an Obituary for GFR as the main marker for kidney function?

Point-of-Care Testing to Differentiate Various Acid-Base Disorders in Chronic Kidney Disease

BACKGROUND

Normal-anion-gap metabolic acidosis (AGMA) and high-anion-gap metabolic acidosis (HAGMA) are two forms of metabolic acidosis, which is a common complication in patients with chronic kidney disease (CKD). The aim of this study is to identify the prevalence of various acid-base disorders in patients with advanced CKD using point-of-care testing (POCT) and to determine the relationship between POCT parameters.

METHODS

In a group of 116 patients with CKD in stages G4 and G5, with a mean age of 62.5 ± 17 years, a sample of arterial blood was taken during the arteriovenous fistula procedure for POCT, which enables an assessment of the most important parameters of acid-base balance, including: pH, base excess (BE), bicarbonate (HCO3-), chloride(Cl-), anion gap (AG), creatinine and urea concentration. Based on this test, patients were categorized according to the type of acidosis-base disorder.

RESULTS

Decompensate acidosis with a pH < 7.35 was found in 68 (59%) patients. Metabolic acidosis (MA), defined as the concentration of HCO3- ≤ 22 mmol/L, was found in 92 (79%) patients. In this group, significantly lower pH, BE, HCO3- and Cl- concentrations were found. In group of MA patients, AGMA and HAGMA was observed in 48 (52%) and 44 (48%) of patients, respectively. The mean creatinine was significantly lower in the AGMA group compared to the HAGMA group (4.91 vs. 5.87 mg/dL, p < 0.05). The AG correlated positively with creatinine (r = 0.44, p < 0.01) and urea (r = 0.53, p < 0.01), but there was no correlation between HCO3- and both creatinine (r = -0.015, p > 0.05) and urea (r = -0.07, p > 0.05). The Cl- concentrations correlated negatively with HCO3- (r = -0.8, p < 0.01).

CONCLUSIONS

The most common type of acid-base disturbance in CKD patients in stages 4 and 5 is AGMA, which is observed in patients with better kidney function and is associated with compensatory hyperchloremia. The initiation of renal replacement therapy was significantly earlier for patients diagnosed with HAGMA compared to those diagnosed with AGMA. The more advanced the CKD, the higher the AG.

Association of tubular solute clearances with the glomerular filtration rate and complications of chronic kidney disease: the Chronic Renal Insufficiency Cohort study

BACKGROUND

The secretion of organic solutes by the proximal tubules is an essential intrinsic kidney function. The degree to which secretory solute clearance corresponds with the glomerular filtration rate (GFR) and potential metabolic implications of net secretory clearance are largely unknown.

METHODS

We evaluated 1240 participants with chronic kidney disease (CKD) from the multicenter Chronic Renal Insufficiency Cohort (CRIC) Study. We used targeted mass-spectrometry to quantify candidate secretory solutes in paired 24-h urine and plasma samples. CRIC study personnel measured GFR using 125I-iothalamate clearance (iGFR). We used correlation and linear regression to determine cross-sectional associations of secretory clearances with iGFR and common metabolic complications of CKD.

RESULTS

Correlations between iGFR and secretory solute clearances ranged from ρ  = +0.30 for hippurate to ρ = +0.58 for kynurenic acid. Lower net clearances of most secretory solutes were associated with higher serum concentrations of parathyroid hormone (PTH), triglycerides and uric acid. Each 50% lower kynurenic acid clearance was associated with a 21% higher serum PTH concentration [95% confidence interval (CI) 15-26%] and a 10% higher serum triglyceride concentration (95% CI 5-16%) after adjustment for iGFR, albuminuria and other potential confounders. Secretory solute clearances were not associated with statistically or clinically meaningful differences in serum calcium, phosphate, hemoglobin or bicarbonate concentrations.

CONCLUSIONS

Tubular secretory clearances are modestly correlated with measured GFR among adult patients with CKD. Lower net secretory clearances are associated with selected metabolic complications independent of GFR and albuminuria, suggesting potential clinical and biological relevance.

Accumulation of uraemic toxins is reflected only partially by estimated GFR in paediatric patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) in childhood is characterised by the accumulation of uraemic toxins resulting in a multisystem disorder that has a negative impact on quality of life. Childhood CKD is predominantly defined by a decrease in glomerular filtration rate, estimated (eGFR) by a single serum measurement of endogenous biomarkers, e.g. creatinine. The objective of this study was to evaluate how accurately eGFR predicts the concentration of uraemic toxins in a paediatric CKD cohort.

METHODS

In 65 children (10.8 [5.1; 14.7] years) with CKD (eGFR 44 [20; 64] mL/min/1.73 m²), serum concentrations were determined of small solutes (uric acid [UA], urea, symmetric dimethylarginine [SDMA], asymmetric dimethylarginine [ADMA]), middle molecules (β2-microglobulin [β2M], complement factor D [CfD]) and protein-bound solutes (p-cresylglucuronide [pCG], hippuric acid, indole acetic acid, indoxyl sulphate [IxS], p-cresylsulfate [pCS] and 3-carboxy-4-methyl-5-propyl-furanpropionic acid [CMPF]). Spearman's correlation coefficients (r) were calculated to correlate uraemic toxin concentrations with three different eGFR equations, based on either serum creatinine or β2M.

RESULTS

Updated Schwartz eGFR was correlated reasonably well with concentrations of creatinine (r = -0.98), urea (r_s = -0.84), SDMA (r = -0.82) and middle molecules CfD and β2M (both r_s = -0.90). In contrast, poor correlation coefficients were found for CMPF (r_s = -0.32), UA (r_s = -0.45), ADMA (r_s = -0.47) and pCG (r_s = -0.48). The other toxins, all protein-bound, had r_s between -0.75 and -0.57. Comparable correlations were found between the three evaluated eGFR equations and uraemic toxin concentrations.

CONCLUSIONS

This study demonstrates that eGFR poorly predicts concentrations of protein-bound uraemic toxins, UA and ADMA in childhood CKD. Therefore, eGFR only partially reflects the complexity of the accumulation pattern of uraemic toxins in childhood CKD.

Establishing the presence or absence of chronic kidney disease: Uses and limitations of formulas estimating the glomerular filtration rate

The development of formulas estimating glomerular filtration rate (eGFR) from serum creatinine and cystatin C and accounting for certain variables affecting the production rate of these biomarkers, including ethnicity, gender and age, has led to the current scheme of diagnosing and staging chronic kidney disease (CKD), which is based on eGFR values and albuminuria. This scheme has been applied extensively in various populations and has led to the current estimates of prevalence of CKD. In addition, this scheme is applied in clinical studies evaluating the risks of CKD and the efficacy of various interventions directed towards improving its course. Disagreements between creatinine-based and cystatin-based eGFR values and between eGFR values and measured GFR have been reported in various cohorts. These disagreements are the consequence of variations in the rate of production and in factors, other than GFR, affecting the rate of removal of creatinine and cystatin C. The disagreements create limitations for all eGFR formulas developed so far. The main limitations are low sensitivity in detecting early CKD in several subjects, e.g., those with hyperfiltration, and poor prediction of the course of CKD. Research efforts in CKD are currently directed towards identification of biomarkers that are better indices of GFR than the current biomarkers and, particularly, biomarkers of early renal tissue injury.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction - manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre-sympathetic but also vasopressin-secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide-dependent effects on neural activity, depletion of nitric oxide and induction of low-grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide-dependent and pro-inflammatory pathways.

Uremic Solutes in Chronic Kidney Disease and Their Role in Progression

Background

To date, over 150 possible uremic solutes have been listed, but their role in the progression of CKD is largely unknown. Here, the association between a selected panel of uremic solutes and progression in CKD patients was investigated.

Methods

Patients from the MASTERPLAN study, a randomized controlled trial in CKD patients with a creatinine clearance between 20 and 70 ml/min per 1.73m², were selected based on their rate of eGFR decline during the first five years of follow-up. They were categorized as rapid (decline >5 ml/min per year) or slow progressors. Concentrations of eleven uremic solutes were obtained at baseline and after one year of follow-up. Logistic regression was used to compare the odds for rapid to slow progression by uremic solute concentrations at baseline. Variability in uremic solute levels was assessed using scatter plots, and limits of variability were calculated.

Results

In total, 40 rapidly and 40 slowly progressing patients were included. Uremic solutes were elevated in all patients compared to reference values for healthy persons. The serum levels of uremic solutes were not associated with rapid progression. Moreover, we observed substantial variability in solute levels over time.

Conclusions

Elevated concentrations of uremic solutes measured in this study did not explain differences in rate of eGFR decline in CKD patients, possibly due to lack of power as a result of the small sample size, substantial between patient variability, and variability in solute concentrations over time. The etiology of intra-individual variation in uremic solute levels remains to be elucidated.

New methods and technologies for measuring uremic toxins and quantifying dialysis adequacy

This publication reviews the currently available methods to identify uremic retention solutes, to determine their biological relevance and to quantify their removal. The analytical methods for the detection of uremic solutes have improved continuously, allowing the identification of several previously unknown solutes. Progress has been accelerated by the development of comprehensive strategies such as genomics, proteomics and the latest "omics" area, metabolomics. Those methodologies will be further refined in future. Once the concentration of solutes of interest is known based on targeted analysis, their biological relevance can be studied by means of in vitro, ex vivo, or animal models, provided those are representative for the key complications of the uremic syndrome. For this to come to pass, rigid protocols should be applied, e.g., aiming at free solute concentrations conform those found in uremia. Subsequently, the decrease in concentration of relevant solutes should be pursued by nondialysis (e.g., by influencing nutritional intake or intestinal generation, using sorbents, modifying metabolism, or preserving renal function) and dialysis methods. Optimal dialysis strategies can be sought by studying solute kinetics during dialysis. Clinical studies are necessary to assess the correct impact of those optimized strategies on outcomes. Although longitudinal studies of solute concentration and surrogate outcome studies are first steps in suggesting the usefulness of a given approach, ultimately hard outcome randomized controlled trials are needed to endorse evidence-based therapeutic choices. The nonspecificity of dialysis removal is however a handicap limiting the chances to provide proof of concept that a given solute or group of solutes has definite biological impact.

Most blood biomarkers related to vitamin status, one-carbon metabolism, and the kynurenine pathway show adequate preanalytical stability and within-person reproducibility to allow assessment of exposure or nutritional status in healthy women and cardiovascular patients

Knowledge of stability during sample transportation and changes in biomarker concentrations within person over time are paramount for proper design and interpretation of epidemiologic studies based on a single measurement of biomarker status. Therefore, we investigated stability and intraindividual vs. interindividual variation in blood concentrations of biomarkers related to vitamin status, one-carbon metabolism, and the kynurenine pathway. Whole blood (EDTA and heparin, n = 12) was stored with an icepack for 24 or 48 h, and plasma concentrations of 38 biomarkers were determined. Stability was calculated as change per hour, intraclass correlation coefficient (ICC), and simple Spearman correlation. Within-person reproducibility of biomarkers was expressed as ICC in samples collected 1-2 y apart from 40 postmenopausal women and in samples collected up to 3 y apart from 551 patients with stable angina pectoris. Biomarker stability was similar in EDTA and heparin blood. Most biomarkers were essentially stable, except for choline and total homocysteine (tHcy), which increased markedly. Within-person reproducibility in postmenopausal women was excellent (ICC > 0.75) for cotinine, all-trans retinol, cobalamin, riboflavin, α-tocopherol, Gly, pyridoxal, methylmalonic acid, creatinine, pyridoxal 5'-phosphate, and Ser; was good to fair (ICC of 0.74-0.40) for pyridoxic acid, kynurenine, tHcy, cholecalciferol, flavin mononucleotide, kynurenic acid, xanthurenic acid, 3-hydroxykynurenine, sarcosine, anthranilic acid, cystathionine, homoarginine, 3-hydroxyanthranilic acid, betaine, Arg, folate, total cysteine, dimethylglycine, asymmetric dimethylarginine, neopterin, symmetric dimethylarginine, and Trp; and poor (ICC of 0.39-0.15) for methionine sulfoxide, Met, choline, and trimethyllysine. Similar reproducibilities were observed in patients with coronary heart disease. Thus, most biomarkers investigated were essentially stable in cooled whole blood for up to 48 h and had a sufficient within-person reproducibility to allow one-exposure assessment of biomarker status in epidemiologic studies. The Western Norway B Vitamin Intervention Trial was registered at clinicaltrials.gov as NTC00354081.

Adjusted Anion Gap Is Associated with Glomerular Filtration Rate Decline in Chronic Kidney Disease

BACKGROUND

Metabolic acidosis is known to accelerate the progression of chronic kidney disease (CKD). However, whether undetermined anions as indicated by the adjusted anion gap (aAG) are associated with estimated glomerular filtration rate (eGFR) decline in patients with CKD is unclear.

METHODS

Data from 42 patients with CKD (baseline eGFR, 7.1-52.0 ml/min/ 1.73 m²) without massive proteinuria (urinary protein-creatinine ratio, UPCR <3.5) were retrospectively analyzed. aAG was calculated from serum sodium, serum chloride, serum bicarbonate, serum albumin, serum potassium, serum calcium and serum phosphate. The association between the percentage of the 6-month change of eGFR (%ΔeGFR/6m) and aAG was examined.

RESULTS

The mean baseline eGFR was 27.5 ± 11.1 ml/min/1.73 m² and the mean %ΔeGFR/6m was 13.8 ± 10.3. UPCR and aAG were 1.13 ± 0.93 and 9.48 ± 1.88, respectively. %ΔeGFR/6m was associated with aAG (r = 0.438, p < 0.005), but not with UPCR (r = 0.194, p = 0.218). In multivariate linear regression analyses, aAG remained significantly associated with %ΔeGFR/6m (β = 0.45, p < 0.01) after controlling for age, baseline eGFR, UPCR and HCO₃^- concentration.

CONCLUSION

These data suggest that aAG appears to be associated with the progression of CKD. aAG might be an independent predictor of CKD progression.

Does the adequacy parameter Kt/V(urea) reflect uremic toxin concentrations in hemodialysis patients?

Hemodialysis aims at removing uremic toxins thus decreasing their concentrations. The present study investigated whether Kt/V_urea, used as marker of dialysis adequacy, is correlated with these concentrations. Predialysis blood samples were taken before a midweek session in 71 chronic HD patients. Samples were analyzed by colorimetry, HPLC, or ELISA for a broad range of uremic solutes. Solute concentrations were divided into four groups according to quartiles of Kt/V_urea, and also of different other parameters with potential impact, such as age, body weight (BW), Protein equivalent of Nitrogen Appearance (PNA), Residual Renal Function (RRF), and dialysis vintage. Dichotomic concentration comparisons were performed for gender and Diabetes Mellitus (DM). Analysis of Variance in quartiles of Kt/V_urea did not show significant differences for any of the solute concentrations. For PNA, however, concentrations showed significant differences for urea (P<0.001), uric acid (UA), p-cresylsulfate (PCS), and free PCS (all P<0.01), and for creatinine (Crea) and hippuric acid (HA) (both P<0.05). For RRF, concentrations varied for β₂-microglobulin (P<0.001), HA, free HA, free indoxyl sulfate, and free indole acetic acid (all P<0.01), and for p-cresylglucuronide (PCG), 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), free PCS, and free PCG (all P<0.05). Gender and body weight only showed differences for Crea and UA, while age, vintage, and diabetes mellitus only showed differences for one solute concentration (UA, UA, and free PCS, respectively). Multifactor analyses indicated a predominant association of concentration with protein intake and residual renal function. In conclusion, predialysis concentrations of uremic toxins seem to be dependent on protein equivalent of nitrogen appearance and residual renal function, and not on dialysis adequacy as assessed by Kt/V_urea. Efforts to control intestinal load of uremic toxin precursors by dietary or other interventions, and preserving RRF seem important approaches to decrease uremic solute concentration and by extension their toxicity.

Renoprotection and the Bardoxolone Methyl Story - Is This the Right Way Forward? A Novel View of Renoprotection in CKD Trials: A New Classification Scheme for Renoprotective Agents

In the June 2011 issue of the New England Journal of Medicine, the BEAM (Bardoxolone Methyl Treatment: Renal Function in CKD/Type 2 Diabetes) trial investigators rekindled new interest and also some controversy regarding the concept of renoprotection and the role of renoprotective agents, when they reported significant increases in the mean estimated glomerular filtration rate (eGFR) in diabetic chronic kidney disease (CKD) patients with an eGFR of 20-45 ml/min/1.73 m(2) of body surface area at enrollment who received the trial drug bardoxolone methyl versus placebo. Unfortunately, subsequent phase IIIb trials failed to show that the drug is a safe alternative renoprotective agent. Current renoprotection paradigms depend wholly and entirely on angiotensin blockade; however, these agents [angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs)] have proved to be imperfect renoprotective agents. In this review, we examine the mechanistic limitations of the various previous randomized controlled trials on CKD renoprotection, including the paucity of veritable, elaborate and systematic assessment methods for the documentation and reporting of individual patient-level, drug-related adverse events. We review the evidence base for the presence of putative, multiple independent and unrelated pathogenetic mechanisms that drive (diabetic and non-diabetic) CKD progression. Furthermore, we examine the validity, or lack thereof, of the hyped notion that the blockade of a single molecule (angiotensin II), which can only antagonize the angiotensin cascade, would veritably successfully, consistently and unfailingly deliver adequate and qualitative renoprotection results in (diabetic and non-diabetic) CKD patients. We clearly posit that there is this overarching impetus to arrive at the inference that multiple, disparately diverse and independent pathways, including any veritable combination of the mechanisms that we examine in this review, and many more others yet to be identified, do concurrently and asymmetrically contribute to CKD initiation and propagation to end-stage renal disease (ESRD) in our CKD patients. We conclude that current knowledge of CKD initiation and progression to ESRD, the natural history of CKD and the impacts of acute kidney injury on this continuum remain in their infancy and call for more research. Finally, we suggest a new classification scheme for renoprotective agents: (1) the single-pathway blockers that block a single putative pathogenetic pathway involved in CKD progression, as typified by ACE inhibitors and/or ARBs, and (2) the multiple-pathway blockers that are able to block or antagonize the effects of multiple pathogenetic pathways through their ability to simultaneously block, downstream, the effects of several pathways or mechanisms of CKD to ESRD progression and could therefore concurrently interfere with several unrelated upstream pathways or mechanisms. We surmise that maybe the ideal and truly renoprotective agent, clearly a multiple-pathway blocker, is on the horizon. This calls for more research efforts from all.

Estimated glomerular filtration rate is a poor predictor of the concentration of middle molecular weight uremic solutes in chronic kidney disease

Background

Uremic solute concentration increases as Glomerular Filtration Rate (GFR) declines. Weak associations were demonstrated between estimated GFR (eGFR) and the concentrations of several small water-soluble and protein-bound uremic solutes (MW<500Da). Since also middle molecular weight proteins have been associated with mortality and cardiovascular damage in Chronic Kidney Disease (CKD), we investigated the association between several eGFR formulae and the concentration of Low Molecular Weight Proteins (LMWP) (MW>500Da).

Materials and Methods

In 95 CKD-patients (CKD-stage 2–5 not on dialysis), associations between different eGFR-formulae (creatinine, CystatinC-based or both) and the natural logarithm of the concentration of several LMWP’s were analyzed: i.e. parathyroid hormone (PTH), Cystatin C (CystC), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), leptin, retinol binding protein (RbP), immunoglobin light chains kappa and lambda (Ig-κ and Ig-λ), beta-2-microglobulin (β₂M), myoglobin and fibroblast growth factor-23 (FGF-23)).

Results

The regression coefficients (R²) between eGFR, based on the CKD-EPI-Crea-CystC-formula as reference, and the examined LMWP’s could be divided into three groups. Most of the LMWP’s associated weakly (R² <0.2) (FGF-23, leptin, IL-6, TNF-α, Ig-κ, Ig-λ) or intermediately (R² 0.2–0.7) (RbP, myoglobin, PTH). Only β₂M and CystC showed a strong association (R² >0.7). Almost identical R²-values were found per LMWP for all eGFR-formulae, with exception of CystC and β₂M which showed weaker associations with creatinine-based than with CystC-based eGFR.

Conclusion

The association between eGFR and the concentration of several LMWP’s is inconsistent, with in general low R²-values. Thus, the use of eGFR to evaluate kidney function does not reflect the concentration of several LMWP’s with proven toxic impact in CKD.