Serum levels of beta-trace protein and glomerular filtration rate--preliminary results

Assessment of glomerular filtration rate in patients with cirrhosis: Available tools and perspectives

Renal dysfunction often complicates the course of liver disease, resulting in higher morbidity and mortality. The accurate assessment of kidney function in these patients is essential to early identify, stage and treat renal impairment as well as to better predict the prognosis, prioritize the patients for liver transplantation and decide whether to opt for simultaneous liver-kidney transplants. This review analyses the available tools for direct or indirect assessment of glomerular filtration rate, focusing on the flaws and strengths of each method in the specific setting of cirrhosis. The aim is to deliver a clear-cut view on this complex issue, trying to point out which strategies to prefer in this context, especially in the peculiar setting of liver transplantation. Moreover, a glance is given at future promising tools for glomerular filtration rate assessment, including new biomarkers and new equations specifically modelled for the cirrhotic population.

Lifting the Veil: Characteristics, Clinical Significance, and Application of β-2-Microglobulin as Biomarkers and Its Detection with Biosensors

Because β-2-microglobulin (β₂M) is a surface protein that is present on most nucleated cells, it plays a key role in the human immune system and the kidney glomeruli to regulate homeostasis. The primary clinical significance of β₂M is in dialysis-related amyloidosis, a complication of end-stage renal disease caused by a gradual accumulation of β₂M in the blood. Therefore, the function of β₂M in kidney-related diseases has been extensively studied to evaluate its glomerular and tubular functions. Because increased β₂M shedding due to rapid cell turnover may indicate other underlying medical conditions, the possibility to use β₂M as a versatile biomarker rose in prominence across multiple disciplines for various applications. Therefore, this work has reviewed the recent use of β₂M to detect various diseases and its progress as a biomarker. While the use of state-of-the-art β₂M detection requires sophisticated tools, high maintenance, and labor cost, this work also has reported the use of biosensor to quantify β₂M over the past decade. It is hoped that a portable and highly efficient β₂M biosensor device will soon be incorporated in point-of-care testing to provide safe, rapid, and reliable test results.

A Review of Specific Biomarkers of Chronic Renal Injury and Their Potential Application in Nonclinical Safety Assessment Studies

A host of novel renal biomarkers have been developed over the past few decades which have enhanced monitoring of renal disease and drug-induced kidney injury in both preclinical studies and in humans. Since chronic kidney disease (CKD) and acute kidney injury (AKI) share similar underlying mechanisms and the tubulointerstitial compartment has a functional role in the progression of CKD, urinary biomarkers of AKI may provide predictive information in chronic renal disease. Numerous studies have explored whether the recent AKI biomarkers could improve upon the standard clinical biomarkers, estimated glomerular filtration rate (eGFR), and urinary albumin to creatinine ratio, for predicting outcomes in CKD patients. This review is an introduction to alternative assays that can be utilized in chronic (>3 months duration) nonclinical safety studies to provide information on renal dysfunction and to demonstrate specific situations where these assays could be utilized in nonclinical drug development. Novel biomarkers such as symmetrical dimethyl arginine, dickkopf homolog 3, and cystatin C predict chronic renal injury in animals, act as surrogates for GFR, and may predict changes in GFR in patients over time, ultimately providing a bridge from preclinical to clinical renal monitoring.

Utility of serum beta-trace protein as a tool for estimating residual kidney function in peritoneal dialysis patients

BACKGROUND

Beta-trace protein (BTP) is a novel marker for residual kidney function (RKF) without need for urinary collection. We aimed to examine its utility as a tool for estimating RKF in incident peritoneal dialysis (PD) patients.

METHODS

This was a post hoc analysis of incident PD patients from the balANZ trial cohort. The outcomes evaluated were trends of serum BTP concentration with time, factors associated with change in BTP using mixed-effect multilevel linear regression and correlation of BTP with mean urinary urea and creatinine clearances (measured glomerular filtration rate (GFR)). Performances of two BTP-derived equations (Shafi-Eqn and Steubl-Eqn) to estimate GFR were evaluated by reporting bias (median difference between estimated and measured GFR), precision (interquartile range of median bias), accuracy (±2 mL/min of measured GFR) and P30 (percentage estimates within 30% of measured GFR) with confidence intervals (CIs) generated by bootstrapping 2000 replicates. The agreement between BTP-estimated GFR and measured GFR was also plotted graphically on Bland-Altman analysis.

RESULTS

The study included 161 PD patients. BTP concentration increased with dialysis vintage and was inversely correlated with measured GFR (r = -0.64). Larger increases in BTP were associated with longer PD vintage and higher dialysate glucose exposure. Biases of BTP-estimated GFRs (Shafi-Eqn and Steubl-Eqn) were 1.2 mL/min/1.73 m² (95% CI 1.0-1.3 mL/min/1.73 m²) and 0.4 mL/min/1.73 m² (95% CI 0.2-0.6 mL/min/1.73 m²), respectively. Both BTP-estimated GFRs had poor precision (3.2 mL/min/1.73 m² (95% CI 2.9-3.5 mL/min/1.73 m²) and 2.8 mL/min/1.73 m² (95% CI 2.5-3.2 mL/min/1.73 m²), respectively) and accuracy of estimates (55% (95% CI 52-60%) and 59% (95% CI 55-63%), respectively). The mean difference of BTP-estimated GFR (Shafi-Eqn and Steubl-Eqn) and measured GFR were -1.14 mL/min/1.73 m² and -0.42 mL/min/1.73 m², respectively, with large limit of agreement on Bland-Altman plot.

CONCLUSIONS

Serum BTP level was inversely related to RKF but neither BTP-estimated GFR equations were sufficiently accurate for routine use in PD patients.

Age appropriate reference intervals for eight kidney function and injury markers in infants, children and adolescents

Objectives

The use of kidney function and injury markers for early detection of drug-related glomerular or tubular kidney injury in infants, children and adolescents requires age-specific data on reference intervals in a pediatric healthy population. This study characterizes serum values for eight kidney function and injury markers in healthy infants, children and adolescents.

Methods

A single center prospective observational study was conducted between December 2018 and June 2019. Serum samples from 142 healthy infants, children and adolescents aged between 0 and ≤15 years were collected. Statistical analyses for eight markers (albumin (ALB), β2-microglobulin (B2M), β-trace protein (BTP), creatinine (SCR), cystatin C (CYSC), kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), uromodulin (URO)) were performed to obtain reference intervals and associations with age, sex and weight were investigated (Pearson correlation, linear and piecewise regression).

Results

ALB and SCR increased with age (p<0.01), whereas B2M, BTP and KIM-1 values decreased with advancing age (p<0.05) in this healthy pediatric study population. CYSC showed dependency on sex (lower concentration in females) and decreased with age until reaching approximately 1.8 years; thereafter an increase with age was seen. NGAL and URO did not show any age-dependency.

Conclusions

This study provides age appropriate reference intervals for key serum kidney function and injury markers determined in healthy infants, children and adolescents. Such reference intervals facilitate the interpretation of changes in kidney function and injury markers in daily practice, and allow early detection of glomerular and tubular injury in infancy, childhood and adolescence.

Does Beta-Trace Protein (BTP) Outperform Cystatin C as a Diagnostic Marker of Acute Kidney Injury Complicating the Early Phase of Acute Pancreatitis?

Acute pancreatitis (AP) belongs to the commonest acute gastrointestinal conditions requiring hospitalization. Acute kidney injury (AKI) often complicates moderately severe and severe AP, leading to increased mortality. Among the laboratory markers proposed for early diagnosis of AKI, few have been studied in AP, including cystatin C and neutrophil gelatinase-associated lipocalin (NGAL). Beta-trace protein (BTP), a low-molecular-weight glycoprotein proposed as an early marker of decreased glomerular filtration, has never been studied in AP. We investigated the diagnostic usefulness of serum BTP for early diagnosis of AKI complicating AP in comparison to previously studied markers. BTP was measured in serum samples collected over the first three days of hospital stay from 73 adult patients admitted within 24 h of mild to severe AP. Thirteen patients (18%) developed AKI in the early phase of AP. Serum BTP was higher in patients who developed AKI, starting from the first day of hospitalization. Strong correlations were observed between BTP and serum cystatin C but not serum or urine NGAL. On admission, BTP positively correlated with endothelial dysfunction. The diagnostic usefulness of BTP for AKI was similar to cystatin C and lower than NGAL. Increased BTP is an early predictor of AKI complicating AP. However, it does not outperform cystatin C or NGAL.

Proteomics and Metabolomics for AKI Diagnosis

Acute kidney injury (AKI) is a severe and frequent condition in hospitalized patients. Currently, no efficient therapy of AKI is available. Therefore, efforts focus on early prevention and potentially early initiation of renal replacement therapy to improve the outcome in AKI. The detection of AKI in hospitalized patients implies the need for early, accurate, robust, and easily accessible biomarkers of AKI evolution and outcome prediction because only a narrow window exists to implement the earlier-described measures. Even more challenging is the multifactorial origin of AKI and the fact that the changes of molecular expression induced by AKI are difficult to distinguish from those of the diseases associated or causing AKI as shock or sepsis. During the past decade, a considerable number of protein biomarkers for AKI have been described and we expect from recent advances in the field of omics technologies that this number will increase further in the future and be extended to other sorts of biomolecules, such as RNAs, lipids, and metabolites. However, most of these biomarkers are poorly defined by their AKI-associated molecular context. In this review, we describe the state-of-the-art tissue and biofluid proteomic and metabolomic technologies and new bioinformatics approaches for proteomic and metabolomic pathway and molecular interaction analysis. In the second part of the review, we focus on AKI-associated proteomic and metabolomic biomarkers and briefly outline their pathophysiological context in AKI.

Novel glomerular filtration markers

Chronic kidney disease is currently assessed by estimated glomerular filtration rate, a mathematical construct based on creatinine or creatinine and cystatin concentration. Creatinine-based equations have improved with standardization efforts and the Modification of Diet in Renal Disease Study (MDRD) and CKD-Epidemiology Collaboration Study (CKD-EPI). Because the measurement of creatinine is subject to interference from non-GFR determinants, alternative markers have long been sought. These have included cystatin C and low molecular weight proteins like β2-microglobulin and beta trace protein. Tubular disease often occurs before glomerular filtration is impaired and investigators have investigated the excretion of other low molecular weight proteins such as Neutrophil Gelatinase-Associated Lipocalin (NGAL) and Kidney Injury Molecule-1 and N-acetyl-β-d-glucosaminidase. While preliminary, there is some evidence linking these analytes with GFR, disease stage and mortality. Although asymmetrical dimethyl arginine, an inhibitor of nitric oxide, has been shown to be associated with progression of renal disease, symmetric dimethyl arginine may be a better marker. Recent work has also explored the potential of microRNA (miRNA) analysis and metabolomics studies to further elucidate this complex pathophysiologic disease process. Investigators hope to improve our ability to detect CKD by the use of test panels, i.e., various marker combinations thereof. Unfortunately, most of these markers lack standardization unlike traditional measures that rely on creatinine and cystatin C measurement.

Estimation of GFR in children using rescaled beta-trace protein

INTRODUCTION

Beta-trace protein (BTP) is a low molecular weight protein, produced mainly in the cerebrospinal fluid. It has been proposed as a marker for kidney function. Recently, a new method for GFR estimation using mean normal values to rescale GFR marker concentrations has been described for creatinine and cystatin C, two commonly used endogenous markers for kidney function. The aim of this study is to apply this approach to BTP in children.

METHOD

We retrospectively analyzed serum concentrations of creatinine, cystatin C and BTP measured during inulin clearance tests in children. BTP was measured using a particle-enhanced immunonephelometric assay (Siemens Healthcare). A novel BTP-based eGFR equation was developed using published normal values for children: eGFR_BTP[ml/min/1.73m²] = 107.3/BTP/Q_BTP with Q_BTP = 0.69. Performance of this equation was compared to the established creatinine-based full age spectrum equation FASage and the cystatin C-based FAScys equations as well as the BTP-based Benlamri equation in terms of bias, % prediction error and P₃₀ and P₁₀ accuracy rates.

RESULTS

322 inulin clearance tests were studied. Overall, our novel equation performed comparably to the creatinine-based FASage and the BTP-based Benlamri equations but was less accurate than FAScys (P₃₀: 79.2 vs 86.3%, p = .008). Combining markers significantly enhanced performance compared to the single marker equations, with the exception of FAScys.

CONCLUSION

Rescaled BTP concentrations are a simple method for estimating GFR in children. However, the additional value of BTP for the estimation of GFR compared to rescaled creatinine and cystatin C still remains to be demonstrated.

Rediscovering Beta-2 Microglobulin As a Biomarker across the Spectrum of Kidney Diseases

There is currently an unmet need for better biomarkers across the spectrum of renal diseases. In this paper, we revisit the role of beta-2 microglobulin (β₂M) as a biomarker in patients with chronic kidney disease and end-stage renal disease. Prior to reviewing the numerous clinical studies in the area, we describe the basic biology of β₂M, focusing in particular on its role in maintaining the serum albumin levels and reclaiming the albumin in tubular fluid through the actions of the neonatal Fc receptor. Disorders of abnormal β₂M function arise as a result of altered binding of β₂M to its protein cofactors and the clinical manifestations are exemplified by rare human genetic conditions and mice knockouts. We highlight the utility of β₂M as a predictor of renal function and clinical outcomes in recent large database studies against predictions made by recently developed whole body population kinetic models. Furthermore, we discuss recent animal data suggesting that contrary to textbook dogma urinary β₂M may be a marker for glomerular rather than tubular pathology. We review the existing literature about β₂M as a biomarker in patients receiving renal replacement therapy, with particular emphasis on large outcome trials. We note emerging proteomic data suggesting that β₂M is a promising marker of chronic allograft nephropathy. Finally, we present data about the role of β₂M as a biomarker in a number of non-renal diseases. The goal of this comprehensive review is to direct attention to the multifaceted role of β₂M as a biomarker, and its exciting biology in order to propose the next steps required to bring this recently rediscovered biomarker into the twenty-first century.

β-trace protein is highly removed during haemodialysis with high-flux and super high-flux membranes

BACKGROUND

Serum β-trace protein (βTP, MW 23-29 kDa) is a marker of GFR impairment in renal patients. Recent papers propose to predict residual renal function (RRF) in maintenance haemodialysis (MHD) patients from serum concentrations of βTP and other small proteins, avoiding the collection of urine. Few data are available on the removal of βTP in patients treated with dialysis membranes with different flux characteristics. The aim of this study was to evaluate the effects of haemodialysis with low-flux, high-flux and super high-flux membranes on serum concentrations of ßTP in MHD patients with null RRF.

METHODS

Serum ßTP concentrations were measured before and after the first dialysis of the week in 51 MDH patients treated by low-flux (n = 24), high-flux (n = 17), or super high-flux (n = 10) membranes. The removal of β2-microglobulin (β2M, MW 11.8), cystatin C (Cys, MW 13.3), urea and creatinine was also analyzed.

RESULTS

Low-flux membranes did not remove βTP, β2M and Cys whose concentration increased at the end of dialysis. High-flux membrane removed more efficiently β2M and Cys than ßTP. Super high-flux membrane had the highest efficiency to remove ßTP: mean reduction ratio (RR) 53.4%, similar to β2M (59.5%), and Cys (62.0%).

CONCLUSIONS

In conclusion, the plasma clearance of small proteins and particularly of βTP is dependent from the permeability of the dialysis membranes Therefore, the reliability of the formulas proposed to predict RRF from serum βTP and other LMWP may be affected by the different permeability of the dialysis membranes.

Urinary β-trace protein: A unique biomarker to screen early glomerular filtration rate impairment

The screening for chronic kidney disease (CKD) patients needs the measurement of serum markers like creatinine. Our previous results indicated that urinary excretion of β-trace protein (BTP), a low-molecular-weight protein (23-29 kDa), is increased in CKD patients from stage 2. The aim of this study was to assess the major determinants of urinary excretion of BTP and to evaluate its feasibility as noninvasive marker of glomerular filtration rate (GFR) impairment.We studied 355 CKD patients (198 males), aged 15 to 83 years, in stable clinical conditions, classified in the different stages of CKD on the basis of GFR (renal clearance of Tc-diethylenetriamine penta-acetic acid). At the same time, we measured serum and urinary creatinine and BTP, and urinary albumin. Urinary excretion of BTP and albumin was expressed as mg/g urinary creatinine. Fractional clearance of BTP was calculated as the ratio of BTP clearance to creatinine clearance (%).Urinary excretion of BTP is mainly determined by its serum concentration and by the level of GFR, and to a lower extent by urinary albumin excretion. In fact, urinary BTP (U-BTP) and fractional clearance of BTP progressively and significantly increased along with the reduction of GFR and the concurrent rise in serum BTP (S-BTP). The relationship of U-BTP with GFR was very similar to that of S-BTP with GFR: U-BTP mirrors S-BTP. The accuracy of U-BTP to screen patients with GFR <90 mL/min/1.73 m was good (area under the curve 0.833), its sensitivity was 76.9%, specificity 80%, and positive predictive value 84.9%. Sensitivity of U-BTP was quite similar to that of S-BTP and serum creatinine.The major determinants of urinary excretion of BTP are S-BTP and GFR. U-BTP may be a suitable noninvasive marker to screen the general population for detection of GFR <90 mL/min/1.73 m.

Applicability of various estimation formulas to assess renal function in Chinese children

BACKGROUND

This study was to evaluate the relative applicability of the most commonly used estimation formulas for renal glomerular filtration rate (GFR) of Chinese children with chronic kidney disease (CKD).

METHODS

One hundred CKD patients of less than 17 years old were divided into two groups by sex which was further categorized into five subgroups based on CKD staging according to the "reference" GFR (rGFR) determined by Tc-99m-DTPA renal dynamic imaging. Four GFR markers including serum cystatin C (CysC), β2-microglobulin, creatinine, and blood urea nitrogen were measured.

RESULTS

Among all four markers, CysC best reflected the extent of glomerular damages for CKD stage 1. The value for estimation of GFR (eGFR) was derived from five different formulas either over-estimated or underestimated GFR as referenced to rGFR, and the extent of deviations was dependent on gender, age and CKD stage. The Counahan-Barratt formula and the Schwartz formula gave the most accurate estimations of GFR for CKD stages 1 and 2-3, respectively regardless of gender and age differences. Receiver operating characteristic analyses indicated that the Counahan-Barratt formula has the highest diagnostic accuracy.

CONCLUSION

The Counahan-Barratt formula provides the best approximation to rGFR, thereby the highest applicability to Chinese children with CKD of different genders, ages and CKD stages.

Estimation of GFR using β-trace protein in children

BACKGROUND AND OBJECTIVES

Sex may affect the performance of small molecular weight proteins as markers of GFR because of differences in fat mass between the two sexes. The hypothesis was that the diagnostic performance of β-trace protein, a novel marker of GFR, would be significantly better in boys than in girls.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

GFR, height, weight, serum creatinine, and β-trace protein were measured in 755 children and adolescents (331 girls) undergoing (99)technetium diethylenetriamine penta-acetic acid renal scans from July of 1999 to July of 2006. Boys and girls were separated into formula generation cohorts (284 boys and 220 girls) and formula validation cohorts (140 boys and 111 girls). GFR-estimating formulas on the basis of β-trace protein, creatinine, and height were derived using stepwise linear regression analysis of log-transformed data. The slope of the regression lines of the sex-specific eGFRs were compared. Bland-Altman analysis was used for testing agreement between (99)technetium diethylenetriamine penta-acetic acid GFR and calculated GFR both with this equation in boys and girls as well as previously established Benlamri, White, and Schwartz formulas.

RESULTS

In the stepwise regression analysis, β-trace protein (R(2)=0.73 for boys and R(2)=0.65 for girls) was more important than creatinine (which increased R(2) to 0.81 for boys and R(2) to 0.75 for girls) and height (which increased R(2) to 0.88 for boys and R(2) to 0.80 for girls) in the data generation groups. GFR can be calculated using the following formulas:[Formula: see text]and[Formula: see text]Bland-Altman analysis showed better performance in boys than in girls. The new formulas performed significantly better than the previous Benlamri, White, and Schwartz formulas with respect to bias, precision, and accuracy.

CONCLUSIONS

Improved and sex-specific formulas for the estimation of GFR in children on the basis of β-trace protein, serum creatinine, and height are now available.

β-Trace protein: a marker of GFR and other biological pathways

β-Trace protein (BTP), also known as lipocalin prostaglandin D2 synthase (L-PGDS; encoded by the PTGDS gene), is a low-molecular-weight glycoprotein and an emerging novel marker of glomerular filtration rate. BTP is an important constituent of cerebral spinal fluid and is found in much lower concentrations in blood. Its serum origin and renal handling remain poorly understood. Unlike serum creatinine, BTP is not physiologically inert. It possesses both ligand-binding and enzymatic properties. BTP catalyzes the conversion of prostaglandin H2 (PGH2) to PGD2. PGD2 is an eicosanoid involved in a variety of important physiologic processes, including platelet aggregation, vasodilation, inflammation, adipogenesis, and bone remodeling. Several studies now have documented BTP's strong association with glomerular filtration rate, end-stage renal disease, cardiovascular disease, and death in a variety of different patient populations. This review provides an overview of the biochemistry, physiology and metabolism, biological functions, and measurement of BTP; summarizes the evidence for BTP as a marker of both kidney function and cardiovascular disease; and then considers the interplay between its biological properties, serum concentration, and patient outcomes.

Beta-trace protein as a marker of GFR--history, indications, and future research

OBJECTIVES

Recent findings suggest that beta-trace protein (BTP), a small molecular weight protein, is at least equal if not superior to serum creatinine as a marker of glomerular filtration rate (GFR), particularly since it is independent from height, gender, age, and muscle mass. The authors sought to summarize knowledge on BTP and its use as a marker of GFR using the most recent literature available.

DESIGN AND METHODS

The authors compiled key articles and all relevant recent literature on this topic. Physical and chemical features of the molecule are described, as well as factors that may affect its expression. The use of BTP in estimating GFR as a whole and in specific patient groups, including pregnant women, neonates and infants, children and adolescents, and patients who have undergone renal transplantation is discussed. The use of BTP as a marker for cardiovascular risk factors is also briefly addressed.

RESULTS

Although its performance in the general population is marginally inferior to cystatin C, studies have suggested that it may be superior in accurately estimating GFR in select patient groups such as pregnant women and neonates.

CONCLUSIONS

This novel marker shows promise, but further research is required to clarify findings from available data.

Comparison of serum concentrations of β-trace protein, β2-microglobulin, cystatin C, and creatinine in the US population

BACKGROUND AND OBJECTIVES

β-trace protein (βTP), β2-microglobulin (β2M), and cystatin C (CysC) have advantages over creatinine for estimating GFR and prognosis. This study compares the distribution of all four markers in the general population and their associations with possible determinants of GFR.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

βTP and β2M were measured in 7596 participants (aged ≥12 years) of the Third National Health and Nutrition Examination Survey (1988-1994). βTP and β2M concentrations and the proportion of persons with elevated (≥99th percentile for young healthy participants) βTP (≥0.81 mg/L), β2M (≥2.80 mg/L), standardized CysC (≥1.03 mg/L), and creatinine (≥1.2 mg/dl for men and ≥1.0 mg/dl for women) were compared across demographic and clinical factors.

RESULTS

Elevated βTP, β2M, and CysC showed stronger associations with age than elevated serum creatinine, the prevalence of elevated levels reaching 47%, 44%, 58%, and 26%, respectively, by age 80 years. βTP, CysC, and creatinine were higher in men but β2M was not associated with sex. Mexican Americans had lower βTP, β2M, CysC, and creatinine compared with non-Hispanic whites. Hypertension and higher C-reactive protein were associated with elevations in all markers, whereas non-Hispanic black race, body mass index, diabetes, smoking status, triglycerides, HDL cholesterol, and education were not associated in a consistent manner across the different markers.

CONCLUSIONS

βTP, β2M, CysC, and creatinine differ in their associations with demographic and clinical factors, suggesting variation in their non-GFR determinants. Future studies should examine these markers with measured GFR to determine their diagnostic and prognostic utility.

Serum β-trace protein and risk of mortality in incident hemodialysis patients

BACKGROUND AND OBJECTIVES

Residual kidney function in dialysis patients is associated with better survival, but there are no simple methods for its assessment. β-Trace protein is a novel endogenous filtration marker of kidney function that is not removed during hemodialysis and may serve as a marker for residual kidney function similar to serum creatinine in patients not on dialysis. The objective of this study was to determine the association of serum β-trace protein with mortality in incident hemodialysis patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Serum β-trace protein was measured in baseline samples from 503 participants of a national prospective cohort study of incident dialysis patients with enrollment during 1995-1998 and follow-up until 2004. Outcomes were all-cause and cardiovascular disease mortality analyzed using Cox regression adjusted for demographic, clinical, and treatment factors.

RESULTS

Serum β-trace protein levels were higher in individuals with no urine output compared with individuals with urine output (9.0±3.5 versus 7.6±3.1 mg/L; P<0.001). There were 321 deaths (159 deaths from cardiovascular disease) during follow-up (median=3.3 years). Higher β-trace protein levels were associated with higher risk of mortality. The adjusted hazard ratio and 95% confidence interval for all-cause mortality per doubling of serum β-trace protein was 1.36 (1.09-1.69). The adjusted hazard ratios (95% confidence intervals) for all-cause mortality in the middle and highest tertiles compared with the lowest tertile were 0.95 (0.69-1.32) and 1.72 (1.25-2.37). Similar results were noted for cardiovascular disease mortality.

CONCLUSIONS

The serum level of β-trace protein is an independent predictor of death and cardiovascular disease mortality in incident hemodialysis patients.

Filtration markers may have prognostic value independent of glomerular filtration rate

Serum levels of creatinine, cystatin C, or β trace protein allow estimation of GFR, but whether these markers contribute additional prognostic information beyond that reflected in GFR is unknown. Here, we analyzed data from the Modification of Diet in Renal Disease study, which provided baseline levels of these markers for 816 participants with a median follow-up of 16.6 years. We examined associations between the reciprocals of these filtration markers and (125)I iothalamate GFR, expressed per SD, with kidney failure and mortality. In univariate analysis, lower GFR and higher levels of each filtration marker associated with a higher risk for all outcomes. After adjustment for GFR in a Cox proportional hazards model, higher creatinine associated with a higher risk for kidney failure but a lower risk for all-cause mortality. Higher cystatin C and β trace protein associated with a higher risk for both kidney failure and all-cause mortality. In models including either cystatin C or β trace protein, the association of GFR with all-cause mortality was no longer significant after the addition of the filtration marker, suggesting the possibility of multicollinearity. In summary, after adjustment for GFR, levels of creatinine, cystatin C, and β trace protein, each remained directly associated with kidney failure but differed with respect to their associations with mortality. These differences may be a result of non-GFR-related associations of filtration markers, residual confounding by GFR, or collinearity between the filtration markers and GFR. β trace protein and cystatin C seem to provide more consistent prognostic information than creatinine.

Comparison of measured GFR, serum creatinine, cystatin C, and beta-trace protein to predict ESRD in African Americans with hypertensive CKD

BACKGROUND

Identification of persons with chronic kidney disease (CKD) who are at highest risk to progress to end-stage renal disease (ESRD) is necessary to reduce the burden of kidney failure. The relative utility of traditional markers of kidney function, including estimated glomerular filtration rate (eGFR) and serum creatinine level, and emerging markers of kidney function, including cystatin C and beta-trace protein (BTP) levels, to predict ESRD and mortality has yet to be established.

STUDY DESIGN

Randomized clinical trial followed by an observational cohort study.

SETTING & PARTICIPANTS

865 African American individuals with hypertensive CKD enrolled in a clinical trial of 2 levels of blood pressure control and 3 different antihypertensive drugs as initial therapy and subsequently followed by an observational cohort study.

PREDICTORS

Quintile of measured GFR (mGFR) by iothalamate clearance, serum creatinine, serum creatinine-based eGFR, cystatin C, and BTP values.

OUTCOMES & MEASUREMENTS

Incidence of ESRD and mortality.

RESULTS

246 participants reached ESRD during a median follow-up of 102 months. The incidence rate of ESRD was higher with higher quintiles of each marker. The association between higher BTP level and ESRD was stronger than those for the other markers, including mGFR. All markers remained significantly associated with ESRD after adjustment for mGFR and relevant covariates (all P < 0.05), with BTP level retaining the strongest association (HR for highest vs lowest quintile, 5.7; 95% CI, 2.2-14.9). Associations with the combined end point of ESRD or mortality (n = 390) were weaker, but remained significant for cystatin C (P = 0.05) and BTP levels (P = 0.004).

LIMITATIONS

The ability of these markers to predict ESRD and mortality in other racial and ethnic groups and in individuals with CKD due to other causes is unknown.

CONCLUSIONS

Plasma BTP and cystatin C levels may be useful adjuncts to serum creatinine level and mGFR in evaluating risk of progression of kidney disease.

Serum and urinary markers of early impairment of GFR in chronic kidney disease patients: diagnostic accuracy of urinary β-trace protein

The screening for chronic kidney diseases (CKD) patients with impaired GFR needs the measurement of serum creatinine (SCr) or cystatin C (SCys). GFR can also be predicted from SCr or SCys with different formulas. The aim of this study, performed in a group of CKD patients with different levels of GFR, was to evaluate the possibility to select the patients with a GFR <90 ml·min(-1)·1.73 m(-2) by means of serum levels and urinary excretion of different low-molecular-weight proteins (LMWP), cystatin C (Cys), β2-microglobulin (β2M), retinol-binding protein (RBP), β-trace protein (BTP), and derived prediction equations for GFR. In the 295 CKD patients (137 women), at all stages of GFR impairment a very high correlation was found between GFR ((99m)Tc-DTPA) and serum Cr, Cys, β2M, and BTP. All these serum markers showed a similar accuracy as indicators of different GFR impairments. RBP had the lowest correlation with GFR and was also significantly less accurate. The different prediction formulas derived from gender, anthropometric data and SCr or S-LMWP had a diagnostic accuracy similar to that of serum Cr, Cys, β2M, and BTP. Urinary albumin was inadequate as an indicator of any level of GFR impairment. Urinary excretion of Cys and β2M increased significantly only in patients with a GFR <30 ml·min(-1)·1.73 m(-2), while urinary BTP increased already at GFR <90 ml·min(-1)·1.73 m(-2). In this selected group of CKD patients, the positive predictive value of urinary BTP for a GFR <90 ml·min(-1)·1.73 m(-2) was 85%, indicating that, in CKD patients, a urine-based test can predict a slight GFR impairment.

Novel algorithm for more accurate calculation of renal function in adults with cancer

BACKGROUND

Cytotoxic agents have a narrow therapeutic window. A high percentage of some of them is renally excreted in unchanged form. Accurate assessment of an individual's glomerular filtration rate (GFR) helps to predict the pharmacokinetic behavior of those drugs more precisely. GFR calculations, however, have their limitations.

OBJECTIVE

To establish a more accurate calculation of renal function over a broad range of constitutive GFR values.

METHODS

Patients with cancer were included in the analysis. Serum levels of cystatin C, creatinine, urea, albumin, and beta-trace protein were measured, and GFR was calculated by 8 mathematical formulas. The results were compared with creatinine clearance (CrCl) calculated from timed urine specimens.

RESULTS

One hundred two patients were evaluated: median age, 57.5 years (range 20-91); females, 52; males, 50; and mean urinary CrCl, 77.0 mL/min. The bias (mean percentage error) was -2% and the precision (mean absolute percentage error) was 23% for the Modification of Diet in Renal Disease (MDRD) estimation of GFR. All equations significantly overestimated CrCl in patients with measured clearance less than 50 mL/min (p < 0.05), with the exception of the modified Salazar-Corcoran formula. All equations underestimated CrCl in patients with measured clearance greater than 100 mL/min. The Wright formula was the least biased and most precise (-5%, 16%, respectively). In patients with measured CrCl 50-100 mL/min, the MDRD calculation had a bias of -4% and a precision of 17%. The Jelliffe and Larsson equations were associated with significant sex bias (p < 0.05).

CONCLUSIONS

These observations suggest that individual GFR values over a broad range cannot be calculated accurately enough with only one selected formula. It may be useful to classify renal function of patients with cancer according to the novel algorithm by using MDRD first and then to subsequently calculate GFR in higher and lower ranges with the Wright and modified Salazar-Corcoran formulas, respectively. This algorithm should be validated using larger numbers of patients.

Biochemistry and Clinical Role of Human Cystatin C

Low molecular-mass plasma proteins play a key role in health and disease. Cystatin C is an endogenous cysteine proteinase inhibitor belonging to the type 2 cystatin superfamily. The mature, active form of human cystatin C is a single non-glycosylated polypeptide chain consisting of 120 amino acid residues, with a molecular mass of 13,343-13,359 Da, and containing four characteristic disulfide-paired cysteine residues. Human cystatin C is encoded by the CST3 gene, ubiquitously expressed at moderate levels. Cystatin C monomer is present in all human body fluids; it is preferentially abundant in cerebrospinal fluid, seminal plasma, and milk. Cystatin C L68Q variant is an amyloid fibril-forming protein with a high tendency to dimerize. It forms self-aggregates with massive amyloid deposits in the brain arteries of young adults, leading to lethal cerebral hemorrhage. The main catabolic site of cystatin C is the kidney: more than 99% of the protein is cleared from the circulation by glomerular ultrafiltration and tubular reabsorption. The diagnostic value of cystatin C as a marker of kidney dysfunction has been extensively investigated in multiple clinical studies on adults, children, and in the elderly. In almost all the clinical studies, cystatin C demonstrated a better diagnostic accuracy than serum creatinine in discriminating normal from impaired kidney function, but controversial results have been obtained by comparing this protein with other indices of kidney disease, especially serum creatinine-based equations. In this review, we present and discuss most of the available data from the literature, critically reviewing conclusions and suggestions for the use of cystatin C in clinical practice. Despite the multitude of clinical data in the literature, cystatin C has not been widely used, perhaps because of a combination of factors, such as a general diffidence among clinicians, the absence of definitive cut-off values, conflicting results in clinical studies, no clear evidence on when and how to request the test, the poor commutability of results, and no accurate examination of costs and of its routine use in a stat laboratory.

Pleural effusion: beta-trace protein in diagnosing ventriculoperitoneal shunt complications

Catheter dysfunction is a common complication with ventriculoperitoneal shunts. Apart from infection, obstruction, and leakage, migration of the shunt tip may cause particular problems. Pleural effusion is easily classified as a shunt complication if a transdiaphragmatic migration of a shunt can be demonstrated. If, however, the tip of the shunt is found adjacent to the diaphragm, it is difficult to decide if the effusion is caused by the ventriculoperitoneal shunt. Different diagnostic methods can be used in this situation. Below we report a case of pleural effusion-without shunt migration-which was revealed to be a shunt complication by quantifying beta-trace protein in the effusion.

Cerebrospinal fluid leakage into the subdural space: possible influence on the pathogenesis and recurrence frequency of chronic subdural hematoma and subdural hygroma

OBJECT

The purpose of this study was to clarify whether cerebrospinal fluid (CSF) leakage into the subdural space is involved in the genesis of chronic subdural hematoma (CSDH) and subdural hygroma (SH) and to clarify whether this leakage of CSF into the subdural space influences the postoperative recurrence rate of CSDH and SH.

METHODS

In this prospective observational study, 75 cases involving patients treated surgically for CSDH (67 patients) or SH (8 patients) were evaluated with respect to clinical and radiological findings at presentation, the content of beta -trace protein (beta TP) in the subdural fluid (betaTPSF) and serum (betaTPSER), and the CSDH/SH recurrence rate. The betaTPSF was considered to indicate an admixture of CSF to the subdural fluid if betaTPSF/betaTPSER>2.

RESULTS

The median beta TPSF level for the whole patient group was 4.29 mg/L (range 0.33-51 mg/L). Cerebrospinal fluid leakage, as indicated by betaTPSF/betaTPSER>2, was found to be present in 93% of the patients with CSDH and in 100% of the patients with SH (p=0.724). In patients who later had to undergo repeated surgery for recurrence of CSDH/SH, the betaTPSF concentrations (median 6.69 mg/L, range 0.59-51 mg/L) were significantly higher (p=0.04) than in patients not requiring reoperation (median 4.12 mg/L, range 0.33-26.8 mg/L).

CONCLUSIONS

As indicated by the presence of betaTP in the subdural fluid, CSF leakage into the subdural space is present in the vast majority of patients with CSDH and SH. This leakage could be involved in the pathogenesis of CSDH and SH. Patients who experience recurrences of CSDH and SH have significantly higher concentrations of betaTPSF at initial presentation than patients not requiring reoperation for recurrence. These findings are presented in the literature for the first time and have to be confirmed and expanded upon by further studies.

Beta-trace protein--a marker of kidney function in children: "Original research communication-clinical investigation"

OBJECTIVES

To determine the pediatric reference interval for serum beta-trace protein (beta-TP) and to compare beta-TP with established LMW markers of GFR, i.e., cystatin C (CysC) and beta(2)-microglobulin (beta(2)-M).

DESIGN AND METHODS

All three LMW markers were measured immunonephelometrically. In 106 children above the age of 2 years without evidence of kidney disease, non-parametric reference intervals were calculated. The relative rise of the GFR marker concentrations above the upper reference was studied in 107 samples from 96 patients covering the entire GFR range.

RESULTS

Above 2 years, the reference range of beta-TP was constant at 0.43-1.04 mg/L. With decreasing Schwartz-GFR, there was a comparable rise in beta-TP and beta(2)-M, while CysC rose less in the group with GFR below 30 mL/min/1.73 m(2) (278+/-49% [CysC] versus 336+/-65% [beta-TP] and 342+/-76% [beta(2)-M]; p=0.043 and 0.027, respectively).

CONCLUSIONS

These data confirm the potential of ss-TP as an endogenous GFR marker in children.

Can cystatin C replace creatinine to estimate glomerular filtration rate? A literature review

BACKGROUND

With the increasing knowledge that estimation of glomerular filtration rate (GFR) from serum creatinine (Scr) has limited value, researchers have developed new equations based on serum cystatin C (Cys C).

AIM

To compare the performance of serum Cys C and Cys C-based GFR equations to Scr and Scr-based GFR equations.

METHODS

A Medline literature search for studies in English.

RESULTS

Fourteen studies in kidney transplant patients and 29 in patients with native kidney disease were identified. 70% of studies on transplants favored Cys C over Scr while 60% favored serum Cys C over Scr in patients with native kidney disease. Three studies in transplant patients and 6 in patients with native kidney disease compared the performances of Cys C- and Scr-based equations. 70% of the studies performed on transplantation favored Cys C, while 85% the studies performed in native kidney diseases showed superiority of Cys C-based equations.

CONCLUSION

A large number of studies favor Cys C over Scr for the estimation of GFR. Still, many reports show no superiority of Cys C over Scr. Consistent with this, more studies are needed to study the performance of Cys C-based GFR equations.

Cerebrospinal fluid and serum concentrations of beta-trace protein during pregnancy

We conducted a prospective observational study among a cohort of 40 term parturients undergoing spinal anaesthesia for elective Caesarean section, to determine the concentration of beta-trace protein in cerebrospinal fluid (CSF) and serum. Serum and CSF samples, taken at the time of dural puncture, were assayed by nephelometry. The mean serum beta-trace protein concentration was 0.39 mg.l(-1) and the mean CSF concentration was 27.9 mg.l(-1), giving a mean ratio of CSF to serum concentration of 76. This ratio is higher than that published for non-pregnant females and for males because of both a higher mean CSF and a lower mean serum beta-trace protein concentration. The concentration correlated positively with both serum creatinine and gestational age. If these concentrations are used to estimate the normal range, we propose that the nephelometric measurement of beta-trace protein might prove a useful diagnostic test for cerebrospinal fluid-cutaneous fistula in parturients.