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. 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. Lay SummaryAccurate diagnosis and treatment of chronic kidney disease–related mineral and bone disorders (CKD-MBDs) hinge on measuring parathyroid hormone (PTH). Unfortunately, current PTH tests often yield inconsistent results, complicating clinical practice. To address this, standardizing PTH measurement methods is essential. Achieving this standardization requires advanced mass spectrometry techniques and understanding whether certain substances in the blood of chronic kidney disease and hemodialyzed patients affect PTH measurements. Recent advances in mass spectrometry revealed that a potentially problematic PTH fragment (7–84) was absent in patients' blood, alleviating concerns. In addition, oxidized PTH was not detected and circulating fragments did not interfere PTH assays. As a result, we explored recalibrating 5 different PTH kits on to a precise liquid chromatography tandem mass spectrometry reference method. The outcomes were promising, aligning PTH results across various immunoassays with the reference method. This calibration process promises more reliable and consistent PTH measurements, ultimately enhancing patient care by reducing result variability. Accurate diagnosis and treatment of chronic kidney disease–related mineral and bone disorders (CKD-MBDs) hinge on measuring parathyroid hormone (PTH). Unfortunately, current PTH tests often yield inconsistent results, complicating clinical practice. To address this, standardizing PTH measurement methods is essential. Achieving this standardization requires advanced mass spectrometry techniques and understanding whether certain substances in the blood of chronic kidney disease and hemodialyzed patients affect PTH measurements. Recent advances in mass spectrometry revealed that a potentially problematic PTH fragment (7–84) was absent in patients' blood, alleviating concerns. In addition, oxidized PTH was not detected and circulating fragments did not interfere PTH assays. As a result, we explored recalibrating 5 different PTH kits on to a precise liquid chromatography tandem mass spectrometry reference method. The outcomes were promising, aligning PTH results across various immunoassays with the reference method. This calibration process promises more reliable and consistent PTH measurements, ultimately enhancing patient care by reducing result variability. Beyond its paramount importance in diagnosing and managing various endocrine conditions such as primary and secondary hyperparathyroidism, hypoparathyroidism, and pseudohypoparathyroidism, the measurement of parathyroid hormone (PTH) is routinely conducted in patients with chronic kidney disease (CKD). The Kidney Disease: Improving Global Outcomes 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease–Mineral and Bone Disorder recommends monitoring PTH levels in CKD patients beginning in CKD G3a and suggests maintaining PTH levels of hemodialyzed (HD) patients at ∼2 to 9 times the upper limit of the normal (ULN) values of the assay used.1Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Update Work GroupKDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD).Kidney Int Suppl (2011). 2017; 7: 1-59Abstract Full Text Full Text PDF PubMed Scopus (1166) Google Scholar Yet, a recommendation that treatments should be based on multiples of a ULN value is quite unique and is a circumvolution because of the lack of standardization of PTH assays.2Cavalier E. Vasikaran S. Bhattoa H.P. et al.The path to the standardization of PTH: is this a realistic possibility? A position paper of the IFCC C-BM.Clin Chim Acta. 2021; 515: 44-51Crossref PubMed Scopus (12) Google Scholar Such a lack of standardization is unfortunately a real source of confusion for clinicians in their daily practice, potentially leading to significant medical errors.3Sturgeon C.M. Sprague S. Almond A. et al.Perspective and priorities for improvement of parathyroid hormone (PTH) measurement—a view from the IFCC Working Group for PTH.Clin Chim Acta. 2017; 467: 42-47Crossref PubMed Scopus (46) Google Scholar,4Souberbielle J.-C. Boutten A. Carlier M.-C. et al.Inter-method variability in PTH measurement: implication for the care of CKD patients.Kidney Int. 2006; 70: 345-350Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar Several reasons contributed so far to the current lack of standardization in PTH assays. The first one is the presence of circulating PTH fragments alongside the bioactive 1 to 84 PTH peptide.5Gardella T.J. Axelrod D. Rubin D. et al.Mutational analysis of the receptor-activating region of human parathyroid hormone.J Biol Chem. 1991; 266: 13141-13146Abstract Full Text PDF PubMed Google Scholar These fragments, which are N-terminal or C-terminal truncated forms of PTH, circulate in the blood owing to liver metabolism of the active peptide or direct secretion by the parathyroid glands.6Segre G.V. Perkins A.S. Witters L. Potts J.T. Metabolism of parathyroid hormone by isolated rat Kupffer cells and hepatocytes.J Clin Invest. 1981; 67: 449-457Crossref PubMed Scopus (75) Google Scholar, 7Hanley D.A. Ayer L.M. Calcium-dependent release of carboxyl-terminal fragments of parathyroid hormone by hyperplastic human parathyroid tissue in vitro.J Clin Endocrinol Metab. 1986; 63: 1075-1079Crossref PubMed Scopus (55) Google Scholar, 8Mayer G.P. Keaton J.A. Hurst J.G. Habener J.F. Effects of plasma calcium concentration on the relative proportion of hormone and carboxyl fragments in parathyroid venous blood.Endocrinology. 1979; 104: 1778-1784Crossref PubMed Scopus (177) Google Scholar These fragments, which are eliminated by the kidney, have a longer half-life than 1 to 84 PTH itself,9Yamashita H. Gao P. Cantor T. et al.Large carboxy-terminal parathyroid hormone (PTH) fragment with a relatively longer half-life than 1-84 PTH is secreted directly from the parathyroid gland in humans.Eur J Endocrinol. 2003; 149: 301-306Crossref PubMed Scopus (31) Google Scholar,10Yamashita H. Cantor T. Uchino S. et al.Sequential changes in plasma intact and whole parathyroid hormone levels during parathyroidectomy for secondary hyperparathyroidism.World J Surg. 2005; 29: 169-173Crossref PubMed Scopus (25) Google Scholar accumulate in the blood of CKD patients,11D'amour P. Lazure C. Labelle F. Metabolism of radioiodinated carboxy-terminal fragments of bovine parathyroid hormone in normal and anephric rats.Endocrinology. 1985; 117: 127-134Crossref PubMed Scopus (30) Google Scholar, 12D'Amour P. Circulating PTH molecular forms: what we know and what we don't.Kidney Int. 2006; 70: S29-S33Abstract Full Text Full Text PDF Scopus (46) Google Scholar, 13Kritmetapak K. Losbanos L.A. Hines J.M. et al.Chemical characterization and quantification of circulating intact PTH and PTH fragments by high-resolution mass spectrometry in chronic renal failure.Clin Chem. 2021; 67: 843-853Crossref PubMed Scopus (16) Google Scholar and potentially interfere with second generation PTH assays (referred as "intact" PTH assay). Indeed, such assays are supposed to recognize, with various cross-reactivities, a family of large C-terminal fragments referred to as "non-(1 to 84)" PTH.14Lepage R. Roy L. Brossard J.H. et al.A non-(1-84) circulating parathyroid hormone (PTH) fragment interferes significantly with intact PTH commercial assay measurements in uremic samples.Clin Chem. 1998; 44: 805-809Crossref PubMed Scopus (335) Google Scholar This is not the case of third generation immunoassays (also referred as "whole" or "bioactive" PTH assays) because such assays incorporate an anti–N-terminal antibody directed toward the first 4 amino acids of the peptide, eliminating the issue of cross-reactivity with PTH fragments.15John M.R. Goodman W.G. Ping G. et al.A novel immunoradiometric assay detects full-length human PTH but not amino-terminally truncated fragments: implications for PTH measurements in renal failure.J Clin Endocrinol Metab. 1999; 84: 4287-4290Crossref PubMed Google Scholar,16Gao P. Scheibel S. D'Amour P. et al.Development of a novel immunoradiometric assay exclusively for biologically active whole parathyroid hormone 1-84: implications for improvement of accurate assessment of parathyroid function.J Bone Miner Res. 2001; 16: 605-614Crossref PubMed Scopus (344) Google Scholar The second reason for the lack of standardization in PTH assays is calibration. Indeed, despite the availability of the World Health Organization International Standard PTH 1-84, human, recombinant (NIBSC [National Institute for Biological Standards and Control] code: 95/646) (WHO 95/646 PTH IS), differences in calibration remain, which can be due to noncommutability of the WHO material and/or its incorrect use by assay manufacturers. Consequently, the recovery of the same amount of the WHO 95/646 PTH IS can range from 100% to >250% depending on the assay used.3Sturgeon C.M. Sprague S. Almond A. et al.Perspective and priorities for improvement of parathyroid hormone (PTH) measurement—a view from the IFCC Working Group for PTH.Clin Chim Acta. 2017; 467: 42-47Crossref PubMed Scopus (46) Google Scholar Finally, the lack of a formally recognized reference measurement procedure, a gold standard method providing true values against which any commercial assay could be calibrated, further contributes to the lack of standardization in PTH assays. In this study, we aimed at evaluating the impact of a recalibration of 5 PTH immunoassays, representing both second and third PTH generations assays, on the liquid chromatography tandem mass spectrometry (LC-MS/MS) candidate reference method we recently developed. This method is indeed calibrated against the WHO 95/646 PTH IS and has the potential to become a reference measurement procedure.17Farré-Segura J. Le Goff C. Lukas P. et al.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.Clin Chem. 2022; 3: 1399-1409Crossref Scopus (9) Google Scholar We used the second and third generation PTH assays from Roche on the cobas, the second and third generation assays from DiaSorin on the LIAISON analyzer, and the third generation PTH assay from Fujirebio on the LUMIPULSE instrument. The characteristics of these methods are detailed in Table 1 for reference. As our gold standard, we used the LC-MS/MS method that we recently developed.17Farré-Segura J. Le Goff C. Lukas P. et al.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.Clin Chem. 2022; 3: 1399-1409Crossref Scopus (9) Google Scholar Notably, this method distinguishes itself by eliminating the use of antibodies during the sample preparation, ensuring complete independence from any cross-reactivity concerns. It is calibrated against the WHO 95/646 PTH IS, providing a robust reference framework. Finally, our LC-MS/MS method has undergone extensive validation for 1 to 84 PTH from 5.7 to 873 ng/l and exhibits a measurement uncertainty of <5.6%.Table 1Characteristics of second and third generation PTH immunoassays as disclosed by the manufacturersMethodIntra-assay CV (%)Interassay CV (%)Expected values (ng/l)GenerationN-terminal antibody and targetC-terminal antibody and targetTraceabilityMeasuring range (ng/l)Roche Cobas Elecsys PTH<1.3<215–65SecondMouse monoclonal26–32Mouse monoclonal37–42Commercially available RIA.Claims a mean recovery of 100% ± 4% of the WHO 95/646 PTH IS1.2–5000Roche Cobas Elecsys PTH (1–84)0.9–7.61.6–11.415–57ThirdMouse monoclonalN-terminal partMouse monoclonalC-terminal partWHO 95/646 PTH IS5.5–2300DiaSorin LIAISONN-TACT PTH Gen II1.3–5.72.8–4.215–87SecondGoat polyclonal1–34Goat polyclonal39–84Internal preparation of human recombinant 1–84 PTH3–1900DiaSorin LIAISON1–84 PTH3.0–5.95.4–9.07–39ThirdPolyclonalN-terminalPolyclonalC-terminalInternal preparation of synthetic human 1–84 PTH4–1800Fujirebio LUMIPULSE G whole PTH1.1–4.11.4–4.15–36ThirdGoat polyclonalGoat polyclonalWHO 95/646 PTH IS4–5000CV, coefficient of variation; IS, International Standard; PTH, parathyroid hormone; RIA, radioimmunoassay; WHO, World Health Organization. Open table in a new tab CV, coefficient of variation; IS, International Standard; PTH, parathyroid hormone; RIA, radioimmunoassay; WHO, World Health Organization. We prepared a calibration panel constituted of 40 pools of leftover ethylenediamine tetraacetic acid (EDTA) plasma samples by carefully mixing at least 10 different leftover samples indiscriminately originating from our daily routine together to constitute a single pool. The samples were selected on the basis of their assigned nominal value determined through our routine method (DiaSorin LIAISON third generation PTH), and pools were constituted to span the measuring range. We then prepared 2 validation panels. For validation panel 1, we selected 138 leftover EDTA plasma samples, 58 from CKD patients (19 from G4 [estimated glomerular filtration rate {eGFRcr} between 15 and 29 ml/min per 1.73 m2], 19 from G3b [eGFRcr between 30 and 44 ml/min per 1.73 m2], and 20 from G3a [eGFRcr between 45 and 59 ml/min per 1.73 m2] categories), 37 from HD patients, and 43 from non-CKD persons (eGFRcr >60 ml/min per 1.73 m2). For validation panel 2, we selected 109 other leftover EDTA plasma samples: 52 from CKD patients (16 from G4, 17 from G3b, and 19 from G3a categories), 37 from HD patients, and 20 from non-CKD patients. The EDTA plasma samples used for panel preparation were stored at −20 °C for less than a month and had not been previously frozen. After preparation, panels were stored at −80 °C until measurement, which occurred within the same month. For each panel, a fresh WHO International Standard Parathyroid Hormone 1-84, human, recombinant (NIBSC code: 95/646) ampoule was used to establish the calibration curve for the LC-MS/MS method. The measurements of the calibration, validation 1, and validation 2 panels were conducted at intervals of a minimum of 3 months, ensuring the use of different lots for immunoassays. All samples were measured in singlicates using both the 5 immunoassays and the LC-MS/MS method. On the basis of the results of the calibration panel, we established regression equations for each of the immunoassays versus LC/MS-MS. We then used these equations to "recalibrate" the immunoassays on the LC-MS/MS method, and we verified the robustness of this calibration on the 2 validation panels. To evaluate the clinical impact of recalibration, we investigated the classification of non-CKD persons and CKD patients using both the LC-MS/MS method and the different immunoassays on the basis of a standardized ULN value. Furthermore, we examined the classification of HD patients before and after recalibration by following the Kidney Disease: Improving Global Outcomes guidelines18Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Work GroupKDIGO clinical practice guideline for the diagnosis, evaluation, prevention, and treatment of chronic kidney disease-mineral bone disorder (CKD-MBD).Kidney Int. 2009; 76: S1-S130Google Scholar and considering 2 to 9 times multiples of the standardized ULN value. Of the 40 constituted pools of the calibration panel, 1 was above the measuring range for the DiaSorin second generation assay (>1900 ng/l) and was thus discarded. The spanning range of the 39 remaining pools was 14 to 533, 16 to 1200, 16 to 436, 8 to 355, 6 to 315, and 5 to 251 ng/l for Roche second generation, DiaSorin second generation, Roche third generation, DiaSorin third generation, Fujirebio third generation, and LC-MS/MS method, respectively. The regression equations of these 39 pools measured with each immunoassay versus LC-MS/MS and the correlations coefficients are presented in Table 2 and Figure 1.Table 2Regression equations and correlation coefficients of the calibration panel's 39 samples for the LC-MS/MS reference methodYXRoche second generationDiaSorin second generationRoche third generationDiaSorin third generationFujirebio third generationLC/MS-MS =0.45X + 0.580.21X + 7.210.60X − 7.10.73X − 1.80.83X + 2.6r20.9730.9880.9910.9890.983LC-MS/MS, liquid chromatography tandem mass spectrometry. Open table in a new tab LC-MS/MS, liquid chromatography tandem mass spectrometry. These equations were then applied to the PTH results of the samples of the 2 validation panels. The results obtained from the 2 sets of samples before and after recalibration are presented in Table 3. Before recalibration, the mean PTH concentrations were 138, 258, 104, 89, 77, and 66 ng/l for Roche second generation, Roche third generation, DiaSorin second generation, DiaSorin third generation, Fujirebio third generation, and LC-MS/MS method, respectively, in validation panel 1. After recalibration, the corresponding values were 63, 62, 57, 67, and 66 ng/l for the 5 immunoassays, respectively. Concerning the samples of validation panel 2, the corresponding mean concentrations before calibration were 82, 157, 288, 111, 95, and 86 ng/l for the same methods. After recalibration, the mean values for immunoassays were 71, 68, 60, 71, and 74 ng/l, respectively.Table 3Characteristics of the 2 validation panels before and after recalibration of the samples on the reference LC-MS/MS methodResultsValidation panel 1Validation panel 2Before recalibrationLC-MS/MSRoche 2nd GenDiaSorin 2nd GenRoche 3rd GenDiaSorin 3rd GenFujirebio3rd GenLC-MS/MSRoche 2nd GenDiaSorin 2nd GenRoche 3rd GenDiaSorin 3rd GenFujirebio 3rd GenMean (ng/l)65.7138257.5104.289.377.081.5156.8287.5111.195.285.8Mean bias (%)104224712791022235717.99.2Overall bias (%)86.981.7Overall CV (%)46.159.0ResultsAfter recalibrationLC-MS/MSRoche 2nd GenDiaSorin 2nd GenRoche 3rd GenDiaSorin 3rd GenFujirebio3rd GenLC-MS/MSRoche 2nd GenDiaSorin 2nd GenRoche 3rd GenDiaSorin 3rd GenFujirebio 3rd GenMean (ng/l)65.762.961.555.667.166.481.571.467.859.871.473.7Mean bias (%)−5.5−6.8−21.5−1.2−0.8−7.2−12.3−24.9−8.9−2.5Overall bias (%)−7.2−11.1Overall CV (%)12.020.52nd Gen, second generation; 3rd Gen, third generation; CV, coefficient of variation; LC-MS/MS, liquid chromatography tandem mass spectrometry. Open table in a new tab 2nd Gen, second generation; 3rd Gen, third generation; CV, coefficient of variation; LC-MS/MS, liquid chromatography tandem mass spectrometry. Before recalibration, the average bias between the immunoassays presenting the highest and lowest biases (i.e., Fujirebio third generation and DiaSorin second generation) compared to LC-MS/MS ranged from +9% to +224% in validation panel 1 and from +9% to +223% in validation panel 2. After recalibration, the average bias decreased to −0.8% and −6.8% in validation panel 1 and from −2.5% to −12.3% in validation panel 2 for these 2 immunoassays. The Roche third generation assay's bias against LC-MS/MS also decreased after recalibration (from +71% to −21.5% in validation panel 1 and from +57% to −24.9% in validation panel 2) but to a lesser extent compared to the other assays. The overall mean bias decreased from 86.9% to −7.2% in validation panel 1 and from 81.7% to −11.1% in validation panel 2 after recalibration. To perform the clinical validation, it was necessary to establish a standardized ULN value. Because a ULN for the LC-MS/MS method had not been established yet, we decided to use the average of the 5 recalibrated ULN values provided for each kit, obtained through the regression equations. The manufacturers' original ULN values were 65, 87, 57, 39, and 36 ng/l for Roche second generation, DiaSorin second generation, Roche third generation, DiaSorin third generation, and Fujirebio third generation, respectively. After recalibration, these values were adjusted to 30, 26, 27, 31, and 31 ng/l, respectively. The average of these recalibrated ULN values was calculated to be 30 ng/l, which was consequently considered as the standardized ULN value to be applied as the reference value for both the LC-MS/MS method and the 5 recalibrated immunoassays. Accordingly, Figures 2 and 3 display the distribution of the results obtained in non-CKD subjects and CKD patients, respectively, before and after recalibration according to the ULN value and Figure 4 displays the results of HD patients according to 2 to 9 times the ULN value before and after recalibration. Regarding these 73 HD patients, using raw values of 60 and 270 ng/l (i.e., 2 × 30 and 9 × 30 ng/l) as targets, 1 patient was misclassified by the Roche second generation assay in the low range (value considered as >60 ng/l whereas it was lower with the LC-MS/MS method) and 5 were misclassified by the Roche third generation assay (values considered as <270 ng/l whereas they were higher). Figure 5 illustrates the representative outcome for a non-CKD subject, a CKD patient, and a HD patient before and after recalibration of the immunoassays using the LC-MS/MS method. Before recalibration, the non-CKD subject exhibited an LC-MS/MS concentration of 27 ng/l. This measurement initially translated to PTH concentrations spanning from 33 ng/l (using the Fujirebio third generation assay) to 77 ng/l (as determined by the DiaSorin second generation assay). However, after recalibration, the same patient's values were refined to a range falling between 22 ng/l (Roche third generation assay) and 30 ng/l (Fujirebio third generation assay). Likewise, in the case of a CKD patient with initial LC-MS/MS PTH concentrations of 81 ng/l and immunoassays PTH concentrations ranging from 99 ng/l (measured using the Fujirebio third generation assay) to 289 ng/l (measured using the DiaSorin second generation assay), the recalibration led to a refined range spanning from 67 ng/l (Roche third generation assay) to 85 ng/l (Fujirebio third generation assay) after recalibration. Finally, an HD patient presenting an LC-MS/MS PTH value of 142 ng/ml initially displayed a PTH concentration range of 181 to 609 ng/mL before recalibration. After recalibration, the PTH concentrations ranged from 117 to 153 ng/l.Figure 3Comparison of the results obtained in chronic kidney disease patients before (left) and after (right) recalibration. The red solid bars correspond to 30 ng/l, which is the average of the 5 recalibrated upper limit of normal values provided for each kit, obtained through the regression equations (measurement units are nanograms per liter). 2nd Gen, second generation; 3rd Gen, third generation; LC-MS/MS, liquid chromatography tandem mass spectrometry.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 4Comparison of the results obtained in hemodialyzed patients before (left) and after (right) recalibration. The red solid bars represent 2 to 9 times the upper limit of normal (ULN) values on the liquid chromatography tandem mass spectrometry (LC-MS/MS) method, determined by the mean of the manufacturers' ULN values obtained after mathematical recalibration (measurement units are nanograms per liter). 2nd Gen, second generation; 3rd Gen, third generation.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 5Radar charts illustrating the variations in parathyroid hormone (PTH) concentrations (measured in nanograms per liter) in 3 different subjects: a non–chronic kidney disease (CKD) subject (left), a CKD patient (middle), and a hemodialyzed patient (right). The data presented include measurements both before (indicated by blue dashed lines) and after (indicated by orange dotted lines) the recalibration of 6 immunoassays using the reference liquid chromatography tandem mass spectrometry (LC-MS/MS) method. The bold and red numbers along the vertical axis represent PTH concentrations (measured in nanograms per liter). 2nd Gen, second generation; 3rd Gen, third generation.View Large Image Figure ViewerDownload Hi-res image Download (PPT) The key finding of this study suggests that it is now feasible to standardize all PTH assays, regardless of the assay methodology (second or third generation immunoassay). This breakthrough has the potential to bring significant improvements in the management of chronic kidney disease–mineral and bone disorder. In 2006, Souberbielle et al. highlighted the misleading nature of the 150 to 300 ng/l National Kidney Foundation Kidney Disease Outcomes Quality Initiative raw values used as targets for PTH in HD patients.4Souberbielle J.-C. Boutten A. Carlier M.-C. et al.Inter-method variability in PTH measurement: implication for the care of CKD patients.Kidney Int. 2006; 70: 345-350Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar,19National Kidney FoundationK/DOQI clinical practice guidelines for bone metabolism and disease in chronic kidney disease.Am J Kidney Dis. 2003; 42: S1-S201PubMed Google Scholar The authors highlighted that despite high correlation among the assays (r values ranging from 0.975 to 0.994), the 15 assays tested (13 second generation and 2 third generation assays) could produce significantly divergent results. This discrepancy had the potential to lead to significant clinical errors, as individual patients might be classified as within, below, or above the Kidney Disease Outcomes Quality Initiative target range depending on the specific assay used. The alarming implication was that contradictory treatment approaches could have been recommended for a single HD patient on the basis of the choice of PTH assay. To address this issue, the Kidney Disease: Improving Global Outcomes proposed using multiples of the ULN value established by assay manufacturers as PTH targets for HD patients. Undoubtedly, this approach significantly reduced the disparities in classifying HD patients based on PTH target ranges.20Cavalier E. Delanaye P. Vranken L. et al.Interpretation of serum PTH concentrations with different kits in dialysis patients according to the KDIGO guidelines: importance of the reference (normal) values.Nephrol Dial Transplant. 2011; 27: 1950-1956Crossref PubMed Scopus (62) Google Scho