C than patients with milder CKD (G1 - G2) [16] [19] .

From the oldest equations, such as Cockcroft & Gault, through MDRD, to the most recent with the Chronic Kidney Disease Epidemiology Consortium (CKD-EPI), the choice of eGFR equation is often not straightforward for the black African population. Indeed, these equations are often obtained on the basis of epidemiological surveys that do not report sufficient data for black subjects due to the lower representation of blacks in European countries. However, scientific societies such as the European Federation of Clinical Chemistry and Laboratory Medicine maintain their support for the CKD-EPI 2009 equation and also recommend increased use of cystatin C due to its independence from race, aiming for more accurate estimates of GFR in various populations [20] [21] . Black subjects tend to have higher muscle mass than Caucasians [22] . Creatinine, being the muscle breakdown product of creatine, is likely to be underestimated in Caucasian subjects [22] .

The 2009 CKD-EPI creatinine equation includes gender, age, creatinine and race [14] [23] . On the other hand, the 2021 CKD-EPI creatinine equation includes age, sex and creatinine, but excludes race [23] [24] . As a result, to avoid over- or underestimating GFRs, several scientific societies are tending to use the CKD-EPI equations published in 2012, which take into account cystatin C alone and in combination with creatinine [14] [25] .

In our study, all patients had end-stage hemodialysis and were clinically classified as stage 5 CKD. The mixed CKD-EPI formula showed 100% of patients in stage 5 (GFR < 15 ml/min/1.73m²), whereas the other two, which took creatinine and cystatin C alone into account, excluded one patient outside this stage. The mixed CKD-EPI formula therefore appeared to be more accurate than that using creatinine and cystatin C alone. The same observation was made by several authors, for whom the bias with the combined creatinine-cystatin C formula was lower than with equations taking creatinine and cystatin C alone into account [18] [24] . The combined formula was therefore more accurate and resulted in a more precise classification of GFR measured as less than 60 ml/min/1.73m² [18] [24] .

Overall, this mixed CKD-EPI-Creat-CystC formula is tending to be proposed by several scientific societies for its accuracy in confirming the diagnosis and classification of CKD, despite the associated costs. One study shows that the cystatin C test costs around 10 times more than the creatinine test, which limits its use in routine practice [8] .

Our study shows that haemodialysis CKD patients with hypertension had significantly higher mean GFR-CKD-EPI values combining creatinine and cystatin C compared with patients without hypertension (CKD-EPI-Creat-CystC with hypertension = 5.72 ± 2.30 mL/min/1.73m² vs CKD-EPI-Creat-CystC without hypertension = 4.16 ± 0.75 mL/min/1.73m², p-value = 0.004). This increase in mixed CKD-EPI in hypertensive patients could be explained by a pronounced deterioration in renal function and blood pressure regulation. For example, some authors described that serum cystatin C levels in patients with uncontrolled hypertension were higher than those in patients with controlled hypertension and no hypertension, although all groups had normal serum creatinine concentrations [26] . These data suggest that the increase in cystatin C associated with uncontrolled hypertension may be manifestations of renal microvascular damage [26] - [29] . In addition, several studies have shown that cGFR and eGFRcys-creat below 85 mL/min/1.73m² were associated with an increased risk of death from any cause and cardiovascular disease [8] [30] - [32] .

In our study of end-stage renal disease patients on haemodialysis, we found that serum cystatin C concentrations correlated with those of biomarkers such as albumin and transthyretin. For estimating GFR, the combined creatinine-cystatin C formula was more accurate in diagnosing the clinical stage and led to a more precise classification of GFR in chronic kidney disease. Also, the mean values of CKD-EPI combining creatinine and cystatin C were significantly elevated in patients with hypertension, suggesting its value in predicting cardiovascular risk.

This study did not receive any specific funding from public, commercial, or nonprofit funding agencies.

The completion of this work has benefited from the invaluable support of many people, including our elders through their guidance (Dr. F. Soudre, Dr. A. Kiba, Dr. R. Karfo, Dr. O. Da) and our mentors through their invaluable advice (Prof. Elie Kabre and Prof. Jean Sakande). I would like to express my deepest gratitude to them all.

We would also like to thank the patients from the nephrology-dialysis department of the SS-TH who agreed to take part in the study.