PTH 1-84 and PTH "7-84" in the noninvasive diagnosis of renal bone disease

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.

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.

PTH assays in dialysis patients: Practical considerations

Parathyroid hormone (PTH) 1-84 is the main biologically active hormone produced by the parathyroid cells. Circulating PTH molecules include the whole PTH 1-84 along with amino (N) and carboxyl (C) terminal fragments. While PTH is the best available noninvasive biomarker to assess bone turnover in dialysis patients, the biological roles of individual circulating PTH fragments are still not completely known. The understanding that there is an enormous variation in the target specificity of currently available PTH assays for different circulating forms of PTH has led to the evolution of assays from first to second then third generation. With a reduction in kidney function, there is a preferential increase in circulating C fragments and non-PTH 1-84 forms, resulting in a decrease in the ratio of PTH 1-84/non-PTH 1-84. However, there are also substantial differences in between-assay measurements, with several fold variations in results. Targets based on multiples of the upper limit of normal (ULN) should be used rather than PTH ranges using absolute iPTH values. To date, the second-generation PTH remains the most widely used assay. Current guidelines recommend following iPTH trends rather than absolute values. Herein, we highlight problems and challenges in PTH assays/measurements and their interpretations in dialysis patients.

Biochemical Testing Relevant to Bone

Laboratory biochemical testing is critical to the clinical understanding of bone disorders. Patients with skeletal diseases have underlying themes in their pathophysiology that would be impossible to detect without biochemical assessment of serum and urine minerals, vitamin D, parathyroid hormone, parathyroid hormone-related peptide, and bone turnover markers. Bone disorders are caused by abnormalities in signaling pathways that affect bone formation and resorption. Therapies for common bone diseases were developed in direct response to underlying biochemical abnormalities.

Oxidation of PTH: in vivo feature or effect of preanalytical conditions?

Background:

Posttranslational oxidation of parathyroid hormone (PTH) modifies its biological activity. Measurement of non-oxidized PTH (n-oxPTH) could be an improvement in assessing PTH status, as intact PTH may rather reflect oxidative stress. However, it is debated whether oxidation of PTH occurs in vivo, or whether it is mainly an in vitro artifact. The aim of this study was to investigate the influence of different preanalytical conditions on the oxidation of PTH within a wide range of plasma PTH concentrations and oxidation propensity.

Methods:

n-oxPTH was separated from its oxidized form using an affinity column capturing the oxidized PTH. n-oxPTH was measured in eluate using commercially available PTH assays. The study included ethylenediaminetetraacetic acid plasma samples from 17 patients undergoing hemodialysis and 32 healthy subjects. We determined effects of storage temperature, time until centrifugation and freeze-thaw cycles. PTH and n-oxPTH concentrations were measured in each sample using six different immunoassays.

Results:

n-oxPTH concentrations remained unchanged up to 180 min until centrifugation, two freeze-thaw cycles or after storage at −20°C or −80°C up to 79 days. Various methods for n-oxPTH and PTH measurements yielded highly comparable results, apart from standardization differences between various PTH and n-oxPTH assays.

Conclusions:

n-oxPTH concentrations were stable under our study conditions, indicating negligible ex vivo oxidation of PTH. In addition, PTH immunoassays have a different sensitivity for n-oxPTH than for total PTH. For this reason, the n-oxPTH/total PTH ratio cannot be used in absence of a n-oxPTH standard. Clinical implications of determining n-oxPTH require additional study.

Pharmacological Management of Secondary Hyperparathyroidism in Patients with Chronic Kidney Disease

Secondary hyperparathyroidism (SHPT) is a common complication of chronic kidney disease (CKD) and is part of the CKD-mineral bone disorder (CKD-MBD). SHPT is associated with increased risk of fracture and mortality; thus, SHPT control is recommended as kidney function declines. Effective SHPT management becomes more difficult once skeletal and cardiovascular adverse effects associated with severe SHPT have become established. However, interventional studies to lower parathyroid hormone (PTH) have so far shown inconsistent results in improving patient-centred outcomes such as mortality, cardiovascular events and fracture. Pharmacological treatment effect on PTH level is also inconsistent between pre-dialysis CKD and dialysis patients, which adds to the complexity of SHPT management. This review aims to give an overview on the pathophysiology, pharmacological and non-pharmacological treatment for SHPT in CKD including some of the limitations of current therapeutic options.

Secondary Hyperparthyroidism: Pathogenesis, Diagnosis, Preventive and Therapeutic Strategies

Uremic secondary hyperparathyroidism is a multifactorial and complex disease often present in advanced stages of chronic kidney disease. The accumulation of phosphate, the increased FGF23 levels, the reduction in active vitamin D production, and the tendency to hypocalcemia are persistent stimuli for the development and progression of parathyroid hyperplasia with increased secretion of PTH. Parathyroid proliferation may become nodular mainly in cases of advanced hyperparathyroidism. The alterations in the regulation of mineral metabolism, the development of bone disease and extraosseous calcifications are essential components of chronic kidney disease-mineral and bone disorder and have been associated with negative outcomes. The management of hyperparathyroidism includes the correction of vitamin D deficiency and control of serum phosphorus and PTH without inducing hypercalcemia. An update of the leading therapeutic tools available for the prevention and clinical management of secondary hyperparathyroidism, its diagnosis, and the main mechanisms and factors involved in the pathogenesis of the disease will be described in this review.

Serum Parathyroid Hormone Concentrations and Clinical Outcomes in ESRD: A Call for Targeted Clinical Trials

Secondary hyperparathyroidism almost universally accompanies end-stage renal disease (ESRD). In some, but not all studies, elevated serum parathyroid hormone (PTH) concentrations are associated with increased fracture rates, cardiovascular disease, and mortality in ESRD. The serum concentration of PTH required for optimal bone health and reduced cardiovascular risk in such patients remains elusive. Recent clinical trials have failed to show substantial changes in morbidity and mortality following reductions of elevated serum PTH concentrations. In this review, we will assess some of the difficulties in evaluating elevated serum PTH concentrations, and their association with skeletal fractures and mortality in ESRD patients. We are of the opinion that in the context of ESRD, elevated PTH concentrations occur in conjunction with other comorbid conditions such as diabetes mellitus, malnutrition, hypertension, volume excess, preexisting heart disease, all of which have prevented establishing a precise association between elevated serum PTH concentrations and global or skeletal outcomes. Age, gender, and racial variability among groups make interpretation exceptionally difficult. Analysis of prevalent ESRD populations with secondary hyperparathyroidism should take all these factors into account. We suggest that future clinical trials which examine the usefulness of reductions in serum PTH concentrations be conducted in age, sex, and racially balanced groups, without or with minimal coexisting confounding disease. Furthermore, trials in such populations should have as their primary outcome a reduction in fractures rather than an alteration in mortality.

[Clinical value of two methods to measure parathyroid hormone in chronic renal insufficiency, considering vitamin D metabolism]

INTRODUCTION

Parathyroid hormone levels provide important information in chronic renal failure.

AIM

To compare parathyroid hormone levels measured by two assays in correlation with vitamin D supply.

METHOD

Parathyroid hormone and 25-hydroxi-vitamin-D were determined in 104 patients (31 patients with chronic renal failure without renal replacement therapy, 36 patients treated with peritoneal dialysis and 37 patients treated with hemodialysis).

RESULTS

Good correlation was found between results of the two parathyroid hormone methods, but the intact parathyroid hormone levels were higher than the biointact values. 87% and 13% of the patients had vitamin-D deficiency and insufficiency, respectively. The frequency of serious vitamin-D deficiency was higher in the peritoneal dialysis than in the hemodialysis group. Intact parathyroid hormone levels were different in dialysed patients having vitamin-D-deficiency and insufficiency, and the difference was higher for the biointact than intact values. Negative correlation was detected between biointact parathyroid hormone and 25-hydroxivitamin-D in the hemodialysis group.

CONCLUSIONS

Biointact parathyroid hormone levels better reflect the vitamin D supply and bone metabolism than intact levels, especially in hemodialysed patients.

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.

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.

PTH--a particularly tricky hormone: why measure it at all in kidney patients?

Plasma parathyroid hormone (PTH) concentrations are commonly measured in the context of CKD, as PTH concentration elevation is typical in this clinical context. Much has been inferred from this raised PTH concentration tendency, both about the state of skeletal integrity and health and also about the potential clinical outcomes for patients. However, we feel that reliance on PTH concentrations alone is a dangerous substitute for the search for, and use of, more precise and reliable biomarkers. In this article, we rehearse these arguments, bringing together patient-level and analytical considerations for the first time.

PTH Assays: Understanding What We Have and Forecasting What We Will Have

Parathyroid hormone (PTH) assays have evolved continuously for the last 50 years. Since the first radioimmunoassay was described in 1963, several assays based on immunological identification have been published (first generation assays). The routine assays used nowadays are immunometric "sandwich-type". They are based on two different monoclonal antibodies, one amino-terminal and the other carboxyl terminal specific. These second generation assays are widely available and adapted to most of the automation platforms. The specificity of the amino terminal antibody defines if the immunometric assay measures only the bioactive PTH circulating form (including the first amino terminal amino acids) or the "intact" PTH, which includes, besides bioactive PTH, other "long" carboxyl-terminal forms, for example, 7-84-PTH. Assays for "intact" PTH are the most commonly available and the potential advantage of the bioactive PTH assays is still debatable. Next generation of assays will be based on different principles, mainly mass spectrometry in samples submitted to a prior purification and fragmentation steps. These assays will provide information about the whole spectra of PTH peptides in circulation, with a significant increase of the information regarding this biologically important peptide hormone.

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

Sclerostin serum levels correlate positively with bone mineral density and microarchitecture in haemodialysis patients

BACKGROUND

Sclerostin is a soluble inhibitor of osteoblast function. Sclerostin is downregulated by the parathyroid hormone (PTH). Here, it was investigated whether sclerostin levels are influenced by intact (i) PTH and whether sclerostin is associated with bone turnover, microarchitecture and mass in dialysis patients.

METHODS

Seventy-six haemodialysis patients and 45 healthy controls were included in this cross-sectional study. Sclerostin, Dickkopf-1 (DKK-1), intact parathyroid hormone (iPTH), vitamin D and markers of bone turnover were analysed. A subset of 37 dialysis patients had measurements of bone mineral density (BMD) using dual-energy X-ray absorptiometry and bone microarchitecture using high-resolution peripheral quantitative computed tomography.

RESULTS

Dialysis patients had significantly higher sclerostin levels than controls (1257 pg/mL versus 415 pg/mL, P < 0.001). Significant correlations were found between sclerostin and gender (R = 0.41), iPTH (R = -0.28), 25-hydroxy-cholecalciferol (R = 0.27) and calcium (R = 0.25). Gender and iPTH remained significantly associated with sclerostin in a multivariate analysis. Sclerostin serum levels were positively associated with BMD at the lumbar spine (R = 0.46), femoral neck (R = 0.36) and distal radius (R = 0.42) and correlated positively mainly with trabecular structures such as trabecular density and number at the radius and tibia in dialysis patients. DKK-1 was related neither to bone measures nor to serologic parameters.

CONCLUSIONS

Considering that sclerostin is an inhibitor of bone formation, the observed positive correlations of serum sclerostin with BMD and bone volume were unexpected. Whether its increase in dialysis patients has direct pathogenetic relevance or is only a secondary phenomenon remains to be seen.

Parathyroid hormone and growth in chronic kidney disease

Growth failure is common in children with chronic kidney disease, and successful treatment is a major challenge in the management of these children. The aetiology is multi-factorial with "chronic kidney disease-metabolic bone disorder" being a key component that is particularly difficult to manage. Parathyroid hormone is at the centre of this mineral imbalance, consequent skeletal disease and, ultimately, growth failure. When other aetiologies are treated, good growth can be achieved throughout the course of the disease when parathyroid hormone (PTH) levels are in the normal range or slightly elevated. A direct correlation between PTH levels and growth has not been convincingly established, and the direct effect of PTH on growth has not been adequately described; furthermore, direct actions of PTH on the growth plate are unproven. The effects of PTH on growth stem from the pivotal role that PTH plays in the development of renal osteodystrophy. In severe secondary hyperparathyroidism, the growth plate is altered and growth is affected. At the other end of the spectrum, with an over-suppressed parathyroid gland, the rate of bone turnover and remodelling is markedly diminished, and some data suggest this is associated with poor growth. Most of the data available suggests that avoiding the development of significant bone disease through the strict control of PTH levels permits good growth. Absolute optimal ranges for PTH that maximise growth or minimise growth failure are not yet established.

Changes in bone mineral parameters, vitamin D metabolites, and PTH measurements with varying chronic kidney disease stages

Vitamin D deficiency is associated with an increased risk of many diseases (skeletal and nonskeletal). Emerging data also associate high concentrations of serum parathyroid hormone (PTH) with morbidity and increased mortality in patients both with and without known chronic kidney disease (CKD). Understanding the relationship between vitamin D and PTH and the determinants of PTH is therefore important. We performed a cross-sectional study of 203 patients with varying stages of CKD randomly recruited from the Renal Unit database at our institution. Detailed case review was performed, and samples of fasting blood were taken for biochemical analyses. We measured standard biochemistry, 25-hydroxyvitamin D (25-OHD), 1,25-OHD, and three PTH measurements [1-84 PTH, total PTH, and derived N-terminal truncated, 7-84 PTH (cPTH)]. Vitamin D deficiency was high, with 86% of patients having 25-OHD levels below 30 ng/ml. Estimated glomerular filtration rate (eGFR) was not associated with 25-OHD levels, whereas 1,25-OHD was lower in those with CKD stage 5 versus stage 4, who were not treated with vitamin D metabolites (18 vs. 65 pg/ml, respectively; P < 0.05). All three PTH measurements increased with worsening eGFR, with this finding being more pronounced in those patients who were not treated with vitamin D metabolites. The slope of the regression line of cPTH on eGFR tended to be steeper, -0.90, compared to -0.81 for total PTH and -0.80 for 1-84 PTH (P = 0.06). The ratio of total PTH to cPTH did decrease significantly through the range of CKD stages (P = 0.03). The determinants of PTH were similar for all three PTH measurements, with eGFR having a strong inverse relationship, with weaker relationships for 25-OHD and ionized calcium on multivariate analyses. We confirm that there is a complex relationship between 25-OHD, eGFR, and PTH. Total PTH, 1-84 PTH, and cPTH increase with increasing CKD stages, with a relatively greater increase in cPTH, although the clinical significance of this finding remains uncertain. The three PTH measurements had similar correlations with the biochemical and clinical variables studied, suggesting that either total PTH or 1-84 PTH can be used in clinical practice when evaluating vitamin D and PTH status.

Heterogeneity of carboxyl-terminal parathyroid hormone circulating forms in patients with hyperparathyroidism due to end stage renal disease

OBJECTIVE

To study carboxyl-terminal (COOH) parathyroid hormone (PTH) circulating forms in patients with hyperparathyroidism due to end stage renal disease (ESRD).

METHODS

An immunometric assay that recognizes both intact and COOH PTH forms was developed. The assay, in conjunction with an intact assay, was used to measure PTH in serum samples obtained from 25 patients with hyperparathyroidism due to ESRD. Samples were also submitted to gel filtration chromatography in a Superdex((R)) 30 1.6 x 60 cm column, and the PTH content in the elution tubes, measured using both assays.

RESULTS

Values from 39.000 to 232.300 ng/mL (mean +/- sd = 101.680 +/- 45.330 ng/mL) were found using the COOH assay (PTH 39-84 was used as standard). Values obtained by the intact PTH assay ranged from 318 to 3.307 ng/mL (1.769 +/- 693 ng/mL) with a correlation between assays of 0.462 (p = 0.02). The elution profile obtained using the COOH assay showed a preponderance of forms with MW ranging from 8.500 to 4.500 daltons. The profiles obtained from the 25 patients were very similar.

CONCLUSIONS

In patients with hyperparathyroidism due to ESRD circulating PTH levels contain a broad range of molecular forms including COOH with MW ranging from 8.500 to 4.500 daltons. These forms are not recognized by the standard intact PTH assays. The correlation of these findings to the clinical aspects of bone disease in ESRD patients remains to be studied.

Can the combination of calcium and parathormone levels above K/DOQI guidelines be used as a marker of adynamic bone disease in African Americans?

BACKGROUND

Patients of African American descent are at risk for the development of adynamic bone disease at parathyroid hormone levels 50% above the K/DOQI guidelines. Since a low bone formation rate is associated with hypercalcemia, attempts to reach one K/DOQI guideline may result in serum calcium levels above another K/DOQI guideline. Calcium levels above K/DOQI guidelines therefore may signal a need to stop parathyroid suppression.

STUDY DESIGN

SETTING AND PARTICIPANTS

Bone biopsies were performed at the East Alabama Medical Center, in Opelika AL, USA on eight patients (four Caucasians, four African Americans) whose parathormone levels and serum calcium levels both exceeded K/DOQI guideline recommendations.

RESULTS

All patients had mild to severe hyperparathyroid bone disease. No variable studied was predictive of the finding.

LIMITATIONS

Small sample size and the unavailability of the original Nichols Diagnostic Institute radioimmunoassay for parathormone.

CONCLUSION

We did not find hypercalcemia predictive of adynamic bone in patients of African American descent at levels of parathormone where low bone formation rates have been documented to occur. Since no parameter predicted bone histology, perhaps bone biopsies will be necessary to distinguish hyperparathyroidism from adynamic bone disease in African Americans with ESRD, hypercalcemia, and moderately elevated levels of PTH. Further studies are needed to determine appropriate therapy.

Intact PTH combined with the PTH ratio for diagnosis of bone turnover in dialysis patients: a diagnostic test study

BACKGROUND

Determination of parathyroid hormone (PTH) level is the most commonly used surrogate marker for bone turnover in patients with stage 5 chronic kidney disease on dialysis therapy (CKD-5D). The objective of this study is to evaluate the predictive value of various PTH measurements for identifying low or high bone turnover rate.

STUDY DESIGN

Diagnostic test study.

SETTINGS & PARTICIPANTS

141 patients with CKD-5D from 15 US hemodialysis centers.

INDEX TESTS

Intact PTH, PTH 1-84, and PTH ratio (ratio of level of PTH 1-84 to level of large carboxy-terminal PTH fragments).

REFERENCE TEST OR OUTCOME

Bone turnover determined using bone histomorphometry.

OTHER MEASUREMENTS

Demographic and treatment-related factors, serum calcium and phosphorus.

RESULTS

Patients presented histologically with a broad range of bone turnover abnormalities. In white patients with CKD-5D (n = 70), PTH ratio <1.0 added to intact PTH level <420 pg/mL increased the positive predictive value for low bone turnover from 74% to 90%. In black patients (n = 71), adding PTH ratio <1.2 to intact PTH level <340 pg/mL increased the positive predictive value for low bone turnover from 48% to 90%. Adding PTH ratio >1.6 to intact PTH level of 340-790 pg/mL increased the positive predictive value for high bone turnover from 56% to 71%.

LIMITATIONS

Because the research protocol called for carefully controlled blood specimen handling, blood drawing and routine specimen handling might be less stringent in clinical practice. By limiting study participation to black and white patients with CKD-5D, we cannot comment on the roles of intact PTH, PTH 1-84, and PTH ratio in other racial/ethnic groups.

CONCLUSION

In black patients with CKD-5D, the addition of PTH ratio to intact PTH measurements is helpful for diagnosing low and high bone turnover. In white patients with CKD-5D, it aids in the diagnosis of low bone turnover.

Optimizing bone health in chronic kidney disease

Phosphocalcic metabolism disorders often complicate chronic kidney disease (CKD) and worsen as kidney function declines, with a consequence on bone structural integrity. The risk of fracture exceeds that of the normal population in both patients with pre-dialysis CKD and end-stage renal disease (ESRD). The increasing incidence of CKD, the high mortality rate induced by hip fracture, the decreased quality of life and economic burden of fragility fracture make the renal bone disorders a major problem of public health around the world. Optimizing bone health in CKD patients should be a priority. Bone biopsy is invasive. Dual-energy X-ray absorptiometry, commonly used to screen individuals at risk of fragility fracture in the general population, is not adequate to assess advanced CKD because it does not discriminate fracture status in this population. New non-invasive three-dimensional high-resolution imaging techniques, distinguishing trabecular and cortical bone, appear to be promising in the assessment of bone strength and might improve bone fracture prediction in this population. Therapeutic intervention in the chronic kidney disease-mineral and bone disorders (CKD-MBD) should begin early in the course of CKD to maintain serum concentration of biological parameters involved in mineral metabolism in the normal recommended ranges, prevent the development of parathyroid hyperplasia, prevent extra-skeletal calcifications and preserve skeletal health. In this paper, we review studies of mineral and bone disorders in patients with CKD and ESRD, the utility of current techniques to assess bone health and the preventive and therapeutic strategies for managing CKD-MBD.

Relation between body mass index and bone mineral density among haemodialysis patients with chronic kidney disease

Renal osteodystrophy is a serious problem for patients with chronic kidney disease. Measurements of bone mineral density, T-score and Z-score were taken in the lumbar region and femur of 73 patients who were being treated on the haemodialysis programme. These measurements were compared with the anthropometric values of weight, height and body mass index (BMI) obtaining a positive correlation between them.

INTRODUCTION

Alterations in the bone mineral metabolism are an important cause of morbidity and mortality among haemodialysis patients with chronic renal failure. Bone mass diminution, together with fracture risk, is a frequent finding in these patients; this fact is explained by different factors, amongst which are those related to their anthropometric values.

MATERIALS AND METHODS

Bone mineral density (BMD) was studied, T-score and Z-score measurements were taken in the neck of the femur, trochanter, intertrochanter, 1/3 of proximal femur, Ward's triangle and L2, L3 and L4 vertebrae; body composition was also studied. With this aim, DXA densitometry was used on 73 haemodialysis patients (40 men and 33 women). The mean of the total haemodialysis time in these patients was 9.7 years. The group showed a very significant positive correlation between BMD, weight, height, BMI, fractures, dialysis time and intact PTH.

CONCLUSIONS

CKD patients undergoing the haemodialysis programme show a significant BMD reduction, which affects both lumbar spine and femur. Weight and height affect BMD and bone change, being thus important factors of prediction for fracture risk. Furthermore, BMI is the main determinant of BMD, a finding that is confirmed in the units in this study and with the evidence described by other authors (Negri et al. (2005).

Critical issues of PTH assays in CKD

Measurement of bioactive parathyroid hormone (PTH) is essential for the optimal management of secondary hyperparathyroidism and its associated bone disorders in chronic kidney disease (CKD) patients. For this purpose, three generations of increasingly specific PTH assays have been developed over the last 4 decades. To date, however, only second-generation PTH assays are most widely used, although these have been shown to cross-react with large PTH fragments having a partially preserved N-structure, mostly PTH(7-84). The newly developed third-generation PTH assays are believed to be the most specific means of measuring PTH(1-84), but their clinical utility remains debatable. More recently, these latter assays have also been shown to react with a new N-form of PTH, which has been identified in patients with severe hyperparathyroidism and parathyroid carcinoma. Progressive research in this area has advanced our understanding considerably regarding the circulating molecular forms of PTH and their pathophysiological roles in bone abnormalities associated with CKD. However, developing an ideal PTH assay continues to be difficult because of key issues such as the reliability of PTH as a surrogate marker for bone turnover, practicality of employing third-generation PTH assays, and unknown biological implications of N-PTH and other PTH fragments. Further research exploring these issues is mandatory to understand and optimally manage parathyroid disorders and bone abnormalities in CKD patients.

Relationship between plasma fibroblast growth factor-23 concentration and bone mineralization in children with renal failure on peritoneal dialysis

CONTEXT

Fibroblast growth factor (FGF)-23 is produced in bone, and circulating levels are markedly elevated in patients with end-stage kidney disease, but the relationship between plasma levels of FGF-23 and bone histology in dialysis patients with secondary hyperparathyroidism is unknown.

OBJECTIVE

The aim of the study was to evaluate the correlation between plasma levels of FGF-23 and bone histology in pediatric patients with end-stage kidney disease who display biochemical evidence of secondary hyperparathyroidism.

DESIGN

We performed a cross-sectional analysis of the relationship between plasma FGF-23 levels and bone histomorphometry.

SETTING

The study was conducted in a referral center.

STUDY PARTICIPANTS

Participants consisted of forty-nine pediatric patients who were treated with maintenance peritoneal dialysis and who had serum PTH levels (1st generation Nichols assay) greater than 400 pg/ml.

INTERVENTION

There were no interventions.

MAIN OUTCOME MEASURE

Plasma FGF-23 levels and bone histomorphometry were measured.

RESULTS

No correlation existed between values of PTH and FGF-23. Bone formation rates correlated with PTH (r = 0.44; P < 0.01), but not with FGF-23. Higher FGF-23 concentrations were associated with decreased osteoid thickness (r = -0.49; P < 0.01) and shorter osteoid maturation time (r = -0.48; P < 0.01).

CONCLUSIONS

High levels of FGF-23 are associated with improved indices of skeletal mineralization in dialyzed pediatric patients with high turnover renal osteodystrophy. Together with other biomarkers, FGF-23 measurements may indicate skeletal mineralization status in this patient population.

Response of different PTH assays to therapy with sevelamer or CaCO3 and active vitamin D sterols

Amino-terminally truncated parathyroid hormone (PTH) fragments are detected to differing degrees by first- and second-generation immunometric PTH assays (PTH-IMAs), and acute changes in serum calcium affect the proportion of these fragments in circulation. However, the effect of chronic calcium changes and different vitamin D doses on these PTH measurements remains to be defined. In this study, 60 pediatric dialysis patients, aged 13.9 +/- 0.7 years, with secondary hyperparathyroidism were randomized to 8 months of therapy with oral vitamin D combined with either calcium carbonate (CaCO(3)) or sevelamer. Serum phosphorus levels did not differ between groups. Serum calcium levels rose from 9.3 +/- 0.1 to 9.7 +/- 0.1 mg/dl during CaCO(3) therapy (p < 0.01 from baseline) but remained unchanged during sevelamer therapy. In the CaCO(3) and sevelamer groups, baseline serum PTH levels (1st PTH-IMA; Nichols Institute Diagnostics, San Clemente, CA) were 964 +/- 75 and 932 +/- 89 pg/ml, and levels declined to 491 +/- 55 and 543 +/- 59 pg/ml, respectively (nonsignificant between groups). Patients treated with sevelamer received higher doses of vitamin D than those treated with CaCO(3). The PTH values obtained by first- and second-generation PTH-IMAs correlated closely throughout therapy and the response of PTH was similar to both PTH-IMAs, despite differences in serum calcium levels.

Differential effects of intermittent PTH(1-34) and PTH(7-34) on bone microarchitecture and aortic calcification in experimental renal failure

PTH(1-84) and PTH(7-84) are elevated in chronic kidney disease (CKD). These peptides, as their shorter analogs PTH(1-34) and PTH(7-34) both promote PTH receptor (PTH1R) internalization but only PTH(1-34) and PTH(1-84) activate the receptor. Here, we examined the effects of intermittent administration of PTH(1-34) and PTH(7-34) on mineral ion metabolism, bone architecture, and vascular calcification in rats with experimental CKD. CKD with or without parathyroidectomy (PTX) was established by 5/6 nephrectomy (NPX) in rats. Animals were divided into 4 groups: Sham PTX+ sham NPX (Sham); PTX+ sham NPX (PTX); Sham PTX+NPX (NPX); PTX+NPX (PTX/NPX). Rats were treated with single daily doses of 40 microg/kg PTH(1-34), PTH(7-34), or vehicle. Creatinine was higher in NPX and Ca lower in PTX and PTX/NPX groups than in Sham or NPX rats. Plasma phosphate was higher in PTX, NPX and PTX/NPX than in Sham rats. PTH(1-34) was more hypercalcemic than PTH(7-34) in PTX rats. Fractional bone volume in rats treated with PTH(1-34) increased significantly in all groups compared to that of vehicle treatment. In addition, trabecular number, thickness and volumetric bone density increased in rats treated with PTH(1-34). In contrast, PTH(1-34) diminished vascular calcification. Bone and renal PTH1R mRNA expression was reduced as much or more in PTX/NPX rats as in NPX alone, whereas PTH(7-34) had no effect on PTH1R expression. Renal but not bone PTH1R mRNA increased in response to PTH(1-34). These findings suggest that PTH(1-34) exerts greater hypercalcemic and anabolic effects in parathyroidectomized and/or nephrectomized rats than does PTH(7-34). There was no evidence for significant bone or vascular actions of PTH(7-34). We conclude that PTH(1-34) protects against vascular calcification and bone demineralization in experimental renal failure.

Cinacalcet reduces the set point of the PTH-calcium curve

The calcimimetic cinacalcet increases the sensitivity of the parathyroid calcium-sensing receptor to calcium and therefore should produce a decrease in the set point of the parathyroid hormone (PTH)-calcium curve. For investigation of this hypothesis, nine long-term hemodialysis patients with secondary hyperparathyroidism were given cinacalcet for 2 mo, the dosage was titrated per a protocol based on intact PTH and plasma calcium concentrations. Dialysis against low- and high-calcium (0.75 and 1.75 mM) dialysate was used to generate curves describing the relationship between PTH and calcium. Compared with precinacalcet levels, cinacalcet significantly reduced mean serum calcium, intact PTH and whole PTH (wPTH; all P < 0.001). The set points for PTH-calcium curves were significantly reduced, and both maximum and minimum levels of PTH (intact and whole) were significantly decreased. The calcium-mediated inhibition of PTH secretion was more marked after cinacalcet treatment. In addition, cinacalcet shifted the inverse sigmoidal curve of wPTH/non-wPTH ratio versus calcium to the left (i.e., less calcium was required to reduce the wPTH/non-wPTH ratio). In conclusion, cinacalcet increases the sensitivity of the parathyroids to calcium, causing a marked reduction in the set point of the PTH-calcium curve, in hemodialysis patients with secondary hyperparathyroidism.

Adynamic bone in patients with chronic kidney disease

Adynamic bone in patients with chronic kidney disease (CKD) is a clinical concern because of its potential increased risk for fracture and cardiovascular disease (CVD). Prevalence rates for adynamic bone are reportedly increased, although the variance for its prevalence and incidence is large. Differences in its prevalence are largely attributed to classification and population differences, the latter of which constitutes divergent groups of elderly patients having diabetes and other comorbidities that are prone to low bone formation. Most patients have vitamin D deficiency and the active form, 1,25-dihydroxyvitamin D, invariably decreases to very low levels during CKD progression. Fortunately, therapy with vitamin D receptor activators (VDRAs) appears to be useful in preventing bone loss, in part, by its effect to stimulate bone formation and in decreasing CVD morbidity, and should be considered as essential therapy regardless of bone turnover status. Future studies will depend on assessing cardiovascular outcomes to determine whether the risk/reward profile for complications related to VDRA and CKD is tolerable.

Chronic kidney disease mineral and bone disorder in children

Childhood and adolescence are crucial times for the development of a healthy skeletal and cardiovascular system. Disordered mineral and bone metabolism accompany chronic kidney disease (CKD) and present significant obstacles to optimal bone strength, final adult height, and cardiovascular health. Decreased activity of renal 1 alpha hydroxylase results in decreased intestinal calcium absorption, increased serum parathyroid hormone levels, and high-turnover renal osteodystrophy, with subsequent growth failure. Simultaneously, phosphorus retention exacerbates secondary hyperparathyroidism, and elevated levels contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves growth and high-turnover bone disease. However, target ranges for serum calcium, phosphorus, and parathyroid hormone (PTH) levels vary according to stage of CKD. Since over-treatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy must be carefully adjusted to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents, including calcium-free phosphate binding agents and new vitamin D analogues, effectively suppress serum PTH levels while limiting intestinal calcium absorption and may provide future therapeutic alternatives for children with CKD.

Evolution of PTH assays

PTH metabolism is complex and the circulating forms include the intact 1-84 molecule as well as several carboxyl-terminal fragments. The first generation of PTH assays included several types of competitive assays, with specificities that spanned carboxyl, mid-region and amino-terminal portions of the molecule. The limitations of these assays and the methodological evolution led to the description of 2nd generation non-competitive immunometric assays for PTH in the late 80's, based on the recognition of the PTH molecule by two different antibodies, one directed against de amino-terminal and other against the carboxyl-terminal segments. The observation that in some circumstances "long" carboxyl-terminal segments were also measured by 2nd generation assays led to the development of 3rd generation assays based on amino-terminal specific antibodies that are specific for the first amino acids, measuring only the molecular forms that activate PTH1R. The practical and cost-benefit advantages of these assays are still debatable. The recent observation that carboxyl-terminal fragments of PTH have biological activity via a distinct receptor than PTH1R, points to the future need of more than one assay in order to evaluate parathyroid hormone function.

Effect of haemodialysis on markers of bone turnover in children

'Intact' parathyroid hormone (iPTH) assays are used to measure serum PTH levels in haemodialysis patients to diagnose and monitor secondary hyperparathyroidism and consequent renal osteodystrophy (ROD); these assays exhibit cross-reactivity with long carboxyl-terminal PTH fragments (C-PTH) that accumulate in end stage renal failure (ESRF) and antagonise the biological activity of the whole molecule, 1-84 PTH. The effects of haemodialysis on C-PTH are not known. We investigated how haemodialysis affects serum concentrations of calcium, iPTH, 1-84 PTH, C-PTH, and other markers of bone turnover; bone-specific alkaline phosphatase (BALP) and type 1 collagen cross-linked telopeptide (CTx). Fifteen patients, mean (range) age 13.9 (4.3-17.6) years, haemodialysed for a median of 16.3 (4-41) months, had pre- and post-dialysis serum samples collected for routine biochemistry, BALP, CTx, iPTH and 1-84 PTH assays. Changes to serum concentrations and relationships between these biochemical surrogate markers of ROD were investigated. Serum phosphate and PTH levels (measured by both assays) fell significantly during dialysis, whereas serum calcium, C-PTH, the 1-84 PTH: C-PTH ratio and BALP and CTx concentrations were not significantly changed. 1-84 PTH levels were related to pre but not post dialysis serum calcium levels and changes to 1-84 PTH levels during dialysis were related to changes in serum calcium levels. 1-84 PTH and iPTH were reduced by haemodialysis, whereas levels of BALP and CTx remained stable post-dialysis. The relationship between BALP and CTx and bone histology requires investigation to determine whether they are more useful markers of bone turnover in this patient group.

Parathyroid hormone assay: problems and opportunities

The assay of parathyroid hormone continues to remain problematic as a result of the presence in the circulation of a variety of parathyroid hormone (PTH) peptides derived from secretion and from peripheral metabolism. The detection of these PTH fragments to varying degrees leads to widely differing results in the various assays used, particularly in the setting of chronic kidney disease, where PTH fragments accumulate as glomerular filtration rate (GFR) falls. The differing results not only lead to problems in comparing values from various laboratories but also limit efforts to develop useful clinical practice guidelines. At the same time, research into the precise identification of the PTH fragments which contribute to the assay problems has uncovered a relatively new area of parathyroid research that has pointed to potential biologic activity of PTH peptides previously thought to be biologically inactive and which may act on a novel PTH receptor. These issues have brought new focus to the difficulties in standardization of PTH assays and have provoked efforts to provide standards to help in the characterization of PTH assays and to facilitate the development of clinical practice guidelines.

Bone health in chronic kidney disease-mineral and bone disease

Chronic kidney disease (CKD) is accompanied by disturbances in calcium, phosphate, vitamin D, and parathyroid hormone (PTH) homeostasis that play an important role in the pathophysiology of renal bone disease. The increased cardiovascular morbidity and mortality observed among patients with CKD has recently been recognized to be associated with these disturbances in mineral metabolism. Thus, disturbances in mineral metabolism observed in renal failure results in a multisystem disorder, making the development of a standardized definition of these disorders a top priority. Therefore, the Board of Directors of Kidney Disease: Improving Global Outcomes proposed to define the broader category of mineral disorders associated with CKD as CKD-mineral and bone disorder (CKD-MBD). This newly proposed definition will include the disorders of mineral metabolism, bone histology (renal osteodystrophy), and the extraskeletal manifestations such as vascular calcification. This new definition and stratification of disease should result in improvement not only in the clinical management of patients but also will facilitate the interpretation and translation of clinical research. Renal osteodystrophy is now considered as 1 component of this disorder and will be defined as a morphologic alteration only, based on unification of the histomorphometric definitions that will include parameters of turnover, mineralization, and volume. An internationally accepted classification system will enable the consensus for bone biopsy evaluation as well as for the role of biomarkers. This article will focus on the newly proposed definitions of bone disease as part of CKD-MBD, based on the complex pathophysiologic process underlying bone disease in CKD stages 2 to 5.