Illumination of Hydroxyl Radical in Kidney Injury and High-Throughput Screening of Natural Protectants Using a Fluorescent/Photoacoustic Probe

The hydroxyl radical (•OH) is shown to play a crucial role in the occurrence and progression of acute kidney injury (AKI). Therefore, the development of a robust •OH probe holds great promise for the early diagnosis of AKI, high-throughput screening (HTS) of natural protectants, and elucidating the molecular mechanism of intervention in AKI. Herein, the design and synthesis of an activatable fluorescent/photoacoustic (PA) probe (CDIA) for sensitive and selective imaging of •OH in AKI is reported. CDIA has near-infrared fluorescence/PA channels and fast activation kinetics, enabling the detection of the onset of •OH in an AKI model. The positive detection time of 12 h using this probe is superior to the 48-hour detection time for typical clinical assays, such as blood urea nitrogen and serum creatinine detection. Furthermore, a method is established using CDIA for HTS of natural •OH inhibitors from herbal medicines. Puerarin is screened out by activating the Sirt1/Nrf2/Keap1 signaling pathway to protect renal cells in AKI. Overall, this work provides a versatile and dual-mode tool for illuminating the •OH-related pathological process in AKI and screening additional compounds to prevent and treat AKI.

β-Trace Protein and β2-Microglobulin do not Improve Estimation of Glomerular Filtration Rate in Kidney Transplant Recipients Compared With Creatinine and Cystatin C

BACKGROUND

Reliable estimates of glomerular filtration rate (eGFR) are important for detecting changes in graft function in kidney transplant recipients. Current eGFR equations are based on plasma creatinine and/or cystatin C; however, these are associated with significant bias. This study investigated if equations based on β-trace protein (BTP) and β2-microglobulin (B2M) performed better than the 2021 Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations based on creatinine and cystatin C among kidney transplant recipients.

METHODS

We included samples and data from the clinical trial CONTEXT. Glomerular filtration rate (GFR) was measured by plasma clearance of an exogenous marker. The eGFR was calculated using the CKD-EPI equations for estimating GFR from BTP and/or B2M and the 2021 CKD-EPI creatinine and creatinine-cystatin C equations. The GFR estimates were evaluated 3 (n = 82) and 12 (n = 64) months after transplant using mean bias, precision, and accuracy. Furthermore, we analyzed the ability of the equations to correctly classify the direction of changes in measured GFR from 3 to 12 months.

RESULTS

Among the BTP- and B2M-based equations, the combined eGFR-BTP-B2M performed best with respect to precision (SD = 7.64 mL/min/1.73 m2) and accuracy (±10% from measured GFR = 36%). The eGFR-BTP-B2M and the eGFR-creatinine-cystatin C (2021) performed similarly when comparing precision, accuracy, and residuals (P = .481). The BTP- and/or B2M-based equations did not perform better than the eGFR-creatinine-cystatin C (2021) in correctly classifying the direction of changes in measured GFR from 3 to 12 months.

CONCLUSIONS

β-trace protein and/or B2M do not improve the estimation of GFR when compared with creatinine- and cystatin C-based 2021 CKD-EPI equations.

Acute effects of lithium augmentation on the kidney in geriatric compared with non-geriatric patients with treatment-resistant depression

INTRODUCTION

Lithium augmentation (LA) of antidepressants is a first-line therapy option for treatment-resistant depression (TRD). Nevertheless, it is rarely used in geriatric patients mostly because of the fear of kidney toxicity. The purpose of this study is to investigate estimated glomerular filtration rate (eGFR) changes and number of acute kidney injuries (AKI) using LA in geriatric compared with non-geriatric patients.

METHODS

In a prospective multicenter cohort study, eGFR changes were measured in 201 patients with unipolar depression (n_{age≥65years}  = 29; n_age<65years  = 172) at baseline and over 2-6 weeks of LA. We used linear mixed models to investigate changes in eGFR upon LA and assessed the number of AKIs, according to the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines.

RESULTS

Both age groups showed a significant eGFR decline over the course of treatment with lower eGFR in geriatric patients. The lithium serum level (interpretable as "effect of LA") had a significant effect on eGFR decline. Both effects (age group and lithium serum level) on eGFR decline did not influence each other, meaning the effect of LA on eGFR decline did not differ between age groups. Two AKIs were observed in the geriatric age group when serum lithium levels exceeded the therapeutic range of >0.8 mmol/L.

CONCLUSION

This is the first study investigating eGFR change and AKI upon LA for TRD in geriatric compared with non-geriatric patients. Our data suggest that LA, as an effective treatment option in geriatric patients, should be closely monitored to avoid AKIs.

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.

Molecular Visualization of Early-Stage Acute Kidney Injury with a DNA Framework Nanodevice

DNA nanomachines with artificial intelligence have attracted great interest, which may open a new era of precision medicine. However, their in vivo behavior, including early diagnosis and therapeutic effect are limited by their targeting efficiency. Here, a tetrahedral DNA framework (TDF)-based nanodevice for in vivo near-infrared (NIR) diagnosis of early-stage AKI is developed. This nanodevice comprises three functional modules: a size-tunable TDF nanostructure as kidney-targeting vehicle, a binding module for the biomarker kidney injury molecule-1 (Kim-1), and a NIR signaling module. The cooperation of these modules allows the nanodevice to be selectively accumulated in injured kidney tissues with high Kim-1 level, generating strong NIR fluorescence; whereas the nanodevice with the proper size can be rapidly cleared in healthy kidneys to minimize the background. By using this nanodevice, the early diagnosis of AKI onset is demonstrated at least 6 h ahead of Kim-1 urinalysis, or 12 h ahead of blood detection. It is envisioned that this TDF-based nanodevice may have implications for the early diagnosis of AKI and other kidney diseases.

Beta-Trace Protein as a Potential Marker of Acute Kidney Injury: A Pilot Study

INTRODUCTION

Acute kidney injury (AKI) is a frequent complication among patients in the intensive care unit (ICU). The limitations of serum Cr (sCr) in timely detecting AKI are well known. Beta-trace protein (BTP) is emerging as a novel endogenous glomerular filtration rate marker. The aim of this study was to explore the role of BTP as a marker of AKI.

METHODS

Patients admitted to the ICU undergoing surgery were included. BTP, sCr, Cystatin C (CysC), and neutrophil gelatinase-associated lipocalin (NGAL) were measured preoperatively, postoperatively (post-op), and at the first (D1) and second (D2) post-op day. AKI was defined as an increase of sCr to ≥1.5-fold from baseline within 2 days after surgery.

RESULTS

Of the 52 patients studied, 10 patients (19%) developed AKI. Patients with AKI were older (69.6 ± 10.7 vs. 58.1 ± 16.7 years, p = 0.043) and had a longer length of ICU stay (13 [IQR 6-49] vs. 6 [IQR 5-8] days, p = 0.032). Between the 2 groups, the evolution of BTP, sCr, CysC, and NGAL over time differed significantly, with overall higher values in the AKI group. ROC analysis for the detection of AKI within 2 days after surgery showed a great accuracy for BTP. The area under the curve (AUC) for BTP post-op; D1; and D2 was, respectively, 0.869 ± 0.049; 0.938 ± 0.035; and 0.943 ± 0.032. The discriminative power of a BTP measurement on D1 was superior in detecting AKI compared to NGAL (adjusted p value = 0.027). We could not detect a significant difference between the AUCs of other biomarkers (NGAL, sCr, and CysC).

CONCLUSION

Serum BTP is a promising marker for diagnosing AKI in ICU patients undergoing surgery.

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.

Formulas Estimating Glomerular Filtration Rate in the Evaluation of Living Kidney Donor Candidates: Comparison of Different Formulas With Scintigraphy-Measured Glomerular Filtration Rate

INTRODUCTION

Estimation of kidney function is crucial in the evaluation of living kidney donor candidates. Despite the multitude of glomerular filtration rate (GFR) formulas, no equation is universal, and none were validated in the population of kidney donors. Novel biomarkers, including beta trace protein (BTP) and cystatin C, are studied to help estimate GFR and improve the safe qualification of living kidney donors.

AIM

This study compares the accuracy of different formulas that estimate GFR with reference scintigraphy-measured GFR in the population of living kidney donor candidates.

MATERIAL AND METHODS

This study enrolled 30 healthy living kidney donor candidates. GFR was measured using the following 11 different formulas. For reference, GFR was assessed using 99m-Technetium-diethylenetriaminepentaacetic acid.

RESULTS

The accuracy of estimation was generally low in all formulas. The strongest correlation between measured GFR (mGFR) and estimated GFR (eGFR) was achieved by the Nankivell formula (R = 0.47, P = .009); however, in the group of patients with a body mass index of >25 kg/m², only the equations based on BTP had a statistically significant correlation with mGFR: White (R = 0.59; P = .016) and Poge (R = 0.53; P = .035). Bland-Altman plots revealed wide limits of agreement between eGFRs and mGFR in all groups of patients.

CONCLUSION

In living kidney donor candidates, GFR estimation formulas should be chosen individually. White formula, which is based on BTP, may be a promising tool in estimating GFR in overweight potential living kidney donor candidates. More than 1 formula and personalized choice of GFR estimation method regarding the given patient should be performed in qualification of kidney donors.

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.

Accurate GFR in obesity-protocol for a systematic review

BACKGROUND

Obesity is increasing globally. Chronic kidney disease (CKD) is strongly associated with obesity. Kidney function is commonly estimated with equations using creatinine (such as CKD-EPI equation) which is a product of muscle metabolism. Decisions about categorizing CKD, planning modality of renal replacement therapies, and adjusting dosages of medications excreted by the kidneys are done using these equations. However, it is not well appreciated that creatinine-based equations may not accurately estimate kidney function in obese individuals. We plan a systematic review of diagnostic studies which will compare estimating equations to actual measured kidney function.

METHODS

We will systematically search electronic bibliographic databases including MEDLINE, EMBASE, and the Cochrane Library with no restrictions on language or specific dates. The search terms will be adapted for the different databases using a combination of Medical Subject Heading and relevant keywords contained in titles and abstracts. Our preliminary search strategy using Cochrane, MEDLINE, and EMBASE databases have identified 190, 1246, and 1660 citations, respectively. For all studies selected, we will extract information on general study characteristics, study participant (age, sex, ethnicity, weight, height, BMI, BSA), type and protocol of reference standard utilized, the index test studied, the methodology of measurement of index test, categories of GFR, the proportion of eGFR within 10, 20, 30, 40, and 50% of measured GFR, and bias between eGFR and measured GFR. If the quality of methods and risk of bias are adequate, we will perform a meta-analysis. We will assess the heterogeneity using the χ 2 and the I 2 statistics to examine whether the estimates from studies included could be pooled. Sensitivity and multivariate meta-regression analyses will be performed to assess the effects of clinical factors and socio-demographic characteristics reported in included studies on the meta-analytic estimates. All analysis will be performed using the Comprehensive Meta-analysis software.

DISCUSSION

This systematic review might help to inform clinicians on the best equation to use in patients with obesity and CKD for staging of CKD and for medication dosing. If no equation is deemed suitable, this review will form a basis for future studies of GFR in obese individuals.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42018104345.

Prospective cohort studies of beta-trace protein and mortality in haemodialysis patients and patients undergoing coronary angiography

Background

Beta-trace protein (BTP) is a low-molecular-weight glycoprotein, which may serve as an endogenous biomarker of kidney function and cardiovascular risk.

Methods

We examined cardiovascular and all-cause mortality according to BTP concentrations in 2962 individuals referred for coronary angiography from the Ludwigshafen Risk and Cardiovascular Health study and in 907 patients with Type 2 diabetes mellitus undergoing haemodialysis from the German Diabetes and Dialysis (4D) study.

Results

Haemodialysis patients had considerably higher median (interquartile range) BTP concentrations [6.00 (4.49-7.96) mg/L] and experienced a 4-fold increased mortality rate compared with coronary angiography patients [BTP concentration: 0.55 (0.44-0.67) mg/L]. After adjustment for age, sex, cardiovascular risk factors and creatinine, 4D patients in the highest quartile (>7.96 mg/L) had a 1.6-fold increased rate of all-cause mortality [hazard ratio (HR) 1.62, 95% confidence interval (CI) 1.19-2.20] compared with the lowest quartile (<4.49 mg/L) (P = 0.002) In patients undergoing coronary angiography, the adjusted HRs (95% CI) for all-cause and cardiovascular mortality were 1.23 (1.0-1.51) and 1.27 (0.99-1.63) in the highest (>0.67 mg/L) compared with the lowest (<0.44 mg/L) quartile (P = 0.043 and 0.062). In both cohorts, the BTP/creatinine ratio was a stronger predictor of all-cause and cardiovascular mortality compared with BTP.

Conclusion

BTP was associated with all-cause mortality independently of renal function in haemodialysis patients. The BTP/creatinine ratio was more predictive for all-cause and cardiovascular mortality in haemodialysis patients and individuals referred for angiography compared with BTP as single marker.

Serum BTP concentrations are not affected by hepatic dysfunction

Background

Beta Trace Protein (BTP) is a promising marker of glomerular filtration rate (GFR). Equations to estimate GFR using BTP have been proposed. Very little is known about BTP’s production and metabolism. It has been hypothesized that the liver metabolizes certain BTP isoforms. As such, hepatic dysfunction may influence serum levels independently of GFR. This would impact on the accuracy and precision of GFR estimates using BTP. The purpose of this study was to assess the impact of cirrhosis on serum BTP concentrations.

Methods

BTP, cystatin C (cysC) and creatinine (Cr) were measured in 99 cirrhotic subjects and in matched controls. BTP/cysC and Cr/cysC ratios were compared between cases and controls. This was repeated after stratification by Child Pugh category. Comparisons of ratios between Child Pugh category A and combined B and C case subjects were also performed.

Results

There were no differences in BTP/cysC ratios between cases and controls for the entire cohort (0.80 vs 0.79) or for any of the Child Pugh categories (p > 0.10). There were significant differences between cases (1.09) and controls (0.73) for the BTP/Cr ratios (p < 0.001). The BTP/Cr ratio was higher in those with more advanced cirrhosis as compared to those with less severe cirrhosis (1.20 vs 1.03, p < 0.01). There were no differences in BTP/cysC ratios between those with less severe and more advanced cirrhosis (p = 0.25).

Conclusions

This study suggests that hepatic dysfunction does not influence serum BTP levels and argues against a significant role for the liver in BTP metabolism. Confirmation in a larger group of patients with advanced cirrhosis is required.

Cystatin C Is a Gender-Neutral Glomerular Filtration Rate Biomarker in Patients with Cirrhosis

BACKGROUND

Lower serum Cr levels in women as compared to men result in underestimation of renal dysfunction and lower model for end-stage liver disease-sodium scores leading to reduced access to liver transplantation in women compared to men with comparable hepatic dysfunction.

AIM

The aim of this study was to determine the gender differences in serum Cr, cystatin C, and other endogenous glomerular filtration rate (GFR) biomarkers, measured and estimated GFR, Cr clearance, and Cr production rates.

METHODS

We measured GFR by iothalamate plasma clearance in 103 patients with cirrhosis and assessed gender differences in GFR, Cr clearance and production rate, serum Cr, cystatin C and other endogenous GFR biomarkers including beta-trace protein, beta-2 microglobulin, and dimethylarginines.

RESULTS

Comparison of men and women showed significantly lower values for mean serum Cr (0.97 vs. 0.82 mg/dl, P = 0.023), and Cr production rate (13.37 vs. 11.02 mg/kg/day, P = 0.022). In contrast to the serum Cr and Cr production rate, men and women exhibited no significant differences in the means of serum cystatin C and other GFR biomarkers, measured GFR, GFR estimated using Cr-cystatin C GFR equation for cirrhosis, measured and estimated Cr clearances. After controlling for age, race, weight, height, and GFR, female gender remained associated with lower serum Cr levels (P = 0.003). Serum cystatin C levels were not associated with gender, age, race, weight, height, C-reactive protein, and history of hypothyroidism.

CONCLUSIONS

Our results suggest that cystatin C and endogenous GFR biomarkers other than Cr, measured GFR, GFR estimated by Cr-cystatin C GFR equation for cirrhosis, measured and estimated Cr clearance minimized between-gender biases in accounting for renal function in patients with cirrhosis. Therefore, serum cystatin C should be measured as a complementary test to serum Cr when renal function is assessed in patients with cirrhosis, particularly in women and those with sarcopenia.

Beta Trace Protein does not outperform Creatinine and Cystatin C in estimating Glomerular Filtration Rate in Older Adults

Despite intense research the optimal endogenous biomarker for glomerular filtration rate (GFR) estimation has not been identified yet. We analyzed if ß-trace protein (BTP) improved GFR estimation in elderly. 566 participants aged 70+ from the population-based Berlin Initiative Study were included in a cross-sectional validation study. BTP, standardized creatinine and cystatin C were measured in participants with iohexol clearance measurement as gold standard method for measured GFR (mGFR). In a double logarithmic linear model prediction of mGFR by BTP was assessed. Analyses with BTP only and combined with creatinine and cystatin C were performed. Additionally, performance of GFR estimating equations was compared to mGFR. We found that the combination of all three biomarkers showed the best prediction of mGFR (r2 = 0.83), whereat the combination of creatinine and cystatin C provided only minimally diverging results (r2 = 0.82). Single usage of BTP showed worst prediction (r2 = 0.67) within models with only one biomarker. Subgroup analyses (arterial hypertension, diabetes, body mass index ≤23 and >30) demonstrated a slight additional benefit of including BTP into the prediction model for diabetic, hypertensive and lean patients. Among BTP-containing GFR equations the Inker BTP-based equation showed superior performance. Especially the use of cystatin C renders the addition of BTP unnecessary.

Beta-trace Protein as a new non-invasive immunological Marker for Quinolinic Acid-induced impaired Blood-Brain Barrier Integrity

Quinolinic acid, a macrophage/microglia-derived excitotoxin fulfills a plethora of functions such as neurotoxin, gliotoxin, and proinflammatory mediator, and it alters the integrity and cohesion of the blood-brain barrier in several pathophysiological states. Beta-trace protein (BTP), a monomeric glycoprotein, is known to indicate cerebrospinal fluid leakage. Thus, the prior aim of this study was to investigate whether BTP might non-invasively indicate quinolinic acid-induced impaired blood-brain barrier integrity. The research hypotheses were tested in three subsamples with different states of immune activation (patients with HCV-infection and interferon-α, patients with major depression, and healthy controls). BTP has also been described as a sensitive marker in detecting impaired renal function. Thus, the renal function has been considered. Our study results revealed highest quinolinic acid and highest BTP- levels in the subsample of patients with HCV in comparison with the other subsamples with lower or no immune activation (quinolinic acid: F = 21.027, p < 0.001 [ANOVA]; BTP: F = 6.792, p < 0.01 [ANOVA]). In addition, a two-step hierarchical linear regression model showed that significant predictors of BTP levels are quinolinic acid, glomerular filtration rate and age. The neurotoxin quinolinic acid may impair blood-brain barrier integrity. BTP might be a new non-invasive biomarker to indicate quinolinic acid-induced impaired blood-brain barrier integrity.

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.

Elevated Serum Beta-Trace Protein Levels are Associated With the Presence of Atrial Fibrillation in Hypertension Patients

Beta-trace protein (BTP) has emerged as a novel biomarker of cardiovascular risk. In this study, the authors aimed to assess the relationship between BTP levels and presence of atrial fibrillation in patients who had controlled hypertension (HTN) and normal renal function. A total of 80 controlled HTN patients with paroxysmal atrial fibrillation (PAF) and 80 age- and sex-matched controls with controlled HTN were enrolled. Serum BTP levels were measured by enzyme-linked immunosorbent assay. BTP levels were found to be significantly higher in patients with PAF (P<.001). Other parameters including mean systolic and diastolic blood pressure values, serum creatinine levels, and glomerular filtration rate were similar between the two groups. Along with left atrial diameter (odds ratio, 1.504; P<.001), BTP levels (odds ratio, 1.015; P<.001) were independently associated with the presence of PAF. BTP levels were increased in controlled HTN patients with PAF compared with controls, and this association was observed within normal renal functions as reflected by normal glomerular filtration rate.

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.

Within-person variability in kidney measures

BACKGROUND

Our objective was to quantify short-term total within-person variability in standard and nontraditional kidney measures using national data.

STUDY DESIGN

Repeated examination study of serum and urine kidney measures.

SETTING & PARTICIPANTS

Participants 18 years or older in the Third National Health and Nutrition Examination Survey (NHANES III) who had repeated blood and urine samples collected during visits occurring approximately 18 days apart.

MEASUREMENTS

Standardized serum creatinine, standardized cystatin C, β-trace protein (BTP), β(2)-microglobulin (B2M), and urine albumin and creatinine. We calculated the within-person coefficient of variation (CV(w)), which includes both biological and analytical variability. We also evaluated the impact of variability on estimates of the prevalence of reduced estimated glomerular filtration rate and albuminuria.

RESULTS

Serum cystatin C level demonstrated the lowest short-term within-person variability (CV(w) = 6.8%). Serum creatinine and B2M levels (CV(w) = 7.6% and 8.4%, respectively) also had low variability. BTP level had the most variability of the serum markers (CV(w) = 11.6%). As expected, urine albumin and urine creatinine measurements showed high variability (CV(w) >30% for both); however, albumin-creatinine ratio performed much better than either measure alone, with CV(w) of 11.3%. The effect of short-term variability on the prevalence of reduced estimated glomerular filtration rate was moderate, with an ~20% lower prevalence when defined based on single measurements compared to repeated application of the same test approximately 18 days apart. Repeated testing for albuminuria had a larger effect, showing a 33% lower prevalence of albuminuria when repeated testing was applied.

LIMITATIONS

Only 2 measurements available. General population with low prevalence of kidney disease.

CONCLUSIONS

Our results suggest that creatinine, cystatin C, and B2M levels have similarly low short-term variability. BTP level was more variable compared with the other serum filtration markers. Urine albumin and creatinine levels were highly variable and may benefit from repeated assessments to reduce the misclassification of albuminuria.

Experimental biomarkers in heart failure: an update

Over the past 5 years, researchers have examined the utility of many experimental heart failure biomarkers that are not yet widely adopted clinically, to complement the role of B-type natriuretic peptide and its precursor. Candidate biomarkers have been identified from several different pathophysiologic categories, including markers of inflammation, myocyte necrosis, renal dysfunction, neurohumoral activation, oxidative stress and raised intracardiac pressure. Indeed, some biomarkers provide prognostic information that is independent of information obtained from conventional clinical and biomarker assessment. Moreover, some biomarkers studied help to identify dominant pathology that may predict responsiveness to specific therapies. Preliminary data also suggest a potential role for the development of comprehensive biomarker profiling models, integrating biomarkers from several categories to refine risk assessment.

Novel filtration markers as predictors of all-cause and cardiovascular mortality in US adults

BACKGROUND

New filtration markers, including β-trace protein (BTP) and β₂-microglobulin (B2M), may, similar to cystatin C, enable a stronger prediction of mortality compared to serum creatinine-based estimated glomerular filtration rate (eGFRcr). We sought to evaluate these mortality associations in a representative sample of US adults.

STUDY DESIGN

Prospective cohort study.

SETTING & PARTICIPANTS

6,445 adults 20 years or older from the Third National Health and Nutrition Examination Survey (1988-1994) with mortality linkage through December 31, 2006.

PREDICTORS

Serum cystatin C, BTP, and B2M levels and eGFRcr categorized into quintiles, with the highest quintile (lowest for eGFRcr) split into tertiles (subquintiles Q5a-Q5c).

OUTCOMES

All-cause, cardiovascular disease, and coronary heart disease mortality.

MEASUREMENTS

Demographic- and multivariable-adjusted Cox proportional hazard models.

RESULTS

During follow-up, 2,392 deaths (cardiovascular, 1,079; coronary heart disease, 605) occurred. Levels of all 4 filtration markers were associated with mortality risk after adjusting for demographics (P trend<0.02). Adjusted for mortality risk factors, compared to the middle quintile, the highest subquintiles for cystatin C (Q5c: HR, 1.94; 95% CI, 1.43-2.62), BTP (Q5c: HR, 2.14; 95% CI, 1.56-2.94), and B2M (Q5c: HR, 2.58; 95% CI, 1.96-3.41) were associated with increased all-cause mortality risk, whereas the association was weaker for eGFRcr (Q5c: HR, 1.31; 95% CI, 0.84-2.04). Associations persisted for the novel markers and not for eGFRcr at eGFRcr ≥60 mL/min/1.73 m². Trends were similar for cardiovascular disease and coronary heart disease mortality.

LIMITATIONS

Single measurements of markers from long-term stored samples.

CONCLUSIONS

The strong association of cystatin C level with mortality compared with serum creatinine estimates is shared by BTP and B2M. This supports the utility of alternative filtration markers beyond creatinine when improved risk prediction related to decreased GFR is needed.

Genome-wide significant locus of beta-trace protein, a novel kidney function biomarker, identified in European and African Americans

BACKGROUND

Beta-trace protein (BTP), measured in serum or plasma, has potential as a novel biomarker for kidney function. Little is known about the genes influencing BTP levels.

METHODS

We conducted a genome-wide association study of log-transformed plasma BTP levels in 6720 European Americans (EAs) and replicated the significant associations in 1734 African Americans (AAs) from the Atherosclerosis Risk in Communities (ARIC) study.

RESULTS

We identified a genome-wide significant locus in EA upstream of Prostaglandin D2 synthase (PTGDS), the gene encoding BTP. Each copy of the A allele at rs57024841 was associated with 5% higher BTP levels (P = 1.2 × 10(-23)). The association at PTGDS was confirmed in AAs (6% higher BTP for each A allele at rs57024841, P = 1.9 × 10(-7)). The index single nucleotide polymorphisms (SNPs) in EAs and AAs explained ∼1.1% of the log(BTP) variance within each population and explained over 30% of the difference in log(BTP) levels between EAs and AAs. The index SNPs at the PTGDS locus in the two populations were not associated with the estimated glomerular filtration rate (eGFR) or the urine albumin creatinine ratio (P > 0.05). We further tested for the associations of BTP with 16 known loci of the eGFR in EA, and BTP was associated with 3 of 16 tested.

CONCLUSIONS

The identification of a novel BTP-specific (non-renal related) locus and the confirmation of several genetic loci of the eGFR with BTP extend our understanding of the metabolism of BTP and inform its use as a kidney filtration biomarker.

Usefulness of β-trace protein and cystatin C for the prediction of mortality in non ST segment elevation acute coronary syndromes

Beta-trace protein (BTP) is a low-molecular mass protein belonging to the lipocalin protein family, which is more sensitive than serum creatinine for detecting impaired renal function. The aims of the present study were to evaluate whether plasma BTP improves the risk stratification of patients with non-ST-segment elevation acute coronary syndromes and to compare it to cystatin C (CysC), serum creatinine, and estimated glomerular filtration rate. Two hundred twenty-six consecutive patients with non-ST-segment elevation acute coronary syndromes were prospectively included. Blood samples were obtained within 24 hours of hospital admission to measure BTP, CysC, and creatinine. The study end point was all-cause death. Over a median follow-up period of 859 days (interquartile range [IQR] 524 to 1,164), 24 patients (10.6%) died. Decedents had higher concentrations of BTP (1.03 mg/L [IQR 0.89 to 1.43] vs 0.74 mg/L [IQR 0.61 to 0.92], p <0.001), CysC (1.16 mg/L [IQR 0.91 to 1.59] vs 0.90 mg/L [IQR 0.76 to 1.08], p = 0.001), and serum creatinine (1.10 mg/L [IQR 0.87 to 1.46] vs 0.94 mg/L [IQR 0.80 to 1.10], p = 0.004) and a lower mean estimated glomerular filtration rate (60 ± 20 vs 80 ± 24 ml/min/1.73 m(2), p <0.001). After multivariate adjustment, BTP and CysC were predictors of all-cause death, while estimated glomerular filtration rate and serum creatinine concentrations did not achieve statistical significance. In stratified analyses according to kidney function, elevated BTP and CysC were associated with a higher risk for all-cause death. Reclassification analyses showed that BTP and CysC added complementary information to Global Registry for Acute Coronary Events (GRACE) risk score. In conclusion, BTP and CysC levels were associated with all-cause death risk and modestly improved prognostic discrimination beyond the GRACE risk score in patients with non-ST segment elevation acute coronary syndromes.

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.

Novel markers of kidney function as predictors of ESRD, cardiovascular disease, and mortality in the general population

BACKGROUND

Cystatin C level predicts mortality more strongly than serum creatinine level. It is unknown whether this advantage extends to other outcomes, such as kidney failure, or whether other novel renal filtration markers share this advantage in predicting outcomes.

STUDY DESIGN

Observational cohort study.

SETTING & PARTICIPANTS

9,988 participants in the Atherosclerosis Risk in Communities (ARIC) Study, a population-based study in 4 US communities, followed for approximately 10 years.

PREDICTORS

Serum creatinine-based estimated glomerular filtration rate calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation (eGFR(CKD-EPI)) and cystatin C, β-trace protein (BTP), and β(2)-microglobulin (B2M) levels.

OUTCOMES

Mortality, coronary heart disease, heart failure, and kidney failure.

RESULTS

Higher cystatin C and B2M concentrations were associated more strongly with mortality (n = 1,425) than BTP level and all were associated more strongly than eGFR(CKD-EPI) (adjusted HR for the upper 6.7 percentile compared with the lowest quintile: 1.6 [95% CI, 1.3-1.9] for eGFR(CKD-EPI), 2.9 [95% CI, 2.3-3.6] for cystatin C level, 1.9 [95% CI, 1.5-2.4] for BTP level, and 3.0 [95% CI, 2.4-3.8] for B2M level). Similar patterns were observed for coronary heart disease (n = 1,279), heart failure (n = 803), and kidney failure (n = 130). The addition of cystatin C, BTP, and B2M levels to models including eGFR(CKD-EPI) and all covariates, including urinary albumin-creatinine ratio, significantly improved risk prediction for all outcomes (P < 0.001).

LIMITATIONS

No direct measurement of GFR.

CONCLUSIONS

B2M and, to a lesser extent, BTP levels share cystatin C's advantage over eGFR(CKD-EPI) in predicting outcomes, including kidney failure. These additional markers may be helpful in improving estimation of risk associated with decreased kidney function beyond current estimates based on eGFR(CKD-EPI).

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.

Preliminary reference intervals for cystatin C and beta-trace protein in preterm and term neonates

OBJECTIVE

To determine the reference intervals for serum cystatin C (CysC) and beta-trace protein (BTP) as markers of renal function in preterm and term neonates.

DESIGN AND METHODS

Blood samples of 128 neonates (34% female) admitted to the NICU were analyzed to determine the levels of serum creatinine (enzymatically), CysC and BTP (nephelometric, Siemens Health Care).

RESULTS

The reference intervals, categorized by age, were reported for the 128 neonates. Median (lower/upper limit) BTP were 1.85 (0.57/3.16) and 1.27 (0.51/2.07) mg/L on days 1 and 3. In keeping with maturation of renal function after birth, CysC and BTP fell from days one to day three after birth, whereas creatinine did not.

CONCLUSION

Our data provides reference intervals for the levels of creatinine, CysC, and BTP in neonates on days 1 and 3 after birth and demonstrates that CysC and BTP reflect neonatal renal function, whereas creatinine reflects maternal renal function.

Biomarkers in chronic kidney disease: a review

Chronic kidney disease (CKD) is a major public health problem. The classification of CKD by KDOQI and KDIGO and the routine eGFR reporting have resulted in increased identification of CKD. It is important to be able to identify those at high risk of CKD progression and its associated cardiovascular disease (CVD). Proteinuria is the most sensitive marker of CKD progression in clinical practice, especially when combined with eGFR, but these have limitations. Hence, early, more sensitive, biomarkers are required. Recently, promising biomarkers have been identified for CKD progression and its associated CVD morbidity and mortality. These may be more sensitive biomarkers of kidney function, the underlying pathophysiological processes, and/or cardiovascular risk. Although there are some common pathways to CKD progression, there are many primary causes, each with its own specific pathophysiological mechanism. Hence, a panel measuring multiple biomarkers including disease-specific biomarkers may be required. Large, longitudinal observational studies are needed to validate candidate biomarkers in a broad range of populations prior to implementation into routine CKD management. Recent renal biomarkers discovered include neutrophil gelatinase-associated lipocalin, kidney injury molecule-1, and liver-type fatty acid-binding protein. Although none are ready for use in clinical practice, it is timely to review the role of such biomarkers in predicting CKD progression and/or CVD risk in CKD.

Hyperfiltration affects accuracy of creatinine eGFR measurement

BACKGROUND AND OBJECTIVES

Surrogate markers such as creatinine, cystatin C (CysC), and beta trace protein (BTP) have been used to estimate GFR (eGFR). The accuracy of eGFR may be altered with hyperfiltration and differences in filtration fraction (FF). It is hypothesized that the accuracy of creatinine for eGFR may be affected by hyperfiltration and different effective renal plasma flow (ERPF).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A total of 127 pediatric patients with various renal diseases underwent simultaneous measurements of GFR using 51Cr-EDTA renal scan and ERPF (131I-hippurate clearance) to calculate the FF (FF=GFR/ERPF). The eGFRs were calculated using the commonly used Schwartz (creatinine), Filler (CysC), and Benlamri (BTP) formulas. Agreement of the eGFRs with the measured isotope GFRs was assessed by Bland-Altman plots. Correlation analysis was performed using nonparametric tests to compare FF with eGFR-GFR.

RESULTS

The 127 children at a median age (with 25th percentile, 75th percentile) of 11.9 (8.5, 14.9) years had a mean 51Cr EDTA-GFR of 100.6±32.1 ml/min per 1.73 m2 and a median 131I-hippurate clearance (ERPF) of 588 (398,739) ml/min per 1.73 m2. Mean FF was 17.7±4.5% with no correlation between the FF and the error (eGFR-GFR) for CysC and BTP eGFR, whereas there was a significant negative correlation between the error for Schwartz eGFR and FF.

CONCLUSIONS

There is a significant negative correlation between the error for the Schwartz eGFR and the FF. CysC and BTP are not affected by differences in FF.

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.

Beta-Trace Protein as a Marker of Renal Dysfunction in Patients with Chronic Kidney Disease: Comparison with Other Renal Markers

Beta-trace protein (BTP), also known as prostaglandin D synthase, is a low-molecular-mass protein which belongs to the lipocalin protein family. It was found to be increased in the serum of patients with renal diseases. The aim of this study was to compare the clinical usefulness of serum levels of beta-trace protein for the detection of renal dysfunction in patients with chronic kidney disease (CKD) with levels of other renal markers: creatinine, cystatin C and β2-microglobulin (B2M). The study included 134 patients with a wide range of renal dysfunction that encompassed all five CKD stages. Obtained data showed that beta-trace protein highly correlated (Spearman test) with creatinine (r = 0.890), cystatin C (r = 0.904) and B2M (r = 0.933) and its levels in serum significantly increased from CKD stage 1 to 5. Furthermore, the values of glomerular filtration rate (GFR) estimated from a BTP-based formula significantly correlated with GFR calculated from creatinine-based and cystatin C-based formulas. ROC analyses showed that BTP had similar diagnostic accuracy for detection of reduced renal function in CKD stages as other renal markers, for estimated GFRs of < 30, < 60 and < 90 mL/min/1.73 m2. The areas under the ROC curves (AUC) for BTP, for these GFR limits, were from 0.983 to 0.917 and they were not significantly different from AUCs for other renal markers. The results of this study showed that BTP may be a useful and reliable serum marker for identifying the magnitude of renal dysfunction in patients with CKD and may have its place beside serum cystatin C and creatinine as an alternative endogenous GFR marker.

Measuring vs estimating glomerular filtration rate in kidney transplantation

Evaluation of kidney function is crucial in the care of kidney transplant recipients and in the design and interpretation of clinical trials in transplantation. Kidney function is most commonly assessed in both instances using serum creatinine concentration or an estimate of glomerular filtration rate (GFR) based on serum creatinine. These are inexpensive, widely available, and easily administered. Both have significant drawbacks, notably with respect to their inability to accurately identify changes in GFR. Novel markers of GFR such as cystatin C and beta-trace protein show promise as accurate and sensitive markers of GFR but have not yet been adequately evaluated in kidney transplantation. In addition, they are relatively expensive compared to creatinine and their assays are not available in most clinical laboratories. Glomerular filtration rate measurement using a variety of different available tracers and techniques is infrequently used in either clinical care or research protocols because of its cost and cumbersomeness. This review will discuss the merits and pitfalls of the various tools available to evaluate GFR in kidney transplantation.