A novel mechanism for skeletal resistance in uremia

Skeletal parathyroid hormone hyporesponsiveness: a neglected, but clinically relevant reality in chronic kidney disease

PURPOSE OF REVIEW

Defining the optimal parathyroid hormone (PTH) target in chronic kidney disease (CKD) is challenging, especially for bone outcomes, due to the substantial variability in the skeleton's response to PTH. Although PTH hyporesponsiveness is as integral a component of CKD-mineral bone disorder as elevated PTH levels, clinical awareness of this condition is limited. In this review, we will discuss factors and mechanisms contributing to PTH hyporesponsiveness in CKD. This knowledge may provide clues towards a personalized approach to treating secondary hyperparathyroidism in CKD.

RECENT FINDINGS

Indicates a link between disturbed phosphate metabolism and impaired skeletal calcium sensing receptor signaling as an important mediator of PTH hyporesponsiveness in CKD. Further, cohort studies with diverse populations point towards differences in mineral metabolism control, rather than genetic or environmental factors, as drivers of the variability of PTH responsiveness.

IN SUMMARY

Skeletal PTH hyporesponsiveness in CKD has a multifactorial origin, shows important interindividual variability, and is challenging to estimate in clinical practice. The variability in skeletal responsiveness compromises PTH as a biomarker of bone turnover, especially when considering populations that are heterogeneous in ethnicity, demography, kidney function, primary kidney disease and mineral metabolism control, and in patients treated with bone targeting drugs.

Determination of parathyroid hormone: from radioimmunoassay to LCMS/MS

Parathyroid hormone (PTH) determination is of paramount importance for the exploration of diseases related with calcium metabolism and for the follow-up of patients suffering from bone and mineral disorders associated with chronic kidney diseases (CKD-MBD). Unfortunately, the biologically active form of PTH, i.e. 1-84 PTH, circulates in the blood stream with many fragments and post-translationally modified forms, which decreases the specificity of immunoassays. The assays used to measure PTH, either from 2nd or 3rd generation, are not standardised, which may lead to interpretation errors and clinical consequences. Reference ranges for PTH have neither been always correctly established and the stability of the peptide is also a matter of concern. Fortunately, these last years, newer techniques using mass spectrometry (either high resolution or triple quadripole) coupled with liquid chromatography have been developed, which will help to standardise the different assays. Indeed, PTH assays standardisation is one of the task of the IFCC Committee for Bone Metabolism. Such standardisation will allow a better consistency in the interpretation of the results and will promote studies aiming at the establishment of correct reference ranges.

Bone Disease in Chronic Kidney Disease and Kidney Transplant

Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD) comprises alterations in calcium, phosphorus, parathyroid hormone (PTH), Vitamin D, and fibroblast growth factor-23 (FGF-23) metabolism, abnormalities in bone turnover, mineralization, volume, linear growth or strength, and vascular calcification leading to an increase in bone fractures and vascular disease, which ultimately result in high morbidity and mortality. The bone component of CKD-MBD, referred to as renal osteodystrophy, starts early during the course of CKD as a result of the effects of progressive reduction in kidney function which modify the tight interaction between mineral, hormonal, and other biochemical mediators of cell function that ultimately lead to bone disease. In addition, other factors, such as osteoporosis not apparently dependent on the typical pathophysiologic abnormalities resulting from altered kidney function, may accompany the different varieties of renal osteodystrophy leading to an increment in the risk of bone fracture. After kidney transplantation, these bone alterations and others directly associated or not with changes in kidney function may persist, progress or transform into a different entity due to new pathogenetic mechanisms. With time, these alterations may improve or worsen depending to a large extent on the restoration of kidney function and correction of the metabolic abnormalities developed during the course of CKD. In this paper, we review the bone lesions that occur during both CKD progression and after kidney transplant and analyze the factors involved in their pathogenesis as a means to raise awareness of their complexity and interrelationship.

Bone Turnover Markers: Basic Biology to Clinical Applications

Bone turnover markers (BTMs) are used widely, in both research and clinical practice. In the last 20 years, much experience has been gained in measurement and interpretation of these markers, which include commonly used bone formation markers bone alkaline phosphatase, osteocalcin, and procollagen I N-propeptide; and commonly used resorption markers serum C-telopeptides of type I collagen, urinary N-telopeptides of type I collagen and tartrate resistant acid phosphatase type 5b. BTMs are usually measured by enzyme-linked immunosorbent assay or automated immunoassay. Sources contributing to BTM variability include uncontrollable components (e.g., age, gender, ethnicity) and controllable components, particularly relating to collection conditions (e.g., fasting/feeding state, and timing relative to circadian rhythms, menstrual cycling, and exercise). Pregnancy, season, drugs, and recent fracture(s) can also affect BTMs. BTMs correlate with other methods of assessing bone turnover, such as bone biopsies and radiotracer kinetics; and can usefully contribute to diagnosis and management of several diseases such as osteoporosis, osteomalacia, Paget's disease, fibrous dysplasia, hypophosphatasia, primary hyperparathyroidism, and chronic kidney disease-mineral bone disorder.

Renal hyperparathyroidism

The number of the patients with chronic kidney disease is now increasing in the world. The pathophysiology of renal hyperparathyroidism is closely associated with Klotho-FGF-endocrine axes, which must be solved definitively as early as possible. It was revealed that the expression of fgf23 is activated by calciprotein particles, which induces vascular ossification. And it is well known that phosphorus overload directly increases parathyroid hormone and hyperparathyroid bone disease develops in those subjects. On the other hand, low turnover bone disease is often recently. Both the patients with chronic kidney disease suffering from hyperparathyroid bone disease or low turnover bone disease are associated with increased fracture risk. Micropetrosis may be one of the causes of increased fracture risk in the subjects with low turnover bone disease. In this chapter, we now describe the diagnosis, pathophysiology and treatments of renal hyperparathyroidism.

1,25-dihydroxyvitamin D as Predictor of Renal Worsening Function in Chronic Kidney Disease. Results From the PASCaL-1,25D Study

Background

Heterogeneous progression of chronic kidney disease (CKD) toward dialysis advocates improving in renal care management. Diagnosis and staging of CKD relies on estimated glomerular filtration rate (eGFR) and albuminuria. Tubular biomarkers emerged as new predictors of worsening renal function (WRF), due to partial inaccuracy of eGFR and existing WRF in non-proteinuric patients. Active vitamin D is synthesized in renal tubules and participates to mineral adaptation in CKD. Circulating 1,25-dihydroxyvitamin D [1,25(OH)₂D] was poorly investigated as a biomarker of endocrine tubular function and predictor of WRF.

Objective

Investigate capability of 1,25(OH)₂D to predict parathormone (PTH) increase and WRF in CKD stage 3-4.

Methods

PASCaL-1,25D was an observational, prospective, monocentric study. Primary outcomes were absolute and 20% increase in PTH, and WRF defined as 20% reduction in eGFR or dialysis initiation at 6 months.

Results

Seventy-one patients completed follow up. Absolute increase in PTH (1-84) was independently predicted by lower 1,25(OH)₂D levels (p = 0.0134). No association was detected between 1,25(OH)₂D and iPTH increase. Higher 1,25(OH)₂D was associated with reduced risk of WRF at univariate analysis [OR 0.89 (95% CI 0.86-0.93), p = 0.006]. The 1,25(OH)₂D/PTH (1-84) ratio was associated with non-significant 84% risk reduction for WRF [OR 0.16 (95% CI 0.06-0.41), p = 0.05]. Low 1,25(OH)₂D reached 100% sensitivity in predicting WRF in CKD stage 3 (AUC 9.909, p < 0.0001) and non-elderly patients (AUC 0.883, p < 0.0001). Machine learning models retained 1,25(OH)₂D/PTH (1-84) as relevant predictor of WRF together with eGFR and albuminuria. Age influenced interaction between renal and mineral biomarkers.

Conclusion

1,25(OH)₂D deserves attention as biomarker of tubular health, and sensible predictor of WRF on the short run among non-elderly patients affected by stage 3 CKD. The 1,25(OH)₂D/PTH (1-84) ratio may represent a composite biomarker of tubular reserve/endocrine response to the transition from adaptive to maladaptive equilibrium in CKD-MBD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Bone responsiveness to parathyroid hormone is negatively associated with parathyroid hormone-lowering drug use in patients undergoing hemodialysis: a cross-sectional study

BACKGROUND

Parathyroid hormone (PTH) acts on bone to indirectly increase the number and activity of osteoclasts. Thus, PTH has a stimulatory effect on bone resorption and upregulates bone turnover. However, the responsiveness of bone to PTH varies widely among patients receiving dialysis. In fact, relative to the serum PTH level, the level of serum tartrate-resistant acid phosphatase-5b (TRACP-5b), a bone resorption marker derived from osteoclasts, varies as well. This study aimed to examine factors related to bone responsiveness to PTH in patients undergoing chronic hemodialysis (HD).

METHODS

This study included patients receiving chronic HD in Kawasaki Municipal Tama Hospital (Kanagawa, Japan) and Yonaha Medical Clinic (Okinawa, Japan) and excluded patients who received HD for less than 6 months, those who received a combination of HD and peritoneal dialysis, and those who had cancer bone metastases or myeloma. The TRACP-5b/intact PTH (iPTH) ratio was created as an index of bone responsiveness to PTH, categorized into tertiles (low, medium, and high), and a cross-sectional study was conducted. P < 0.05 indicated statistically significant differences.

RESULTS

One hundred and six patients were analyzed. Age (P = 0.010), body mass index (BMI) (P = 0.003), use of calcium-sensing receptor (CaSR) agonists (P = 0.008), use of vitamin D receptor activators (VDRAs) (P = 0.012), plasma iPTH level (P < 0.001), serum 1,25(OH)₂D level (P = 0.003), and serum TRACP-5b level (P < 0.001) were significantly different among the three categories. In the single linear regression analysis, age (P = 0.016), corrected serum calcium level (P = 0.007), and ln [1,25(OH)₂D] (P = 0.044) showed a significant positive correlation with ln [TRACP-5b/iPTH], whereas BMI (P = 0.026), use of CaSR agonists (P = 0.001), use of VDRAs (P = 0.009), and serum phosphorus level (P = 0.018) showed a significant negative correlation. Upon conducting multiple linear regression analysis incorporating significant variables in the single linear regression analysis, a significant negative correlation was observed between the TRACP-5b/iPTH ratio and intravenous administration of a CaSR agonist (etelcalcetide) and/or a VDRA (calcitriol or maxacalcitol) in all the adjusted models.

CONCLUSIONS

Bone responsiveness to PTH is negatively correlated with the intravenous administration of a CaSR agonist and/or a VDRA in patients undergoing chronic HD.

Effects of parathyroidectomy on plasma PTH fragments and heart rate variability in stage 5 chronic kidney disease patients

INTRODUCTION

Circulating intact parathyroid hormone (iPTH) levels include full-length (1-84) PTH and long C-PTH fragments, but primarily (7-84) PTH, which have been reported to have antagonistic effects on the bones and kidneys. However, their effects on the cardiovascular system remain unclear. In this study, the relationships between the plasma PTH fragments levels and heart rate variability (HRV) in stage 5 chronic kidney disease (CKD5) patients are explored. Furthermore, the effects of parathyroidectomy (PTX) on the above indices are investigated.

METHODS

In this cross-sectional study, 164 healthy controls and 354 CKD5 patients, including 208 secondary hyperparathyroidism (SHPT) subgroup with PTX, were enrolled. Circulating (7-84) PTH levels were calculated by subtracting plasma (1-84) PTH levels from iPTH levels. The HRV parameters were measured using a 24-hour Holter.

RESULTS

The baseline levels of plasma iPTH, (1-84) PTH, and (7-84) PTH in the CKD5 patients were 930.40 (160.65, 1792.50) pg/mL, 448.60 (99.62, 850.45) pg/mL, and 468.20 (54.22, 922.55) pg/mL, respectively. In the CKD5 patients, plasma (1-84) PTH levels were independently correlated with the standard deviation of the normal-to-normal R-R intervals (SDNN) and the standard deviation of the five-minute average of the normal R-R intervals (SDANN). With a median follow up time of 6.50 months after PTX in the SHPT patients (n = 30), improved SDNN and SDANN markers were related with decreased (1-84) PTH levels. Furthermore, an improved SDNN was related with decreased (7-84) PTH levels.

CONCLUSIONS

The CKD5 patients' baseline (1-84) PTH levels were correlated with the SDNN and SDANN. After PTX, an improved SDNN was related with decreased (1-84) PTH and (7-84) PTH levels, while improved SDANN was related with decreased (1-84) PTH levels. No antagonistic effects of (1-84) PTH and (7-84) PTH on HRV were found in the CKD5 patients.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Chemical Characterization and Quantification of Circulating Intact PTH and PTH Fragments by High-Resolution Mass Spectrometry in Chronic Renal Failure

BACKGROUND

The precise concentrations of full-length parathyroid hormone (PTH1-84) and the identity and concentrations of PTH fragments in patients with various stages of chronic renal failure are unknown.

METHODS

We developed a liquid chromatography-high resolution mass spectrometry (LC-HRMS) method to characterize and quantify PTH1-84 and PTH fragments in serum of 221 patients with progressive renal dysfunction. Following capture by matrix-bound amino-terminal or carboxyl-terminal region-specific antibodies and elution from matrix, PTH1-84 and PTH fragments were identified and quantitated using LC-HRMS. PTH was simultaneously measured using an intact PTH (iPTH) immunoassay.

RESULTS

Full-length PTH1-84 and 8 PTH fragments (PTH28-84, 34-77, 34-84, 37-77, 37-84, 38-77, 38-84, and 45-84) were unequivocally identified and were shown to increase significantly when an eGFR declined to ≤17-23 mL/min/1.73m2. Serum concentrations of PTH1-84 were similar when measured by LC-HRMS following capture by amino-terminal or carboxyl-terminal immunocapture methods. In patients with an eGFR of <30 mL/min/1.73 m2, serum PTH concentrations measured using LC-HRMS were significantly lower than PTH measured using an iPTH immunoassay. PTH7-84 and oxidized forms of PTH1-84 were below the limit of detection (30 and 50 pg/mL, respectively).

CONCLUSIONS

LC-HRMS identifies circulating PTH1-84, carboxyl-terminal PTH fragments, and mid-region PTH fragments, in patients with progressive renal failure. Serum PTH1-84 and its fragments markedly rise when an eGFR decreases to ≤17-23 mL/min/1.73 m2. PTH concentrations measured using LC-HRMS tend to be lower than those measured using an iPTH immunoassay, particularly in severe chronic renal failure. Our data do not support the existence of circulating PTH7-84 and oxidized PTH1-84.

Parathyroid Hormone Concentrations in Maintenance Hemodialysis: Longitudinal Evaluation of Intact and Biointact Assays

Rationale & Objective

Management of chronic kidney disease mineral and bone disorder requires parathyroid hormone (PTH) concentrations. “Biointact” PTH immunoassays detect “whole” PTH (wPTH), whereas “intact” immunoassays measure PTH plus PTH fragments (iPTH). We aimed to determine whether longitudinal changes in PTH concentrations can be evaluated using biointact and intact immunoassays alike.

Study Design

Open noninterventional longitudinal cohort study.

Setting & Participants

PTH concentrations were measured quarterly up to 5 times in 102 hemodialysis patients.

Predictors & Tests Compared

Age, sex, phosphate levels, and others as clinical predictors for PTH trend. Tests compared were iPTH immunoassays from Siemens and Roche and wPTH immunoassays from Roche and DiaSorin.

Outcomes

PTH concentration trend; regression equations; test bias.

Analytical Approach

Predictive regression-to-the-mean model for PTH slope; Bland-Altman plots, Passing-Bablok regression, and reference change values for test comparisons.

Results

wPTH concentrations were similar with both immunoassays (wPTH-Roche = 11.7 + 0.97 × wPTH-DiaSorin, r = 0.99; mean ± 1.96 SD bias, 8.2 ± 43.3 pg/mL [17.5% ± 40.9%], by Bland-Altman plots). iPTH-Siemens concentrations were higher than iPTH-Roche concentrations (iPTH-Siemens = −5.4 + 1.33 × iPTH-Roche, r = 0.99; mean ± 1.96 SD bias, 84.0 ± 180.2 pg/mL [21.1% ± 29.8%], by Bland-Altman plots). iPTH-Roche and iPTH-Siemens concentrations were 2- and 2.5-fold higher than wPTH concentrations, respectively. Full agreement among all 4 immunoassays in detecting both significant and insignificant changes in PTH concentrations, upward or downward from one quarter to the next, was reached in 87% of consecutive measurements. In a predictive model, baseline PTH concentrations > 199 pg/mL (wPTH-Roche), 204 pg/mL (wPTH-DiaSorin), 386 pg/mL (iPTH-Roche), and 417 pg/mL (iPTH-Siemens) correctly predicted declining PTH concentration trend in 62% to 68% of patients, but age, sex, hemodialysis vintage, and calcium and phosphate levels were no significant predictors.

Limitations

Limited number of immunoassays, only 59 patients attended all quarterly samplings.

Conclusions

wPTH-Roche and wPTH-DiaSorin concentrations were similar, while iPTH was higher than wPTH concentrations. The iPTH-Siemens immunoassay is either higher calibrated or detects more fragments than iPTH-Roche. However, longitudinal PTH concentration changes largely coincided with all tested immunoassays.

Comparison of Pre- and Post-transplant Parathyroidectomy in Renal Transplant Recipients and the Impact of Parathyroidectomy Timing on Calcium Metabolism and Renal Allograft Function: A Retrospective Single-Center Analysis

BACKGROUND

The effect of parathyroidectomy (PTx) timing on serum calcium (Ca) levels and renal functions in renal transplant recipients with severe hyperparathyroidism (HPT) remains unclear. We retrospectively aimed to investigate and compare the clinical data of patients who underwent pre- and post-transplant PTx and elucidated the impact of PTx timing on serum Ca levels and renal graft outcomes after renal transplantation (RTx).

METHODS

During January 2000-December 2016, 53 and 55 patients underwent post-transplant PTx (Post-RTx group) and pretransplant PTx (Pre-RTx group), respectively. The serum Ca levels and estimated glomerular filtration rate (eGFR) were assessed in both groups.

RESULTS

At the end of the follow-up, the serum Ca levels were significantly higher and the incidence of hypocalcemia was significantly lower in the Pre-RTx group than in the Post-RTx group [9.5 vs. 8.9 mg/dL, P < 0.001; 14.5% vs. 34.0%, P = 0.024]. The decrease in the eGFR 12-36 months after RTx was more significant in the Post-RTx group than in the Pre-RTx group (-13.8% vs. -0.9%; P = 0.001). A logistic regression involving age, sex, dialysis period, and serum parathormone level revealed that post-transplant PTx is an independent risk factor for persistent hypocalcemia at the end of the follow-up (P = 0.034) and for a >20% decrease in the eGFR 12-36 months after RTx (P = 0.029).

CONCLUSIONS

In renal transplant candidates with severe HPT, pretransplant PTx should be considered to prevent persistent hypocalcemia and deterioration of the renal graft function.

Management of Renal Osteodystrophy in Peritoneal Dialysis Patients

The term “renal osteodystrophy” encompasses all forms of metabolic bone disease found in dialysis patients. The primary approach to the treatment of renal osteodystrophy in peritoneal dialysis (PD) patients is similar to that in hemodialysis patients. However, the increased prevalence of adynamic bone histology, together with the difficulty in judging calcium balance, the inability to practicably give intravenous vitamin D, and the clearance of vitamin D and parathyroid hormone via dialysate require a different therapeutic approach in PD compared to hemodialysis patients. Clearly, more comparative studies of new agents are needed to find the optimal approach to achieving the K/DOQI guidelines in PD patients. The unique aspects of the approach to renal osteodystrophy in PD patients are the focus of this review.

Changes in the whole/intact parathyroid hormone ratio and their clinical implications in patients with chronic kidney disease

BACKGROUND

Although a previous study has reported the relationship between intact parathyroid hormone (iPTH) and whole parathyroid hormone (wPTH) levels in patients undergoing dialysis, the w/i PTH ratio (whole/intact PTH ratio) among predialysis chronic kidney disease (CKD) patients remains unclear. The present study therefore aimed to examine the relationship between w/i PTH ratio and kidney function and determine other factors affecting the w/i PTH ratio.

METHODS

An observational study including 773 predialysis CKD patients was conducted. The correlation between w/i PTH ratio and kidney function, as well as clinical factors at different CKD stages, were assessed using univariate and multivariate analyses. In addition, the relationship between w/i PTH ratio and composite renal outcome [kidney transplantation, dialysis, and 30% decline in estimated glomerular filtration rate (eGFR)] was examined.

RESULTS

The w/i PTH ratio decreased as the CKD stage progressed. Patients in each CKD stage (1/2, 3, 4, and 5) had a w/i PTH ratio of 0.85, 0.81, 0.78, and 0.59, respectively. The inflection point in the correlation lines between eGFR and w/i PTH ratio was 24.1 mL/min/1.73 m². In multivariate analysis, the w/i PTH ratio was significantly correlated with serum calcium levels only in the CKD5 group and with eGFR in the CKD3, CKD4 and CKD5 group. Furthermore, w/i PTH ratio, eGFR, serum phosphate levels, and urinary protein/creatinine ratio were determined to be significant independent predictors for composite renal outcome.

CONCLUSIONS

Our study demonstrated that changes in the w/i PTH ratio were associated with kidney function, abnormal mineral metabolism, and renal outcome.

PTH and PTHrP Actions on Bone

Parathyroid hormone (PTH), PTH-related peptide (PTHrP), PTHR, and their cognate G protein-coupled receptor play defining roles in the regulation of extracellular calcium and phosphate metabolism and in controlling skeletal growth and repair. Acting through complex signaling mechanisms that in many instances proceed in a tissue-specific manner, precise control of these processes is achieved. A variety of direct and indirect disease processes, along with genetic anomalies, can cause these schemes to become dysfunctional. Here, we review the basic components of this regulatory network and present both the well-established elements and emerging findings and concepts with the overall objective to provide a framework for understanding the elementary aspects of how PTH and PTHrP behave and as a call to encourage further investigation that will yield more comprehensive understanding of the physiological and pathological steps at play, with a goal toward novel therapeutic interventions.

Bone biomarkers in de novo renal transplant recipients

Successful kidney transplantation (partly) corrects the physiologic and metabolic abnormalities driving chronic kidney disease - mineral and bone disorders. At the same time, renal transplant recipients are exposed to immunosuppressive agents that may affect bone metabolism. Bone biomarkers have been suggested as surrogates of or adjuncts to bone biopsy and imaging techniques to assess bone health and to classify risk of bone loss and fractures. Bone biomarkers may be classified as circulating factors that affect bone metabolism (commonly referred to as bone metabolism markers) or that reflect bone cell number and/or activity (commonly referred to as bone turnover markers). A growing body of evidence shows that successful renal transplantation has a major impact on both bone metabolism and bone turnover. Analytical issues, including the cross-reactivity with fragments, complicate the interpretation of bone biomarkers, especially in the setting of a rapid changing kidney function, as is the case after successful renal transplantation. Overall, bone turnover seems to decline following renal transplantation, but inter-individual variability is substantial. Preliminary evidence indicates that bone biomarkers may be useful in guiding mineral and bone therapy in renal transplant recipients.

Parathyroid Hormone Measurement in Chronic Kidney Disease: From Basics to Clinical Implications

Accurate measurement of parathyroid hormone (PTH) is crucial for therapeutic decision-making in patients with chronic kidney disease-mineral and bone disorder (CKD-MBD). The second-generation PTH assays, often referred to as “intact PTH” assays, are the current standard and most available assays in clinical practice. However, intact PTH assays measure both full-length biologically active PTH and heterogeneous PTH fragments in the circulation, providing the equivocal value of PTH measurement in patients with CKD-MBD. Due to the variability of PTH assays, preanalytical sample errors, and the phenomenon of end-organ PTH hyporesponsiveness, current CKD-MBD guidelines recommend a wide range for serum PTH targets (2–9 the upper normal limit of the intact PTH assay) in dialysis patients to diminish the risk of developing adynamic bone disease. Nevertheless, a sizeable proportion of CKD patients still experience renal osteodystrophy despite having serum PTH levels within the recommended range. The primary cause of this inconsistency is the analytical interference of various PTH fragments and oxidized PTH forms that considerably accumulate in CKD patients. Therefore, a new mass spectrometry-based assay, which is capable of specifically measuring the whole spectra of PTH fragments, can potentially improve diagnostic accuracy for renal osteodystrophy. However, the effects of different PTH fragments on bone metabolism, vascular calcification, and mortality in CKD patients warrant further research.

Parathyroid Hormone Fragments: New Targets for the Diagnosis and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder

As a common disorder, chronic kidney disease (CKD) poses a great threat to human health. Chronic kidney disease-mineral and bone disorder (CKD-MBD) is a complication of CKD characterized by disturbances in the levels of calcium, phosphorus, parathyroid hormone (PTH), and vitamin D; abnormal bone formation affecting the mineralization and linear growth of bone; and vascular and soft tissue calcification. PTH reflects the function of the parathyroid gland and also takes part in the metabolism of minerals. The accurate measurement of PTH plays a vital role in the clinical diagnosis, treatment, and prognosis of patients with secondary hyperparathyroidism (SHPT). Previous studies have shown that there are different fragments of PTH in the body's circulation, causing antagonistic effects on bone and the kidney. Here we review the metabolism of PTH fragments; the progress being made in PTH measurement assays; the effects of PTH fragments on bone, kidney, and the cardiovascular system in CKD; and the predictive value of PTH measurement in assessing the effectiveness of parathyroidectomy (PTX). We hope that this review will help to clarify the value of accurate PTH measurements in CKD-MBD and promote the further development of multidisciplinary diagnosis and treatment.

Higher Proportion of Non-1-84 PTH Fragments in Peritoneal Dialysis Patients Compared to Hemodialysis Patients Using Solutions Containing 1.75 mmol/l Calcium

Background: The prevalence of low- turnover bone disease (LTBD) in peritoneal dialysis (PD) patients is higher than in hemodialysis (HD) patients. LTBD patients may be at risk for vascular calcification, and cardiovascular disease. Current therapy for chronic kidney disease metabolic bone disorders (CKD-MBD) is guided by biochemical parameters, as bone biopsy is not used in routine clinical care. Methods: We assessed intact PTH (iPTH: 1-84PTH plus non-1-84PTH), 1-84PTH, and the 1-84PTH/non-1-84PTH ratio in 129 hemodialysis and 73 PD prevalent patients dialyzed with solutions containing 1.75 mmol/L calcium. Results: Hemodialysis and PD patients presented similar iPTH and tCa values and prevalence of putative LTBD as defined according to KDOQI iPTH cut-off levels or 1-84 PTH levels. However, iCa accounted for a higher percentage of tCa in PD (53%) than in hemodialysis (39%) p < 0.001, and the 1-84PTH/non-1-84PTH ratio was lower in PD than in hemodialysis patients (0.44 ± 0.12) vs. (0.60 ± 0.10), p < 0.001. The prevalence of putative LTBD when using the coexistence of 1-84PTH/non-1-84PTH ratio < 1.0 and iPTH < 420 pg/m, was higher in PD than in hemodialysis patients (73 vs. 16% respectively, p < 0.001). In a multivariate logistic regression analysis, dialysis modality was the main determinant of the 1-84PTH/non-1-84PTH ratio. Conclusion: Solutions containing 1.75 mmol/L calciums are associated to a higher proportion of non-1-84PTH fragments in PD than in HD patients. Different analytical criteria result in widely different estimates of LTBD prevalence, thus impairing the ability of clinicians to optimize therapy for CKD-MBD.

Effect of uremic toxin-indoxyl sulfate on the skeletal system

Chronic kidney disease-mineral bone disorders (CKD-MBD) exhibit abnormalities in the circulating mineral levels, vitamin D metabolism, and parathyroid function that contribute to the formation of a bone lesion. The uremic toxin, indoxyl sulfate (IS), accumulates in the blood in cases of renal failure and leads to bone loss. The bone and renal responses to the action of the parathyroid hormone (PTH) are progressively decreased in CKD in spite of increasing PTH levels, a condition commonly called PTH resistance. There is a high prevalence of low bone turnover or adynamic bone disease in the early stages of CKD. This could be due to the inhibition of bone turnover, such as in PTH resistance, reduced active vitamin D levels, diabetes, aluminum, and, increased IS. With an increase in IS, there is a decrease in the osteoblast Wnt/b-catenin signaling and increase in the expression of Wnt signaling inhibitors, such as sclerostin and Dickkopf-1 (DKK1). Thus, a majority of early CKD patients exhibit deterioration of bone quality owing to the action of IS, this scenario could be termed uremic osteoporosis. However, this mechanism is complicated and not fully understood. With progressive deterioration in the renal function, IS accumulates along with persistent PTH secretion, potentially leading to high-turnover bone disease because high serum PTH levels have the ability of overriding peripheral PTH resistance and other inhibitory factors of bone formation. Finally, it leads to deterioration in bone quantity with prominent bone resorption in end stage renal disease. Uremic toxins adsorbents may decelerate oxidative stress and improve bone health in CKD patients. This review article focuses on IS and bone loss in CKD patients.

Analytical verification and method comparison of the ADVIA Centaur® Intact Parathyroid Hormone assay

OBJECTIVES

This study is the first verification of the novel iPTH Siemens ADVIA Centaur® Intact Parathyroid Hormone (iPTHm) chemiluminescence immunoassay based on monoclonal antibodies. We also compared the iPTH results obtained using this assay with the previous ADVIA Centaur® Parathyroid Hormone assay (iPTHp) based on polyclonal antibodies.

DESIGN AND METHODS

The analytical performance study of iPTHm assay included LoD, LoQ, intra- and inter-assay reproducibility, and linearity. A comparison study was performed on 369 routine plasma samples. The results were analyzed independently for patients with normal and abnormal GFR, as well as patients on hemodialysis. In addition, clinical concordance between assays was assessed. Finally, we studied PTH stability of plasma samples at 4°C.

RESULTS

For the iPTHm assay LoD and LoQ were 0.03pmol/L and 0.10pmol/L, respectively. Intra- and inter-assay CV were between 2.3% and 6.2%. Linearity was correct in the range from 3.82 to 203.08pmol/L. Correlation studies showed a good correlation (r=0.99) between iPTHm and iPTHp, with bias of -2.55% (IC -3.48% to -1.62%) in the range from 0.32 to 117.07pmol/L. Clinical concordance, assessed by Kappa Index, was 0.874. The stability study showed that differences compared to basal iPTH concentration did not exceed 20% in any of the samples analyzed.

CONCLUSIONS

The iPTHm assay demonstrated acceptable performance and a very good clinical concordance with iPTHp assay, currently used in our laboratory. Thus, the novel iPTHm assay can replace the previous iPTHp assay, since results provided by both assays are very similar. In our study, the stability of iPTH is not affected by storage up to 14days.

Clinical practice recommendations for treatment with active vitamin D analogues in children with chronic kidney disease Stages 2-5 and on dialysis

In patients with chronic kidney disease (CKD), renal synthesis of active vitamin D [1,25-dihydroxyvitamin D (1,25(OH)2D)] declines and is associated with hypocalcaemia, secondary hyperparathyroidism and the spectrum of CKD-mineral and bone disorder (MBD). In advanced CKD, active vitamin D analogues, including alfacalcidol, calcitriol and paricalcitol, are routinely administered. There are few studies on the use of vitamin D analogues in children with CKD and on dialysis. It is difficult to define bone-specific outcomes that can guide treatment with active vitamin D analogues in children with CKD-MBD. A core working group (WG) of the European Society for Paediatric Nephrology (ESPN) CKD-MBD and Dialysis WGs has developed recommendations for the use of active vitamin D therapy in children with CKD and on dialysis. A second document in parallel with this one covers treatment recommendations for native vitamin D therapy. The WGs have performed an extensive literature review to include systematic reviews and randomized controlled trials in adults and children with CKD and prospective observational studies in children with CKD. The Grading of Recommendation, Assessment, Development and Evaluation (GRADE) system was used to develop and grade the recommendations. In the absence of applicable study data, the opinion of experts from the ESPN CKD-MBD and Dialysis WGs is provided, but clearly GRADE-ed as such and must be carefully considered by the treating physician and adapted to individual patient needs as appropriate.

Review: Therapy for the altered mineral metabolism of chronic kidney disease: implications for vascular calcification

The development of chronic kidney disease (CKD) is associated with poor outcomes largely due an increased likelihood of adverse cardiovascular events. Many factors are playing a role in cardiovascular disease in CKD including the development of vascular calcification (VC). Studies have indeed shown that the presence of VC is associated with decreased survival among patients with CKD. The pathogenesis of VC is itself multi-factorial. LikeLy playing a significant role is the altered mineral metabolism of CKD. Management of the altered mineral metabolism in CKD is quite difficult and may also play a role in the pathogenesis of VC. In this manuscript, we will review the pathogenesis of the altered mineral metabolism of CKD, its management, and how both may play a role in the development of VC.

Adynamic bone disease: from bone to vessels in chronic kidney disease

Adynamic bone disease (ABD) is a well-recognized clinical entity in the complex chronic kidney disease (CKD)-mineral and bone disorder. Although the combination of low intact parathyroid hormone (PTH) and low bone alkaline phosphatase levels may be suggestive of ABD, the gold standard for precise diagnosis is histomorphometric analysis of tetracycline double-labeled bone biopsies. ABD essentially is characterized by low bone turnover, low bone volume, normal mineralization, and markedly decreased cellularity with minimal or no fibrosis. ABD is increasing in prevalence relative to other forms of renal osteodystrophy, and is becoming the most frequent type of bone lesion in some series. ABD develops in situations with reduced osteoanabolic stimulation caused by oversuppression of PTH, multifactorial skeletal resistance to PTH actions in uremia, and/or dysregulation of Wnt signaling. All may contribute not only to bone disease but also to the early vascular calcification processes observed in CKD. Various risk factors have been linked to ABD, including calcium loading, ageing, diabetes, hypogonadism, parathyroidectomy, peritoneal dialysis, and antiresorptive therapies, among others. The relationship between low PTH level, ABD, increased risk fracture, and vascular calcifications may at least partially explain the association of ABD with increased mortality rates. To achieve optimal bone and cardiovascular health, attention should be focused not only on classic control of secondary hyperparathyroidism but also on prevention of ABD, especially in the steadily growing proportions of diabetic, white, and elderly patients. Overcoming the insufficient osteoanabolic stimulation in ABD is the ultimate treatment goal.

A potential kidney-bone axis involved in the rapid minute-to-minute regulation of plasma Ca2+

Background

Understanding the regulation of mineral homeostasis and function of the skeleton as buffer for Calcium and Phosphate has regained new interest with introduction of the syndrome “Chronic Kidney Disease-Mineral and Bone Disorder”(CKD-MBD). The very rapid minute-to-minute regulation of plasma-Ca²⁺ (p-Ca²⁺) takes place via an exchange mechanism of Ca²⁺ between plasma and bone. A labile Ca storage pool exists on bone surfaces storing excess or supplying Ca when blood Ca is lowered. Aim was to examine minute-to-minute regulation of p-Ca²⁺ in the very early phase of acute uremia, as induced by total bilateral nephrectomy and to study the effect of absence of kidneys on the rapid recovery of p-Ca²⁺ from a brief induction of acute hypocalcemia.

Methods

The rapid regulation of p-Ca²⁺ was examined in sham-operated rats, acute nephrectomized rats(NX), acute thyroparathyrectomized(TPTX) rats and NX-TPTX rats.

Results

The results clearly showed that p-Ca²⁺ falls rapidly and significantly very early after acute NX, from 1.23 ± 0.02 to 1.06 ± 0.04 mM (p < 0.001). Further hypocalcemia was induced by a 30 min iv infusion of EGTA. Control groups had saline. After discontinuing EGTA a rapid increase in p-Ca²⁺ took place, but with a lower level in NX rats (p < 0.05). NX-TPTX model excluded potential effect of accumulation of Calcitonin and C-terminal PTH, both having potential hypocalcemic actions. Acute TPTX resulted in hypercalcemia, 1.44 ± 0.02 mM and less in NX-TPTX rats,1.41 ± 0.02 mM (p < 0.05). Recovery of p-Ca²⁺ from hypocalcemia resulted in lower levels in NX-TPTX than in TPTX rats, 1.20 ± 0.02 vs.1.30 ± 0.02 (p < 0.05) demonstrating that absence of kidneys significantly affected the rapid regulation of p-Ca²⁺ independent of PTH, C-PTH and CT.

Conclusions

P-Ca²⁺ on a minute-to-minute basis is influenced by presence of kidneys. Hypocalcemia developed rapidly in acute uremia. Levels of p-Ca²⁺, obtained during recovery from hypocalcemia resulted in lower levels in acutely nephrectomized rats. This indicates that kidneys are of significant importance for the ‘set-point’ of p-Ca²⁺ on bone surface, independently of PTH and calcitonin. Our results point toward existence of an as yet unknown factor/mechanism, which mediates the axis between kidney and bone, and which is involved in the very rapid regulation of p-Ca²⁺.

News on biomarkers in CKD-MBD

The increased awareness of the potential role played by mineral and bone disorder in the appearance of cardiovascular disease in renal patients has produced research efforts aimed at discovering possible pathogenic links. Accordingly, the diagnostic significance of the classic bone markers of mineral disorders and of the new markers in the setting of chronic kidney disease-mineral and bone disorders (CKD-MBD) needs to be re-evaluated along with increasing information. In this article we include classic markers of bone metabolism and some of the noncollagenous bone proteins that are gaining experimental and clinical significance in CKD-MBD. Among classic markers of secondary hyperparathyroidism and of renal osteodystrophy, we analyzed parathyroid hormone, alkaline phosphatase, tartrate-resistant acid phosphatase, and bone collagen-derived peptides. We underlined, for each, the relevance of parent proteins (peptides or isoforms) that affect assay methods and, eventually, the diagnostic or prognostic significance. Also, we considered their relationship with cardiovascular mortality. Among the numerous noncollagenous bone proteins, we examined matrix Gla protein (MGP), osteocalcin (OC), osteoprotegerin, and the small integrin-binding ligand N-linked glycoprotein family. For MGP and OC we report the relevant involvement with the process of calcification (MGP) and with glucose and energy metabolism (OC). Both of these proteins require vitamin K to become active and this is a specific problem in renal patients who frequently are deficient of this vitamin. Finally, recent acquisitions on the fascinating family of the small integrin-binding ligand N-linked glycoprotein proteins are recapitulated briefly to underline their potential clinical interest and their complex involvement with all aspects of CKD-MBD. Their diagnostic role in clinical practice awaits further studies.

Laboratory abnormalities in CKD-MBD: markers, predictors, or mediators of disease?

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is characterized by bone abnormalities, vascular calcification, and an array of laboratory abnormalities. The latter classically include disturbances in the parathyroid hormone/vitamin D axis. More recently, fibroblast growth factor 23 (FGF23) and klotho also have been identified as important regulators of mineral metabolism. Klotho deficiency and high circulating FGF23 levels precede secondary hyperparathyroidism in CKD patients. Levels of FGF23 and parathyroid hormone increase along the progression of CKD to maintain mineral homeostasis and to overcome end-organ resistance. It is hard to define when the increase of both hormones becomes maladaptive. CKD-MBD is associated with adverse outcomes including cardiovascular disease and mortality. This review summarizes the complex pathophysiology of CKD-MBD and outlines which laboratory abnormalities represent biomarkers of disease severity, which laboratory abnormalities are predictors of cardiovascular disease, and which laboratory abnormalities should be considered (direct) uremic toxins exerting organ damage. This information may help to streamline current and future therapeutic efforts.

Fibroblast growth factor 23

There is growing interest in the role of fibroblast growth factor 23 (FGF23) in various diseases of disordered mineral metabolism. In chronic kidney disease (CKD), where biochemical evidence of mineral disturbances is especially common, FGF23 measurement has been advocated as an early and sensitive marker for CKD-related bone disease. In this setting, FGF23 analysis may also improve the discrimination of risk of adverse renal and cardiovascular outcomes and aid targeting of those patients that are likely to benefit from interventions. Nonetheless, while the physiological relevance of FGF23 in the control of mineral metabolism is now firmly established, relatively little attention has been paid to important preanalytical and analytical aspects of FGF23 measurement that may impact on its clinical utility. Here we review these issues and discuss the suitability of FGF23 testing strategies for routine clinical practice. The current ‘state-of-the-art’ enzyme-linked immunosorbent assay methods for FGF23 measurement show poor agreement due to differences in FGF23 fragment detection, antibody specificity and calibration. Such analytical variability does not permit direct comparison of FGF23 measurements made with different assays and is likely to at least in part account for some of the inconsistencies noted between observational studies. From a clinical perspective, the lack of concordance has implications for the development of standardized reference intervals and clinical decision limits. Finally, the inherent assay-dependent biological variability of plasma FGF23 concentration can further complicate the interpretation of results and the design of FGF23-based testing protocols. Currently, it would be premature to consider incorporating FGF23 measurements into standard testing repertoires.

Guideline bone disease

In the current issue of Blood Purification, Palomares et al. [Blood Purif 2013;36:122-131] bemoan the poor level of compliance in dialysis units in achieving compliance with KDOQI and KDIGO bone and mineral guideline targets. These targets are based almost completely on observational data and rely on an obsolete assay for PTH. The so-called intact PTH assay measures both 1-84 PTH and 7-84 PTH; the latter has been demonstrated to possess biological activity that is antagonistic to that of 1-84 PTH. The assay cannot reliably distinguish high from low bone turnover in the target ranges suggested by the guideline panels. Targeting these ranges leads to an increased incidence of adynamic bone disease, higher calcium and phosphorus, and likely poor patient outcomes.

Variability in parathyroid hormone assays confounds clinical practice in chronic kidney disease patients

BACKGROUND

Intact parathyroid hormone (iPTH) measurements are used to guide therapy in renal patients, but variability in results can occur depending on the assay used. This study has investigated iPTH assay variation in North West England and paired data with regional audit data to determine clinical relevance of assay variability.

METHODS

Thirty-seven haemodialysis patients had blood taken (EDTA plasma, and serum), and samples were processed at 17 laboratories that analyse iPTH for North West dialysis patients. Correction factors were calculated and applied to the iPTH assay results to enable direct comparisons. These correction factors were also applied to Regional Audit data to determine if iPTH assay variability explains the variation in unit performance in achieving PTH targets.

RESULTS

The iPTH results from the 37 patients were significantly different when either analysed by different assays and/or different laboratories (P < 0.001). The Abbott Architect method consistently produced the highest iPTH results. Of the 37 patients, between 49% and 65% would achieve the Kidney Disease: Improving Global Outcomes (KDIGO) iPTH target depending on the assay used. When results were adjusted using correction factors, 21% of the patients would require a change of management according to guidelines. Data from all haemodialysis units submitted for the regional audit were adjusted to the Roche assay and this led to a small change in achievement of KDIGO iPTH targets in individual units when compared to each other.

CONCLUSIONS

A combination of iPTH assay variability and diversity in clinical management leads to variation in achieving iPTH targets. Both need to be improved and/or standardized to improve patient care.

The relationship between intact PTH and biointact PTH (1-84) with bone and mineral metabolism in pre-dialysis chronic kidney disease (CKD)

OBJECTIVES

Abnormalities in PTH are implicated in the pathogenesis of bone abnormalities in chronic kidney disease (CKD)-mineral bone disorder (CKD-MBD). PTH concentrations are important in clinical decision and management. This emphasises the importance of providing an assay which measures biologically active PTH. We compared concentrations of intact PTH with biointact PTH (1-84) in CKD and end stage renal disease (ESRD) and investigated the relationship between the 2 PTH assays with bone and mineral laboratory parameters and bone mineral density (BMD) in CKD.

DESIGN AND METHODS

We assessed 140 patients (61 in ESRD and 79 with CKD stages 1-4) in this cross-sectional study. We measured biointact PTH (1-84) as well as routine biochemical parameters on all subjects. In the CKD cohort, bone turnover markers; bone alkaline phosphatase (BAP) and tartrate resistant acid phosphatase (TRACP)-5b and bone mineral density (BMD) were also determined.

RESULTS

In ESRD, intact PTH concentration was significantly higher compared to biointact PTH (1-84) (422 [443] v/s 266 [251] pg/mL, (p<0.001) with an average bias of 60%. In CKD, intact PTH concentration was also higher compared to biointact PTH (1-84) (79[55] v/s 68[49] pg/mL p<0.001) with an average bias of 18%. Only the biointact PTH (1-84) assay showed any significant correlation with serum calcium concentrations (r=-0.26, p<0.05) and phosphate (r=0.25, p<0.05) in CKD. Following multilinear regression analysis and adjustment for all significant co-variables, only eGFR, BAP and 25 (OH)vitamin remained significantly associated with intact PTH and biointact PTH (1-84). The strength of association was stronger between BAP and biointact PTH (1-84) (biointact PTH (1-84): p=0.007, intact PTH: p=0.01). In adjusted analyses, only biointact PTH (1-84) was significantly associated with BMD at the fore-arm (FARM) (p=0.049).

CONCLUSIONS

The study confirms the differences between intact PTH and biointact PTH (1-84) in ESRD. Whilst there may be similarities in the diagnostic ability of both intact and biointact PTH (1-84), our data suggest that biointact PTH (1-84) assay may better reflect bone metabolism and BMD in CKD. Further longitudinal studies are needed.

Renale osteodystrophie

The incidence of renal osteodystrophy (ROD) increases with deteriorating kidney function, affecting virtually every patient on chronic dialysis treatment. ROD can persist after kidney transplantation and may be aggravated by immunosuppressants, mainly glucocorticoids. Fracture risk, including hip fractures, is markedly elevated in patients with renal disease compared to the general population. Depending on the type of ROD, high or low bone turnover can be found. Because of poor positive and negative predictive values of serological markers of bone turnover and limited technical capabilities of various bone imaging modalities, the only reliable method to correctly classify ROD is the transiliac bone biopsy. Elevated bone turnover can be successfully treated with active vitamin D, cinacalcet, or parathyreoidectomy, but all of these therapies may lead to oversuppression of bone metabolism. Currently, no specific therapy is available for low turnover bone disease. Bisphosphonates can be a therapeutic option for selected patients after renal transplantation.

Searching the optimal PTH target range in children undergoing peritoneal dialysis: new insights from international cohort studies

The treatment of the mineral and bone disorder associated with chronic kidney disease (CKD-MBD) remains a major challenge in pediatric patients. The principal aims of therapeutic measures are not only to prevent the debilitating skeletal complications and to achieve normal growth but also to preserve long-term cardiovascular health. Serum parathyroid hormone (PTH) levels are used as a surrogate parameter of bone turnover. Whereas it is generally accepted that serum calcium and phosphate levels should be kept within the range for age, current pediatric consensus guidelines differ markedly with respect to the optimal PTH target range and operate on a limited evidence base. Recently, the International Pediatric Dialysis Network (IPPN) established a global registry collecting detailed clinical and biochemical information, including data relevant to CKD-MBD in children on chronic peritoneal dialysis (PD). This review highlights the current evidence basis regarding the optimal PTH target range in pediatric CKD patients, and re-assesses the current guidelines in view of the outcome data collected by the IPPN registry. Based on a comprehensive evaluation of CKD-MBD outcome measures in this global patient cohort, a PTH target range of 1.7-3 times the upper limit of normal (i.e. 100-200 pg/ml) appears reasonable in children undergoing chronic PD.

Bone disease in pediatric chronic kidney disease

Children with long-standing chronic kidney disease (CKD) display clinical symptoms of bone disease, including bony deformities and fractures, which contribute to long-standing disability. Abnormalities in skeletal mineralization occur in a substantial proportion of this population and may contribute to chronic morbidity. Underscoring the potential contribution of parameters other than bone turnover to bone disease in CKD, a new definition for renal osteodystrophy (ROD), emphasizing the assessment of three key histologic descriptors, i.e., bone turnover (T), mineralization (M), and volume (V) (TMV), has been recommended in the assessment of all patients with CKD. Although bone biopsy is the only available method for assessing all three recommended areas of bone histology, this invasive procedure is not routinely used in any clinical setting; thus, a true understanding of the prevalence of abnormal turnover, defective mineralization, and altered bone volume throughout the course of CKD is limited. Recent data, however, have shed light on the progression of renal ROD throughout the course of CKD, including its early stages, as well as on the alterations in cell biology that accompany ROD.