A novel immunoradiometric assay detects full-length human PTH but not amino-terminally truncated fragments: implications for PTH measurements in renal failure

Unlocking the mysteries of n-oxPTH: implications for CKD patients

Parathyroid hormone (PTH) is a pivotal hormone that regulates serum calcium and phosphate and is closely associated with higher risk of cardiovascular disease and mortality in patients with chronic kidney disease (CKD). PTH can undergo oxidation at methionine 8 and methionine 18 of the molecule. This oxidation process leads to a lower binding affinity to the PTH receptor due to molecular refolding, particularly for PTH oxidized at methionine 8. Although, the oxidation of PTH has been reported for several decades, it is only recently that a method has been developed to detect non-oxidized PTH (n-oxPTH) levels. The utilization of this assay enables the precise detection of n-oxPTH levels and facilitates the evaluation of their correlation with poor prognosis in patients with CKD. However, the current available clinical research findings indicate that n-oxPTH does not demonstrate clinical superiority over iPTH. Here, we provide a comprehensive review on the mechanism of PTH oxidation, the n-oxPTH assay method, and its correlation with iPTH and clinical outcomes.

Rare genetic disorders that impair parathyroid hormone synthesis, secretion, or bioactivity provide insights into the diagnostic utility of different parathyroid hormone assays

PURPOSE OF REVIEW

Parathyroid hormone (PTH) is the major peptide hormone regulator of blood calcium homeostasis. Abnormal PTH levels can be observed in patients with various congenital and acquired disorders, including chronic kidney disease (CKD). This review will focus on rare human diseases caused by PTH mutations that have provided insights into the regulation of PTH synthesis and secretion as well as the diagnostic utility of different PTH assays.

RECENT FINDINGS

Over the past years, numerous diseases affecting calcium and phosphate homeostasis have been defined at the molecular level that are responsible for reduced or increased serum PTH levels. The underlying genetic mutations impair parathyroid gland development, involve the PTH gene itself, or alter function of the calcium-sensing receptor (CaSR) or its downstream signaling partners that contribute to regulation of PTH synthesis or secretion. Mutations in the pre sequence of the mature PTH peptide can, for instance, impair hormone synthesis or intracellular processing, while amino acid substitutions affecting the secreted PTH(1-84) impair PTH receptor (PTH1R) activation, or cause defective cleavage of the pro-sequence and thus secretion of a pro- PTH with much reduced biological activity. Mutations affecting the secreted hormone can alter detection by different PTH assays, thus requiring detailed knowledge of the utilized diagnostic test.

SUMMARY

Rare diseases affecting PTH synthesis and secretion have offered helpful insights into parathyroid biology and the diagnostic utility of commonly used PTH assays, which may have implications for the interpretation of PTH measurements in more common disorders such as CKD.

Unveiling a new era with liquid chromatography coupled with mass spectrometry to enhance parathyroid hormone measurement in patients with chronic kidney disease

Precise determination of circulating parathyroid hormone (PTH) concentration is crucial to diagnose and manage various disease conditions, including the chronic kidney disease-mineral and bone disorder. However, the lack of standardization in PTH assays is challenging for clinicians, potentially leading to medical errors because the different assays do not provide equivalent results and use different reference ranges. Here, we aimed to evaluate the impact of recalibrating PTH immunoassays by means of a recently developed LC-MS/MS method as the reference. Utilizing a large panel of pooled plasma samples with PTH concentrations determined by the LC-MS/MS method calibrated with the World Health Organization (WHO) 95/646 International Standard, five PTH immunoassays were recalibrated. The robustness of this standardization was evaluated over time using different sets of samples. The recalibration successfully reduced inter-assay variability with harmonization of PTH measurements across different assays. By recalibrating the assays based on the WHO 95/646 International Standard, we demonstrated the feasibility for standardizing PTH measurement results and adopting common reference ranges for PTH assays, facilitating a more consistent interpretation of PTH values. The recalibration process aligns PTH results obtained from various immunoassays with the LC-MS/MS method, providing more consistent and reliable measurements. Thus, establishing true standardization across all PTH assays is crucial to ensure consistent interpretation and clinical decision-making.

Parathyroid hormone reference intervals in adults using second- and third-generation assays

Parathyroid hormone (PTH) is a routine biochemical analysis, and it varies whether a second- or third-generation assay is used. Information on the levels obtained with different assays and evidence to substantiate local assay-specific reference ranges are important to inform clinical practice. Prior to a shift from the second- to the third-generation PTH assay (Cobas 8000, Roche Diagnostics) in the Department of Clinical Biochemistry, Aalborg University Hospital, Aalborg, Denmark, a total of 59 EDTA-plasma samples were collected for method comparison (Passing-Bablok). Furthermore, 120 EDTA-plasma samples were randomly obtained from adult blood donors and used for the establishment of reference intervals using the third-generation PTH assay (Cobas 8000, Roche Diagnostics) and two second-generation assays (Atellica, Siemens Healthineers; Alinity, Abbott Laboratories). Method comparison (Cobas 8000, Roche Diagnostics) showed lower levels with the third-generation (y) as compared to the second-generation assay (x) depending on the measurement range (PTH < 10 pmol/L: y = 0.8 (95% CI: 0.7; 0.9) x + 0.3 (95% CI: 0.2; 0.5), PTH ≥ 10 pmol/L: y = 0.6 (95% CI: 0.5; 0.6) x + 3.2 (95% CI: 1.1; 5.2)). Method-specific reference intervals (2.5 and 97.5 percentiles) after the exclusion of samples (n = 31) with 25-hydroxy-vitamin D below 50 nmol/L were: 1.8-8.5 pmol/L (second-generation, Atellica, Siemens Healthineers); 2.4-10.9 pmol/L (second-generation, Alinity, Abbott Laboratories), and 1.8-7.0 pmol/L (third-generation, Cobas 8000, Roche Diagnostics). PTH levels with second- and third-generation assays are not interchangeable. Clinicians should be informed when a laboratory assay is changed, and method-specific reference ranges are needed.

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.

Preanalytical considerations in parathyroid hormone measurement

Automated immunoassays used to evaluate parathyroid function are vulnerable to different types of interference, which can affect clinical practices. This review provides a detailed overview of the six main types of interference known to affect the measurement of parathyroid hormone (PTH): heterophilic antibodies, biotin, PTH fragments, oxidized PTH (oxPTH), phosphorylated PTH, and some preanalytical factors. Because the prevalence of some of these conditions has been reported to approach 11.7%, and the frequency of testing for parathyroid function is important, the scale of the problem might be tremendous. Potential interference in parathyroid function testing should always be suspected whenever clinical or biochemical discrepancies arise. Their identification typically relies on additional laboratory tests, including method comparison, serial dilution, blocking reagent studies, affinity adsorption, and polyethylene glycol precipitation. Moreover, some of these issues can be mitigated with the development of mass spectrometry. This review also evaluated the clinical impact of parathyroid interference on immunoassays, including misdiagnosis, inappropriate parathyroidectomy; and delay in receiving appropriate therapy. Hence, strong communication should be maintained between the clinician and laboratory to avoid such scenarios.

Full-length versus intact PTH concentrations in pseudohypoparathyroidism type 1 and primary hyperparathyroidism: clinical evaluation of immunoassays in individuals from China

PURPOSE

The application of the third-generation parathyroid hormone (PTH) assay [PTH(1-84) assay] for evaluating PTH levels in patients with pseudohypoparathyroidism type-1 (PHP1) is less popular than the second-generation assay. Therefore, we aimed at examining the conformity between the PTH(1-84) assay and the intact PTH (iPTH) assay, specifically examining their performance in individuals with PHP1 versus individuals with primary hyperparathyroidism (PHPT), compared to healthy controls.

METHODS

PTH(1-84) and iPTH assay were performed in patients with PHP1, patients with PHPT, and healthy volunteers. ∆PTH%, PTH(1-84)/iPTH (3^rd/2^nd ratio), iPTH/upper limit of normal (ULN), and PTH (1-84)/ULN of each group were calculated for comparison. Linear regression, Kappa conformity test, and Bland-Altman analysis of ∆PTH/mean of iPTH and PTH(1-84) (percent bias) plotted against the mean of iPTH and PTH(1-84) were performed to determine the conformance of PTH(1-84) assay with iPTH assay.

RESULTS

A total of 54 patients with PHP1, 127 patients with PHPT, and 65 healthy volunteers were enrolled in this study. All the three groups showed strong linear relationship between iPTH and PTH (1-84) (r² = 0.9661, 0.7733, and 0.9575, respectively). No significant differences were noted in 3^rd/2^nd ratio (median 0.76 vs. 0.72) between the PHP1 and PHPT groups (p > 0.05). Conformity examination showed the Kappa value was 0.778 and 0.395 for PHP1 and PHPT groups respectively. No difference in the Kappa values was found between PHP1A and PHP1B subgroups. Bland-Altman plot demonstrated that the proportion of data points that were plotted within mean ± 1.96 SD in PHP1, PHPT and normal control groups were 96.3%, 93.7%, and 98.5%, respectively. The mean percent bias of the three groups were 26.1%, 31.2%, and 17.0%, respectively. The range of mean ± 1.96 SD of percent bias of the three groups were 2.2%-50.0%, -14.3%-76.6%, and 6.7%-27.2%, respectively.

CONCLUSION

Although iPTH and PTH(1-84) values were both lower in the present PHP1 cohort than in the PHPT cohort, there appear to be differences in the relative agreement between both immunoassays, and in the relationship between the two values, especially in comparison to healthy controls. Whether these differences are due to differential accumulation of C-terminal fragments or other factors requires further study.

Hypoparathyroidism: Genetics and Diagnosis

This narrative report summarizes diagnostic criteria for hypoparathyroidism and describes the clinical presentation and underlying genetic causes of the nonsurgical forms. We conducted a comprehensive literature search from January 2000 to January 2021 and included landmark articles before 2000, presenting a comprehensive update of these topics and suggesting a research agenda to improve diagnosis and, eventually, the prognosis of the disease. Hypoparathyroidism, which is characterized by insufficient secretion of parathyroid hormone (PTH) leading to hypocalcemia, is diagnosed on biochemical grounds. Low albumin-adjusted calcium or ionized calcium with concurrent inappropriately low serum PTH concentration are the hallmarks of the disease. In this review, we discuss the characteristics and pitfalls in measuring calcium and PTH. We also undertook a systematic review addressing the utility of measuring calcium and PTH within 24 hours after total thyroidectomy to predict long-term hypoparathyroidism. A summary of the findings is presented here; results of the detailed systematic review are published separately in this issue of JBMR. Several genetic disorders can present with hypoparathyroidism, either as an isolated disease or as part of a syndrome. A positive family history and, in the case of complex diseases, characteristic comorbidities raise the clinical suspicion of a genetic disorder. In addition to these disorders' phenotypic characteristics, which include autoimmune diseases, we discuss approaches for the genetic diagnosis. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Validation of an LC-MS/MS Method Using Solid-Phase Extraction for the Quantification of 1-84 Parathyroid Hormone: Toward a Candidate Reference Measurement Procedure

BACKGROUND

Parathyroid hormone (PTH) measurement is important for patients with disorders of calcium metabolism, including those needing bone-turnover monitoring due to chronic kidney disease-mineral bone disorder. There are currently 2 generations of PTH immunoassays on the market, both having cross-reactivity issues and lacking standardization. Therefore, we developed an LC-MS/MS higher-order method for PTH analysis.

METHODS

The method was calibrated against the international standard for 1-84 PTH (WHO 95/646). Antibody-free sample preparation with the addition of an isotope-labeled internal standard was performed by solid-phase extraction. Extracts were analyzed by LC-MS/MS. EDTA-K2 plasma was used throughout the development and validation. Bias and uncertainty sources were tested according to ISO 15193. Clinical Laboratory Standards Institute guidelines and reference measurement procedures were consulted for the design of the validation. Patient samples and external quality controls were compared between LC-MS/MS and 2 third-generation immunoassays.

RESULTS

The method was validated for 1-84 PTH from 5.7 to 872.6 pg/mL. The interassay imprecision was between 1.2% and 3.9%, and the accuracy ranged from 96.2% to 103.2%. The measurement uncertainty was <5.6%. The comparison between LC-MS/MS and the immunoassays showed a proportional bias but moderate to substantial correlation between methods.

CONCLUSIONS

This LC-MS/MS method, which is independent of antibodies, is suitable for a wide range of PTH concentrations. The obtained analytical performance specifications demonstrate that development of a reference measurement procedure will be possible once a higher order reference standard is available.

Should clinical laboratories adapt to the reality of chronic kidney disease in the determination of parathyroid hormone?

Objectives

The contribution of the clinical laboratory to diagnostics is increasingly important since a great deal of clinical decisions rely on laboratory test results.

Content

Parathyroid hormone (PTH) measurement presents a considerable analytical variability due to the heterogeneity of its circulating forms and the antigenic configuration of the different assays commercially available. Such variability may have an impact on pathological conditions associated with significant increases in circulating PTH, as it is the case of chronic kidney disease (CKD).

Summary

Despite the recent identification of new molecules involved in bone and mineral disorders associated with CKD, such as klotho or the fibroblastic factor 23 (FGF23), nephrologists still base their clinical decisions on PTH concentrations. The problem is that unawareness of these analytical considerations may cause errors in the clinical interpretation of test results.

Outlook

This systematic review addresses these issues from the clinical laboratory perspective and proposes new approaches related to PTH method selection and result expression. These new strategies will help laboratory medicine specialists and nephrologist better determine the status of CKD patients.

The path to the standardization of PTH: Is this a realistic possibility? a position paper of the IFCC C-BM

Parathyroid hormone (PTH) determination is of greatest importance for patients suffering from parathyroid gland disorders and for the follow-up of bone turnover in patients suffering from chronic kidney disease (CKD). Two generations of PTH assays are simultaneously present on the market for PTH quantification. As these assays are not yet standardized, this results in a significant level of confusion in the care of CKD patients. One key objective of the IFCC Committee for Bone Metabolism is to improve this situation. In this position paper, we will highlight the current state of PTH testing and propose a pathway to ultimately overcome issues resulting from PTH assay variability.

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

Parathyroid hormone is an essential regulator of extracellular calcium and phosphate. PTH enhances calcium reabsorption while inhibiting phosphate reabsorption in the kidneys, increases the synthesis of 1,25-dihydroxyvitamin D, which then increases gastrointestinal absorption of calcium, and increases bone resorption to increase calcium and phosphate. Parathyroid disease can be an isolated endocrine disorder or part of a complex syndrome. Genetic mutations can account for diseases of parathyroid gland formulation, dysregulation of parathyroid hormone synthesis or secretion, and destruction of the parathyroid glands. Over the years, a number of different options are available for the treatment of different types of parathyroid disease. Therapeutic options include surgical removal of hypersecreting parathyroid tissue, administration of parathyroid hormone, vitamin D, activated vitamin D, calcium, phosphate binders, calcium-sensing receptor, and vitamin D receptor activators to name a few. The accurate assessment of parathyroid hormone also provides essential biochemical information to properly diagnose parathyroid disease. Currently available immunoassays may overestimate or underestimate bioactive parathyroid hormone because of interferences from truncated parathyroid hormone fragments, phosphorylation of parathyroid hormone, and oxidation of amino acids of parathyroid hormone.

Hypoparathyroidism

BACKGROUND

Hypoparathyroidism is a rare endocrine disorder characterized by hypocalcemia and low or undetectable levels of parathyroid hormone.

METHODS

This review is an evidence-based summary of hypoparathyroidism in terms of relevant pathophysiological, clinical, and therapeutic concepts.

RESULTS

Many clinical manifestations of hypoparathyroidism are due to the lack of the physiological actions of parathyroid hormone on its 2 major target organs: the skeleton and the kidney. The skeleton is inactive, accruing bone without remodeling it. The kidneys lose the calcium-conserving actions of parathyroid hormone and, thus, excrete a greater fraction of calcium. Biochemical manifestations, besides hypocalcemia and low or undetectable levels of parathyroid hormone, include hyperphosphatemia and low levels of 1,25-dihydroxyvitamin D. Calcifications in the kidney, brain, and other soft tissues are common. Removal of, or damage to, the parathyroid glands at the time of anterior neck surgery is, by far, the most likely etiology. Autoimmune destruction of the parathyroid glands and other genetic causes represent most of the other etiologies. Conventional treatment with calcium and active vitamin D can maintain the serum calcium level but high doses may be required, adding to the risk of long-term soft tissue calcifications. The advent of replacement therapy with recombinant human PTH(1-84) represents a major step in the therapeutics of this disease.

CONCLUSIONS

Advances in our knowledge of hypoparathyroidism have led to greater understanding of the disease itself and our approach to it.

Oxidation of parathyroid hormone

Parathyroid hormone (PTH) is the key hormone regulating calcium homeostasis and, as such, is an important diagnostic and prognostic marker. Although the measurement of PTH has greatly improved over the past few decades, oxidation status thereof is unaccounted for in currently used assays. PTH can be oxidized on methionine residues located at amino acid positions 8 and 18. This is a relevant post-translational modification as, due to refolding of the molecule, it results in the decreased ability to activate the PTH1 receptor. Although this loss of activity after oxidation was observed as early as 1934, only recently a method was developed to measure and distinguish non-oxidized PTH (n-oxPTH) from oxidized PTH. This method creates exciting possibilities for studying more specifically the role of n-oxPTH in physiology and pathology. Therefore, it can now be explored what the clinical implications of measuring n-oxPTH will be. Herein, we review the available evidence of the effect of oxidation on the biological activity of PTH. We also discuss studies examining the mechanism of PTH oxidation in vivo and efforts to stabilize synthetic PTH ex vivo for therapeutic applications. Lastly, the available studies regarding the clinical significance of n-oxPTH are evaluated and future directions discussed.

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.

Clinical presentation and management of hypoparathyroidism

The clinical manifestations of hypoparathyroidism are variable and can involve almost any organ system. The main clinical features of the hypoparathyroidism are typically signs or symptoms due to neuromuscular irritability owing to low serum calcium level. In addition to hypocalcemia, hyperphosphatemia can contribute to long-term complications, including extra-skeletal calcifications. Bone turnover markers are generally decreased, and bone mass density is usually normal-increased compared to age- and gender-matched controls. It is still unclear whether or not these bone features could have an impact on the risk of fracture. Impaired renal function is a common complication described in patients treated for hypoparathyroidism. Other complications include premature cataracts, seizures, basal ganglia calcifications, and cardiac arrhythmias. Lastly, some clinical studies have also reported a reduced quality of life of patients with hypoparathyroidism. Increased awareness of the clinical manifestations of this disease is important to improve its clinical management.

Hypoparathyroidism

Hypoparathyroidism is a disease characterized by inadequately low circulating concentrations of parathyroid hormone (PTH) resulting in low calcium levels and increased phosphate levels in the blood. Symptoms of the disease result from increased neuromuscular irritability caused by hypocalcaemia and include tingling, muscle cramps and seizures. The most common cause of the disease is inadvertent removal of, or injury to, the parathyroid glands during neck surgery, followed by genetic, idiopathic and autoimmune aetiologies. Conventional treatment includes activated vitamin D and/or calcium supplements, but this treatment does not fully replace the functions of PTH and can lead to short-term problems (such as hypocalcaemia, hypercalcaemia and increased urinary calcium excretion) and long-term complications (which include nephrocalcinosis, kidney stones and brain calcifications). PTH replacement has emerged as a new treatment option. Clinical trials using human PTH(1-34) and PTH(1-84) showed that this treatment was safe and effective in studies lasting up to 6 years. Recombinant human PTH(1-84) has been approved in the United States and Europe for the management of hypoparathyroidism; however, its effect on long-term complications is still being evaluated. Clinical practice guidelines, which describe the consensus of experts in the field, have been published and recognize the need for more research to optimize care. In this Primer, we summarize current knowledge of the prevalence, pathophysiology, clinical presentation and management of hypoparathyroidism.

How the reference values for serum parathyroid hormone concentration are (or should be) established?

Well-validated reference values are necessary for a correct interpretation of a serum PTH concentration. Establishing PTH reference values needs recruiting a large reference population. Exclusion criteria for this population can be defined as any situation possibly inducing an increase or a decrease in PTH concentration. As recommended in the recent guidelines on the diagnosis and management of asymptomatic primary hyperparathyroidism, PTH reference values should be established in vitamin D-replete subjects with a normal renal function with possible stratification according to various factors such as age, gender, menopausal status, body mass index, and race. A consensus about analytical/pre-analytical aspects of PTH measurement is also needed with special emphasis on the nature of the sample (plasma or serum), the time and the fasting/non-fasting status of the blood sample. Our opinion is that blood sample for PTH measurement should be obtained in the morning after an overnight fast. Furthermore, despite longer stability of the PTH molecule in EDTA plasma, we prefer serum as it allows to measure calcium, a prerequisite for a correct interpretation of a PTH concentration, on the same sample. Once a consensus is reached, we believe an important international multicentre work should be performed to recruit a very extensive reference population of apparently healthy vitamin D-replete subjects with a normal renal function in order to establish the PTH normative data. Due to the huge inter-method variability in PTH measurement, a sufficient quantity of blood sample should be obtained to allow measurement with as many PTH kits as possible.

Calcium and Bone Metabolism Indices

Calcium and inorganic phosphate are of critical importance for many body functions, thus the regulations of their plasma concentrations are tightly controlled by the concerted actions of reabsorption/excretion in the kidney, absorption in the intestines, and exchange from bone, the major reservoir for calcium and phosphate in the body. Parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D (1,25(OH)₂D) control calcium homeostasis, whereas PTH, 1,25(OH)₂D, and bone-derived fibroblast growth factor 23 (FGF 23) control phosphate homeostasis. Hypoparathyroidism can cause hypocalcemia and hyperphosphatemia, whereas deficient vitamin D actions can cause osteomalacia in adults and rickets in children. Hyperparathyroidism, alternatively, can cause hypercalcemia and hypophosphatemia. Laboratory tests of calcium, phosphate, PTH, and 25-hydroxyvitamin D are very useful in the diagnosis of abnormalities associated with calcium and/or phosphate metabolisms. Bone is constantly remodeled throughout life in response to mechanical stress and a need for calcium in extracellular fluids. Metabolic bone diseases such as osteoporosis, osteomalacia in adults or rickets in children, and renal osteodystrophy develop when bone resorption exceeds bone formation. Bone turnover markers (BTM) such as serum N-terminal propeptide of type I procollagen (P1NP) and C-terminal collagen cross-link (CTX) may be useful in predicting future fracture risk or monitoring the response to anti-resorptive therapy. There is a need to standardize sample collection protocols because certain BTMs exhibit large circadian variations and tend to be influenced by food intakes. In the United States, a project to standardize BTM sample collection protocols and to establish the reference intervals for serum P1NP and serum CTX is ongoing. We anticipate the outcome of this project to shine lights on the standardization of BTM assays, sample collection protocols, reference intervals in relation to age, sex, and ethnic origins, and clinical utilities of BTMs. This review will briefly discuss the regulations of calcium and phosphate homeostasis, laboratory's role in the diagnosis, and monitoring of bone and calcium metabolism, as well as the usefulness and controversies of the utilities of BTMs in the diagnosis and monitoring of metabolic bone diseases.

A Homozygous [Cys25]PTH(1-84) Mutation That Impairs PTH/PTHrP Receptor Activation Defines a Novel Form of Hypoparathyroidism

Hypocalcemia and hyperphosphatemia are encountered in idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism type Ib (PHP1B). In contrast to PHP1B, which is caused by resistance toward parathyroid hormone (PTH), the genetic defects leading to IHP impair production of this important regulator of mineral ion homeostasis. So far, only five PTH mutations were shown to cause IHP, each of which is located in the hormone's pre-pro leader segment and thus impair hormone secretion. In three siblings affected by IHP, we now identified a homozygous arginine-to-cysteine mutation at position 25 (R25C) of the mature PTH(1-84) polypeptide; heterozygous family members are healthy. Depending on the assay used for evaluating these patients, plasma PTH levels were either low or profoundly elevated, thus leading to ambiguities regarding the underlying diagnosis, namely IHP or PHP1B. Consistent with increased PTH levels, recombinant [Cys25]PTH(1-84) and wild-type PTH(1-84) were secreted equally well by transfected COS-7 cells. However, synthetic [Cys25]PTH(1-34) was found to have a lower binding affinity for the PTH receptor type-1 (PTH1R) than PTH(1-34) and consequently a lower efficiency for stimulating cAMP formation in cells expressing this receptor. Consistent with these in vitro findings, long-term infusion of [Cys25]PTH(1-34) resulted only in minimal calcemic and phosphaturic responses, despite readily detectable levels of [Cys25]PTH(1-34) in plasma. The mineral ion abnormalities observed in the three IHP patients are thus most likely caused by the inherited homozygous missense PTH mutation, which reduces bioactivity of the secreted hormone. Based on these findings, screening for PTH(1-84) mutations should be considered when clinical and laboratory findings are consistent with PHP1B, but GNAS methylation changes have been excluded. Differentiating between IHP and PHP1B has considerable implications for genetic counseling, therapy, and long-term outcome because treatment of IHP patients with inappropriately high doses of active vitamin D and calcium can contribute to development of nephrocalcinosis and chronic kidney disease.

Reversed whole PTH/intact PTH ratio as an indicator of marked parathyroid enlargement: five case studies and a literature review

Parathyroid hormone (PTH) levels detected by intact PTH assays are generally higher than those detected by the whole PTH assay because the latter does not detect non-(1–84) PTH fragments, mainly PTH (7–84). Rare exceptions to this rule have been reported in patients with severe primary or secondary hyperparathyroidism and parathyroid carcinoma. Overproduction of an N-form of PTH other than PTH (1–84) has been observed in the sera of these patients. We report five additional cases with the reversed whole PTH/intact PTH ratio associated with severe hyperparathyroidism in haemodialysis patients. Three patients demonstrated enlargement of a single hypervascular gland, whereas the other two had undergone surgical parathyroidectomy and later showed recurrent hyperparathyroidism due to progressive autograft hyperplasia. In the case of a single enlarged gland, the pathological pattern and heterogeneous expression of parathyroid adenomatosis 1/cyclin D1 suggested it to be a single nodule of uraemic hyperparathyroidism rather than sporadic primary adenoma. These cases suggested that the reversed whole PTH/intact PTH ratio could be an indicator of marked parathyroid enlargement. Further studies are required to elucidate the clinical significance of the reversed whole PTH/intact PTH ratio in haemodialysis patients.

AXT914 a novel, orally-active parathyroid hormone-releasing drug in two early studies of healthy volunteers and postmenopausal women

Antagonism of the calcium-sensing receptor in the parathyroid gland leads to parathyroid hormone (PTH) release. Calcilytics are a new class of molecules designed to exploit this mechanism. In order to mimic the known bone-anabolic pharmacokinetic (PK) profile of s.c. administered PTH, such molecules must trigger sharp, transient and robust release of PTH. The results of two early clinical studies with the orally-active calcilytic AXT914, a quinazolin-2ne derivative are reported. These were GCP-compliant, single and multiple dose studies of PK/PD and tolerability in healthy volunteers and postmenopausal women. The first study, examined single ascending doses (4 to 120 mg) and limited multiple doses (60 or 120 mgq.d. for 12 days) of AXT914. The second study was a randomized, double-blind, active- and placebo-controlled, 4-week repeat-dose parallel group study of healthy postmenopausal women (45 and 60 mg AXT914, placebo, 20 μg Forteo/teriparatide/PTH(1-34) fragment). AXT914 was well tolerated at all doses and reproducibly induced the desired PTH-release profiles. Yet, 4 weeks of 45 or 60 mg AXT914 did not result in the expected changes in circulating bone biomarkers seen with teriparatide. However total serum calcium levels increased above baseline in the 45 and 60 mg AXT914 treatment groups (8.0% and 10.7%, respectively), compared to that in the teriparatide and placebo groups (1.3% and 1.0%, respectively). Thus the trial was terminated after a planned interim analysis due to lack of effect on bone formation biomarkers and dose-limiting effects on serum calcium. In conclusion, AXT914 was well tolerated but the observed transient and reproducible PTH-release after repeat oral administration of AXT914 which showed an exposure profile close to that of s c. PTH, did not translate into a bone anabolic response and was associated with a persistent dose-related increase in serum calcium concentrations.

The relationship between serum 25-hydroxy vitamin D, parathormone and bone mineral density in Indian population

OBJECTIVES

Vitamin D deficiency (VDD) is a global problem. Not all patients with VDD have clinical manifestations or secondary hyperparathyroidism. We studied the interaction between serum 25-hydroxy vitamin D (25OHD), parathormone (PTH) and bone mineral density (BMD) in Indian adolescents and adults.

DESIGN

Population survey.

PATIENTS

A total of 1829 adolescents and 1346 adults aged 50 years and above were analysed in this study.

MEASUREMENTS

Serum biochemistry, 25OHD, PTH and BMD were estimated. Subjects were grouped according to quartiles of serum PTH. VDD was defined as severe (25OHD ≤ 5 ng/ml), moderate (25OHD ≤ 10 ng/ml) and mild (25OHD ≤ 20 ng/ml) and secondary hyperparathyroidism (SHPT) when serum PTH levels >65 pg/ml.

RESULTS

Only 30-40% of subjects with moderate and severe VDD, respectively, had SHPT. BMD decreased from Quartile 1 to Quartile 4 of PTH at all sites among adolescents and adults, with only a marginal decline in serum 25OHD levels between these quartiles. Further, within each PTH quartile, there was no difference in BMD according to categories of VDD. Analysing BMD in the different PTH quartiles, the PTH cut-offs beyond which BMD showed a significant decline, was 35 pg/ml in adolescents and 53 pg/ml in adults.

CONCLUSIONS

Less than half of the subjects with VDD have SHPT. BMD levels start to decline at PTH values currently considered to be normal. These data suggest the need to redefine SHPT in different age groups keeping in mind the relationship between PTH and BMD. This may also influence the decision to supplement subjects with VDD.

Comparison between a second and a third generation parathyroid hormone assay in hemodialysis patients

OBJECTIVE

Third generation parathyroid hormone (PTH) assays are new generation assays that do not recognize the PTH7-84 fragment whereas second generation assays detect both PTH1-84 and PTH7-84 fragments. Despite the excellent correlation between both assays in chronic renal failure (CRF) subjects, the mean PTH levels are typically 50% lower with the third compared to the second generation assays. The assessment of third generation PTH assays has not been extensively studied in hemodialysis subjects. The purpose of our study was to compare a third generation PTH assay to a second generation one in a population of hemodialysis subjects.

MATERIALS AND METHODS

92 haemodialysis subjects (36 women and 56 men) with a mean age of 67±12.9 years were included in this study. Anthropometric and clinical parameters (Body Mass Index (BMI) and blood pressure) were measured. Second and third generation PTH assays (Cis biomedical and Diasorin respectively) were performed in each subject. In addition, the following biochemical tests were measured: 25-hydroxyvitamin D (25-(OH)D), 1,25-hydroxyvitamin D (1,25-(OH)2D), crosslaps and alkaline phosphatase.

RESULTS

The mean second and third generation PTHs are respectively 211±205 pg/ml and 151±164 pg/ml. The mean third generation PTH values are 28.4% lower compared to the second generation ones. Both methods are strongly correlated (r=0.923, p<0.001). This correlation persisted without any significant difference after controlling for gender, age, BMI and Blood Pressure. However, the difference between both methods increases when baseline PTH increases. Each of the second and third generation method is significantly correlated with hemodialysis duration (p<0.01), crosslaps (p<0.001), alkaline phosphatase (p<0.05), but not with age, BMI, Blood Pressure, 25-(OH)D or 1,25-(OH) 2D levels.

CONCLUSION

Our results show that both second and third generation PTH methods are strongly correlated in hemodialysis patients mainly when PTH values are low. However, the difference between both methods increases when PTH values are high. More research is needed to establish which method is the gold standard when PTH values are high.

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. Early increases in bone and plasma fibroblast growth factor 23 (FGF23) are associated with early defects in skeletal mineralization. Later in the course of CKD, secondary hyperparathyroidism--caused by a combination of declining calcitriol values and phosphate retention--results in high-turnover renal osteodystrophy whereas increased levels of both phosphate and FGF23 contribute to cardiovascular disease. Treatment of hyperphosphatemia and secondary hyperparathyroidism improves high-turnover bone disease but fails to correct defects in skeletal mineralization. Because overtreatment may result in adynamic bone disease, growth failure, hypercalcemia, and progression of cardiovascular calcifications, therapy therefore must be titrated carefully to maintain optimal serum biochemical parameters according to stage of CKD. Newer therapeutic agents and new treatment paradigms may suppress serum PTH levels effectively while limiting intestinal calcium absorption and skeletal FGF23 stimulation and may provide future therapeutic alternatives for children with CKD.

Comparison of the Elecsys PTH(1-84) assay with four contemporary second generation intact PTH assays and association with other biomarkers in chronic kidney disease patients

OBJECTIVES

The COBAS Elecsys PTH(1-84) assay is a novel, electro-chemiluminescence immunoassay that exclusively measures full-length parathyroid hormone (PTH). The aim of this study is to compare the automated biointact Elecsys PTH(1-84) assay with four contemporary, iPTH assays in chronic kidney disease (CKD) patients.

DESIGN AND METHODS

We compared the Elecsys PTH(1-84) assay with four iPTH assays (Siemens ADVIA Centaur, Ortho Clinical Diagnostics (OCD) VITROS, Beckman Access2, Abbott ARCHITECT) in the measurement of PTH in 83 local CKD patients. Majority of the patients (44) had CKD but were not on dialysis, 15 were on hemodialysis, 15 were on peritoneal dialysis, and 9 were post-renal transplant. The precision performance and correlation of the assays were determined. PTH(1-84) concentrations were correlated with calcium, phosphate, alkaline phosphatase, hemoglobin, HbA1c and lipid concentrations.

RESULTS

The Elecsys PTH(1-84) assay showed comparable precision and good correlation with the iPTH assays. Although the four different iPTH assays correlated well with each other, there was significant discrepancy among assays. The discrepancy among assays increased with increasing PTH concentrations. The ADVIA Centaur and ARCHITECT assays measured significantly higher PTH concentrations than the VITROS and Access2 assays. PTH(1-84) showed a positive association with phosphate and alkaline phosphatase and an inverse association with HbA1c. There was no significant association with lipid concentrations.

CONCLUSIONS

The third generation Elecsys PTH(1-84) assay had comparable precision performance and correlated well with second generation iPTH assays. However, significant discrepancy was found among the four iPTH assays in measuring iPTH in CKD patients.

Switching between parathormone (PTH) assays: the impact on the diagnosis of renal osteodystrophy

Clinical guidelines for decision-making in chronic kidney disease (CKD) consider parathormone (PTH) levels. The measured PTH values differ if novel full length PTH(1-84) assays are used instead of earlier intact iPTH assays. In this study we analyzed how the classification of CKD patients alters when iPTH assays are switched to PTH(1-84) assays.

Plasma samples were collected prior to dialysis sessions from 110 consecutive CKD patients on maintenance hemodialysis. PTH levels were determined with iPTH assays (Elecsys, Architect and DiaSorin Liaison N-tact) and PTH(1-84) assays (Elecsys and Liaison). Using KDIGO guidelines patients were classified as being below, above and in the recommended target range (RTR) of PTH. The results of classification with different assays were evaluated and, a novel calculation method of RTR was implemented.

The prevalence of patients with PTH in RTR is comparable with each assay, but the individual patients differed. PTH(1-84) Elecsys and Liaison assays classified more patients as being below RTR than iPTH Elecsys and Architect but not Liaison N-tact assay (27.3%, 22.7% vs. 41%, 31.8%, and 36.4%, respectively). In turn, PTH(1-84) Elecsys and Liaison assays identified less CKD patients with PTH above the RTR than iPTH except N-tact assays (6.4%, 10% vs. 16.3%, 19%, and 6.3%, respectively). Using our calculation method, our discrimination values for PTH(1-84) assays to achieve classification identical to that with iPTH Elecsys were lower than those recommended by the manufacturer.

Current guidelines for the treatment of secondary hyperparathyroidism in CKD should consider the type of assays used for PTH measurement. Each laboratory should assess its own RTR for PTH tests to achieve comparable classification. The presented calculation is simple, it mimics an everyday situation, switching from one assay to another one, and provides useful RTR values for PTH tests.

Diagnosis and management of parathyroid cancer

Parathyroid cancer is rare, but often fatal, as preoperative identification of malignancy against the backdrop of benign parathyroid disease is challenging. Advanced genetic, laboratory and imaging techniques can help to identify parathyroid cancer. In patients with clinically suspected parathyroid cancer, malignancy of any individual lesion is established by three criteria: demonstration of metastasis, specific ultrasonographic features, and a ratio >1 for the results of third-generation:second-generation parathyroid hormone assays. Positive findings for all three criteria dictate an oncological surgical approach, as appropriate radical surgery can achieve a cure. Mutation screening pinpoints associated conditions and asymptomatic carriers. Molecular profiling of tumour cells can identify high-risk features, such as differential expression of specific micro-RNAs and proteins, and germ line mutations in CDC73, but is unsuitable for preoperative assessment owing to the potential risks associated with biopsy. A validated, histopathology-based prognostic classification can identify patients in need of close follow-up and adjuvant therapy, and should prove valuable to stratify clinical trial cohorts: low-risk patients rarely die from parathyroid cancer, even on long-term follow-up, whereas 5-year mortality in high-risk patients is around 50%. This insight has improved the approach to parathyroid cancer by enabling risk-adapted surgery and follow-up.

Parathyroid hormone measurement in chronic kidney disease--an evolving issue for the nephrologist and the clinical laboratorist: minireview

Parathyroid hormone (PTH) is the polypeptide hormone produced by the parathyroid glands, which plays a central role in calcium homeostasis. Circulating PTH must be measured regularly in patients with chronic kidney disease (CKD)--mineral and bone disorders (MBD) to monitor and to adapt treatment with the aim of maintaining PTH levels within a defined narrow range of optimal values for each stage of CKD. Often, for the nephrologists, it is not easy to determine what PTH levels are clinically appropriate. Moreover, the PTH determination also shows many criticisms from the laboratory point of view and there is a clear need to standardize PTH measurements in every phase of the process: pre-analytical, analytical and post-analytical. In this review, all these aspects are summarized with particular reference to the most recent opportunities to improve PTH assays quality on the whole. To this aim, a closer cooperation between nephrologists and clinical laboratories is undoubtedly necessary.

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.

Mineral and bone disorders in children with chronic kidney disease

As children with chronic kidney disease (CKD) have a long lifespan, optimal control of bone and mineral homeostasis is essential not only for the prevention of debilitating skeletal complications and for achieving adequate growth but also for preserving long-term cardiovascular health. As the growing skeleton is highly dynamic and at particular risk of deterioration, close control of bone and mineral homeostasis is required in children with CKD. However, assessment of bone disease is hampered by the limited validity of biochemical parameters-major controversy exists on key issues such as parathyroid hormone target ranges and the lack of useful imaging techniques. The role of newly discovered factors in bone and mineral homeostasis, such as fibroblast growth factor 23, is not yet established. Even though scientific evidence is limited in children with CKD, ergocalciferol or cholecalciferol supplementation and the use of calcium-free phosphate binders is recommended. The new drug cinacalcet is highly promising; however, pediatric experience is still limited to observational data and the effect of cinacalcet on longitudinal growth and pubertal development is unknown. Randomized, controlled trials are underway, including studies of cinacalcet pharmacokinetics and pharmacodynamics in infants.

Rapid intraoperative parathyroid hormone assay in the surgical management of hyperparathyroidism

CONTEXT

Historically, successful surgical management of primary hyperparathyroidism has required bilateral exploration of the neck. By confirming complete removal of hypersecreting tissue, an intraoperative parathyroid hormone (IO-PTH) assay allows use of a more limited procedure.

OBJECTIVE

Our objective was to evaluate the utility of IO-PTH assay used in 32 parathyroid explorations versus conventional bilateral exploration used before the advent of IO-PTH assays.

METHODS

Minimally invasive parathyroidectomy (MIP) was used. Plasma samples were obtained at several intervals and were analyzed for IO-PTH by use of a rapid immunochemiluminescent assay (ICMA). Outcomes were assessed by univariate inferential testing, yielding one-tailed t-test results.

RESULTS

The study group had a mean plasma IO-PTH level decrease of 87% at ten minutes after excision. All 32 patients who underwent MIP using IO-PTH monitoring had successful surgery. At last postoperative follow-up examination, all 32 patients were normocalcemic. There were statistically significant decreases in duration of surgery, length of hospital stay, and surgery cost.

CONCLUSIONS

IO-PTH levels predicted the postoperative outcome for all patients studied, can provide valuable information to surgeons, and can decrease the duration of surgery and hospital stay.

Comparison of three different immunoassay methods for the evaluation of intact parathyroid hormone levels in hemodialysis patients

BACKGROUND

Intact parathyroid hormone (iPTH) assays react with the non-(1-84) molecular form of PTH. This form behaves as a carboxy-terminal fragment and accumulates during renal failure. We wanted to examine the variation of iPTH levels between the more commonly used different immunoassay methods in hemodialysis patients.

METHODS

Our study was designed to compare three commercial second-generation immunoassays based on electrochemiluminescent immunoassay (ECLIA), enzyme immunoassay (EIA) and immunoradiometric assay (IRMA) for intact PTH. The serum samples from 88 patients were collected and the iPTH concentrations measured.

RESULTS

The median iPTH (IRMA) concentration (99 pg/mL) was lower than both median iPTH (ECLIA) concentration (290.5 pg/ml; p < 0.001) and iPTH (EIA) concentration (369 pg/mL; p < 0.001). The Bland-Altman graphs, which are plots of the percentage differences between the two methods against their mean, suggested that the IRMA methods are not in agreement with the other methods.

CONCLUSION

It would be useful to reduce the variability among the methods with the use of a more standardized calibrator and of the same specific antibodies that only recognize the active PTH molecule.

The calcemic response to continuous parathyroid hormone (PTH)(1-34) infusion in end-stage kidney disease varies according to bone turnover: a potential role for PTH(7-84)

CONTEXT

Factors contributing to PTH resistance in dialysis patients remain elusive.

OBJECTIVES

The study assessed the skeletal and biochemical response to 46 h of PTH(1-34) infusion in dialysis patients.

DESIGN

The study was a prospective, controlled assessment of response to PTH(1-34).

SETTING

The study was performed at the University of California, Los Angeles, General Clinical Research Center.

PARTICIPANTS

Nineteen dialysis patients and 17 healthy volunteers were studied.

INTERVENTION

PTH(1-34) was infused at a rate of 8 pmol/kg x h for 46 h. Bone biopsy was performed in all dialysis patients.

MAIN OUTCOME MEASURES

Serum calcium, phosphorus, 1,25-dihydroxyvitamin D, PTH (four separate assays), and FGF-23 were determined at baseline and h 7, 23, 35, and 46 of the infusion.

RESULTS

Serum calcium levels rose in healthy volunteers (9.2 +/- 0.1 to 11.9 +/- 0.3 mg/dl; P < 0.01) and in dialysis patients with adynamic/normal bone turnover (9.0 +/- 0.3 to 10.7 +/- 0.7 mg/dl; P < 0.05) but did not change in dialysis patients with high bone turnover. Serum phosphorus levels declined in healthy volunteers (3.9 +/- 0.1 to 3.5 +/- 0.1 mg/dl; P < 0.05) but increased in all dialysis patients (6.7 +/- 0.4 to 8.0 +/- 0.3 mg/dl; P < 0.05). Full-length PTH(1-84) declined in all subjects; however, PTH(7-84) fragments declined only in healthy subjects and in dialysis patients with normal/adynamic bone but remained unchanged in dialysis patients with high bone turnover.

CONCLUSIONS

The skeleton of dialysis patients with high bone turnover is resistant to the calcemic actions of PTH. PTH(7-84) may contribute to this phenomenon.

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.

The opposing actions of the two parathyroid hormones, 1-84 PTH and 7-84 PTH: improvement in renal bone and calcium metabolism management

Bone biopsy, while invasive, is the gold standard for assessing bone status. According to published bone biopsy studies, half of the end-stage renal disease patients have adynamic bone disease. Compared to high-bone-turnover disease, adynamic bone disease has the higher mortality and is associated with arterial calcification. The treatment for high-bone-turnover disease is divergent from the treatment for adynamic bone disease. The parathyroid hormone (PTH) assay has been relied on as the routine, noninvasive diagnostic method to assess bone status. According to bone biopsy studies, the intact PTH assay has been demonstrated as ineffective at differentiating adynamic bone disease from normal and high-bone-turnover disease. For example, bone biopsy studies found the normal range for iPTH to be 451 to 1339 pg/mL and the range for adynamic bone disease to be 400 to 919 pg/mL. Intact PTH measures the sum of the two PTH hormones 1-84 PTH and 7-84 PTH. Specific 1-84 PTH assays neglect the role of the 7-84 PTH hormone, which is to lower bone turnover. According to independent bone biopsy studies, the 1-84 PTH/7-84 PTH ratio is 94% accurate in identifying adynamic bone disease and 94% accurate in assessing bone-turnover status.

Selected reaction monitoring-mass spectrometric immunoassay responsive to parathyroid hormone and related variants

BACKGROUND

Parathyroid hormone (PTH) assays able to distinguish between full-length PTH (PTH1-84) and N-terminally truncated PTH (PTH7-84) are of increasing significance in the accurate diagnosis of endocrine and osteological diseases. We describe the discovery of new N-terminal and C-terminal PTH variants and the development of selected reaction monitoring (SRM)-based immunoassays specifically designed for the detection of full-length PTH [amino acid (aa)1-84] and 2 N-terminal variants, aa7-84 and aa34-84.

METHODS

Preparation of mass spectrometric immunoassay pipettor tips and MALDI-TOF mass spectrometric analysis were carried out as previously described. We used novel software to develop SRM assays on a triple-quadrupole mass spectrometer. Heavy isotope-labeled versions of target peptides were used as internal standards.

RESULTS

Top-down analysis of samples from healthy individuals and renal failure patients revealed numerous PTH variants, including previously unidentified aa28-84, aa48-84, aa34-77, aa37-77, and aa38-77. Quantitative SRM assays were developed for PTH1-84, PTH7-84, and variant aa34-84. Peptides exhibited linear responses (R(2) = 0.90-0.99) relative to recombinant human PTH concentration limits of detection for intact PTH of 8 ng/L and limits of quantification of 16-31 ng/L depending on the peptide. Standard error of analysis for all triplicate measurements was 3%-12% for all peptides, with <5% chromatographic drift between replicates. The CVs of integrated areas under the curve for 54 separate measurements of heavy peptides were 5%-9%.

CONCLUSIONS

Mass spectrometric immunoassays identified new clinical variants of PTH and provided a quantitative assay for these and previously identified forms of PTH.

[Biology of primary hyperparathyroidism: selective venous sampling]

The diagnosis of primary hyperparathyroidism (PHP) is chemical: high level of Parathormone (PTH) in conjunction with hypercalcaemia. In borderline cases with sub-normal plasma PTH and calcium, an oral calcium load test could allow a differential diagnosis from other causes of high PTH. Imaging is required only for PHP. Selective venous sampling can help in localizing a parathyroid adenoma in difficult cases by PTH cartography in the following situations: imaging in favour of an ectopic mediastinal gland or a deep cervical adenoma, persistent or recurrent PHP after first failed surgery with negative neck exploration or unsatisfactory in case of another hypersecreting gland, PHP well diagnosed with indeterminate imaging, symptomatic PHP with normal PTH and negative imaging. Venous blood sampling performed in a vascular radiological department with a quick PTH assay can reveal an area of maximum secretion potentially linked to a nodule localized by previous ultrasound coupled to scintigraphy, giving thus a "biological imaging" study. The association of imaging and biology is an efficient procedure enabling localization of an area of abnormal PTH secretion and characterization of the level of PTH secretion. The area with the highest gradient of PTH concentration can help to protocol CT and MR examination.

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.

Predictors of intra-operative parathyroid hormone decline in subjects operated for primary hyperparathyroidism by minimally invasive parathyroidectomy

BACKGROUND

The predictors of intra-operative PTH (IOPTH) decline during minimally invasive parathyroidectomy (MIP) for primary hyperparathyroidism have been but poorly studied.

MATERIALS AND METHODS

This retrospective study included 108 patients who underwent MIP for a single adenoma. Serum calcium and phosphorus were measured before surgery and 1 day post-operatively. IOPTH was measured before (intra-operative preincision or PTHt0) and 10 min after removal of the adenoma (PTHt10). The Modification of Diet in Renal Disease (MDRD) equation was used to estimate the glomerular filtration rate. The weight of the adenoma was assessed in all the subjects.

RESULTS

The sex ratio female/male was 5.37 with a mean age of 57.3 yr. The mean pre- and postoperative values were for calcium 2.80 and 2.19 mmol/l, respectively (p<0.0001) and for phosphorus 0.90 and 1.16 mmol/l, respectively (p<0.0001). The PTH dropped from a mean value of 184.8 to 50.8 pg/ml 10 min after adenoma resection with a mean drop of 69.7%. Thirteen patients (12%) did not achieve a PTH fall of more than 50%. In a bivariate analysis, age, an MDRD<60 ml/min and weight of adenoma were inversely associated with IOPTH fall (p=0.009, p=0.004, and p<0.001, respectively) while gender, body mass index, hypertension, diabetes, pre-operative phosphorus and calcium had no significant effects. In the multivariate analysis, age, weight of adenoma, and MDRD were still independent negative predictors of the IOPTH fall (p=0.01, p=0.018, and p<0.001, respectively).

CONCLUSION

Our results suggest that during MIP the presence of a parathyroid adenoma with a high weight, in an elderly subject or in a subject with altered renal function, will result in a lesser degree of IOPTH fall.