Harmonization of commercial assays for PINP; the way forward

A Systematic Review of the Circadian Rhythm of Bone Markers in Blood

There exists a marked circadian variation for several bone markers (BM), which is influenced by endogenous as well as exogenous factors including hormones, physical activity, and fasting. Consequently, was the aim of this review to provide an overview of the knowledge of the circadian variation of BM and which factors influence this rhythmicity. A systematic search of PubMed was performed for studies evaluating the circadian variation of BM and which factors influence this rhythmicity. The studies were screened for eligibility by a set of predetermined criteria including a list of relevant BM and a minimum study duration of 24 h with at least 3 blood samples of which two should be at least 6 h apart. In total were 29 papers included. There exists a marked circadian variation for most BM including Carboxy-terminal Cross-Linked Telopeptide of Type I Collagen (CTX) and osteocalcin (OC) with nighttime or early morning peak. Pro-collagen Type I N-terminal Propeptide (PINP) and PTH also showed circadian rhythm but with less amplitude. The inter-osteoblast-osteoclast regulatory markers such as OPG, RANKL, FGF23, and sclerostin showed no circadian rhythm. The markers were differently affected by exogenous factors like fasting, which greatly reduced the circadian variation of CTX but did not affect PINP or OC. The marked circadian variation and the factors which influence the rhythmicity, e.g., fasting are of great consequence when measuring BM. To reduce variation and heighten validity should circadian variation and fasting be kept in mind when measuring BM.

Bone Turnover Markers in Children: From Laboratory Challenges to Clinical Interpretation

Bone turnover markers (BTMs) have been developed many years ago to study, in combination with imaging techniques, bone remodeling in adults. In children and adolescents, bone metabolism differs from adults since it implies both growth and bone remodeling, suggesting an age- and gender-dependent BTM concentration. Therefore, specific studies have evaluated BTMs in not only physiological but also pathological conditions. However, in pediatrics, the use of BTMs in clinical practice is still limited due to these many children-related specificities. This review will discuss about physiological levels of BTMs as well as their modifications under pathological conditions in children and adolescents. A focus is also given on analytical and clinical challenges that restrain BTM usefulness in pediatrics.

The harmonization issue in laboratory medicine: the commitment of CCLM

The analytical quality of the clinical laboratory results has shown a significant improvement over the past decades, thanks to the joint efforts of different stakeholders, while the comparability among the results produced by different laboratories and methods still presents some critical issues. During these years, Clinical Chemistry and Laboratory Medicine (CCLM) published several papers on the harmonization issue over all steps in the Total Testing Process, training an important number of laboratory professionals in evaluating and monitoring all the criticisms inherent to the pre-analytical, as well as analytical and post analytical phases: from the consensus statement on the most informative testing in emergency setting, to the prevention and detection of hemolysis or to patients identification and tube labeling procedures, as far as to different approaches to harmonize hormones measurements or to describe new reference methods or to harmonize the laboratory report. During these years the commitment of the journal, devoted to the harmonization processes has allowed to improve the awareness on the topic and to provide specific instruments to monitor the rate of errors and to improve patients safety.

Intra-Individual Changes in Total Procollagen-Type 1 N-terminal Propeptide in a Korean Adult Population

We aimed to investigate intra-individual changes in total procollagen-type 1 N-terminal pro-peptide (P1NP), a biochemical marker of bone turnover, to understand patient populations and test utilization in a Korean adult population while considering different definitions of least significant changes by sex, age, and medical institution type. Overall, 31,501 P1NP tests were performed on 24,644 Korean adults (3389 men and 21,255 women) with a median age of 68.9 years (interquartile range, IQR, 61.2–77.2) for osteoporosis evaluation. Among these, 1331 (5.4%) patients (127 men and 1204 women) underwent ≥3 follow-up P1NP measurements. The median follow-up period was 12.5 months (IQR, 11.7–15.9). Among 1331 patients, 64.4% experienced a decrease in P1NP and 35.6% experienced an increase in P1NP during follow-up. Among these, the proportion of patients who experienced serum P1NP changes ≥14.4% from baseline was 92.3%, and the proportion of patients who achieved ≤40 ng/mL (a median level of premenopausal Korean women) during follow-up was 31.8%. The overall proportion of patients that experienced a serum P1NP change exceeding the least significant change during follow-up was not significantly different by the type of medical institution.

Patients With Cirrhosis Have Elevated Bone Turnover but Normal Hepatic Production of Osteoprotegerin

CONTEXT

Severe osteodystrophy is common in patients with liver dysfunction. Markers of bone metabolism may help in early diagnosis of osteodystrophy and in understanding underlying pathophysiological mechanisms.

OBJECTIVE

To elucidate changes in bone metabolism associated with cirrhosis and to determine the route of elimination for the markers.

METHODS

Case-control study at a public university hospital. Fifty-nine patients with cirrhosis (47 alcoholic and 12 nonalcoholic cirrhosis) and 20 controls were included. Participants underwent catheterization of the femoral artery, and the hepatic, renal, and femoral veins with collection of blood from all 4 sites. Regional arteriovenous differences in concentrations of bone metabolism markers were determined: procollagen of type I collagen propeptide (PINP), C-terminal cross-linking telopeptide of type I collagen (CTX), osteocalcin, tartrate-resistant acid phosphatase isoform 5b (TRAcP5b), osteoprotegerin (OPG), and sclerostin and correlated with degree of disease (Child-Pugh classification).

RESULTS

PINP concentration was higher (median: 87.9 µg/L) in patients with cirrhosis than in controls (52.6 µg/L) (P = .001), while hepatic extraction was lower (4.3% vs 14.5%) (P < .001). Both CTX and TRAcP5b were higher in patients with cirrhosis (340 ng/L and 3.20 U/L) than in controls (215 ng/L and 1.60 U/L) (P < .001 and P < .0001). Hepatic sclerostin extraction was lower in patients with cirrhosis (14.6%) than in controls (28.7%) (P < .0001). In both groups OPG showed a hepatic release rate (production) of 6%.

CONCLUSION

Patients with cirrhosis have increased bone resorption, but unaltered bone formation. Sclerostin is eliminated through the liver while OPG is produced in the liver. Bone markers may prove useful in evaluating bone turnover in patients with cirrhosis.

Assessing bone formation in patients with chronic kidney disease using procollagen type I N-terminal propeptide (PINP): The choice of assay makes a difference

BACKGROUND

Measurement of procollagen type I N-terminal propeptide (PINP) concentration in serum reflects the rate of type I collagen synthesis and can therefore be used as a bone formation marker. There are two methods of PINP quantification; the first measures the trimeric propeptide (intact PINP) and the second measures both the trimeric and monomeric propeptides (total PINP). Trimeric PINP is excreted via hepatic endothelial cells, whereas monomeric PINP is cleared renally. Therefore, in renal failure, the total assay has a positive bias with respect to the intact assay, due to monomeric PINP accumulation. The aim of this study was to compare the performance of both assays across all stages of chronic kidney disease.

METHODS

Serum was taken from male (n = 111) and female (n = 105) patients attending a metabolic bone clinic, and these were partitioned into stages of chronic kidney disease 1-5. Each serum sample was analysed using the Roche electrochemiluminescence immunoassay for total PINP and the Immunodiagnostic Systems chemiluminescence immunoassay for intact PINP.

RESULTS

Passing-Bablok regression analysis comparing both methods showed that with advancing chronic kidney disease there was a proportional positive bias affecting the total assay when compared with the intact assay. This proportional positive bias was statistically significant for chronic kidney disease stages 3b, 4 and 5.

CONCLUSIONS

Based on this method comparison study, usage of the total PINP assay should be avoided in chronic kidney disease stages 3b, 4 and 5 (eGFR ≤44 mL/min/1.73 m²) and instead an intact assay used as the total assay overestimates PINP concentrations due to monomeric PINP accumulation.

A Multicenter Study to Evaluate Harmonization of Assays for C-Terminal Telopeptides of Type I Collagen (ß-CTX): A Report from the IFCC-IOF Committee for Bone Metabolism (C-BM)

BACKGROUND

Biochemical bone turnover markers are useful tools to assess bone remodeling. C-terminal telopeptide of type I collagen (ß-CTX) has been recommended as a reference marker for bone resorption in research studies.

METHODS

We describe the results of a multicenter study for routine clinical laboratory assays for ß-CTX in serum and plasma. Four centers (Athens GR, Copenhagen DK, Liege BE and Sheffield UK) collected serum and plasma (EDTA) samples from 796 patients presenting to osteoporosis clinics. Specimens were analyzed in duplicate with each of the available routine clinical laboratory methods according to the manufacturers' instructions. Passing-Bablok regressions, Bland-Altman plots, V-shape evaluation method, and Concordance correlation coefficient for ß-CTX values between serum and plasma specimens and between methods were used to determine the agreement between results. A generalized linear model was employed to identify possible variables that affected the relationship between the methods. Two pools of serum were finally prepared and sent to the four centers to be measured in 5-plicates on 5 consecutive days with the different methods.

RESULTS

We identified significant variations between methods and between centers although comparison results were generally more consistent in plasma compared to serum. We developed univariate linear regression equations to predict Roche Elecsys®, IDS-iSYS, or IDS ELISA ß-CTX results from any other assay and a multivariable model including the site of analysis, the age, and weight of the patient. The coefficients of determination (R²) increased from approximately 0.80 in the univariate model to approximately 0.90 in the multivariable one, with the site of analysis being the major contributing factor. Results observed on the pools also suggest that long-term storage could explain the difference observed with the different methods on serum.

CONCLUSION

Our results show large within- and between-assay variation for ß-CTX measurement, particularly in serum. Stability of the analyte could be one of the explanations. More studies should be undertaken to overcome this problem. Until harmonization is achieved, we recommend measuring ß-CTX by the same assay on EDTA plasma, especially for research purposes in large pharmacological trials where samples can be stored for long periods before they are assayed.

Role of Bone Turnover Markers in Osteoporosis Therapy

Bone turnover markers fill a clinical need that improves comprehensive care of metabolic bone health and osteoporosis. Creating a standard process for drawing them that reduces modifiable variability improves their precision and clinical usefulness. Creating a standard process for interpreting them by applying statistical significance improves their clinical applicability. Understanding what causes them to increase and decrease can help elucidate secondary causes of osteoporosis. Monitoring them can assess patient adherence to therapy for a silent disease that will progressively become louder with an aging global population.

Analytical considerations and plans to standardize or harmonize assays for the reference bone turnover markers PINP and β-CTX in blood

Procollagen type I N-propeptide (PINP) and the C-terminal telopeptide of type I collagen (β-CTX) in blood have been designated as reference bone turnover markers in osteoporosis by the International Osteoporosis Foundation (IOF) and International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). The IFCC Committee on Bone Metabolism (C-BM) has examined current commercial assays and performed a multicentre study to examine the agreement between assays for PINP and β-CTX in serum and plasma. The results of these studies will inform our work towards the harmonization of PINP assays and the standardization of β-CTX assays in blood, with the development of common calibrators and reference measurement procedures in collaboration with the reagent manufacturing industry. Successful achievement of these goals will help develop universally acceptable practice guidelines for the management of osteoporosis with the inclusion of common reference intervals and treatment targets for PINP and β-CTX.

The Role of PINP in Diagnosis and Management of Metabolic Bone Disease

Serum procollagen type I N-propeptide (PINP) is designated the reference marker of bone formation in osteoporosis; the reference marker for resorption is C-terminal telopeptide of type I collagen (CTX). PINP has very low circadian and biological variation, is not affected by food intake, and is very stable in serum after venepuncture. The two automated commercial assays for PINP provide similar results in subjects with normal renal function, allowing reference intervals to be used interchangeably. Bone turnover markers (BTM) are currently not recommended for fracture risk assessment and therefore not included in fracture risk calculators. In the management of osteoporosis, the main utility of BTM including PINP is for monitoring therapy, both antiresorptive as well as anabolic agents; monitoring is thought to help improve adherence. PINP as well as CTX may also be used in assessing offset of drug action following a pause in bisphosphonate therapy, to help decide when to re-instate therapy, or following cessation of denosumab therapy to assess efficacy of follow-on bisphosphonate therapy. PINP may also be used in the diagnosis of Paget's disease of bone as well as in monitoring response to therapy and for recurrence. Although BTM other than bone alkaline phosphatase are currently not recommended for use in metabolic bone disease of chronic kidney disease, PINP measured by assays specific to the intact molecule has potential in this condition. Further studies are needed to examine this area, as well as in malignant bone disease.

European Biological Variation Study (EuBIVAS): within- and between-subject biological variation estimates of β-isomerized C-terminal telopeptide of type I collagen (β-CTX), N-terminal propeptide of type I collagen (PINP), osteocalcin, intact fibroblast growth factor 23 and uncarboxylated-unphosphorylated matrix-Gla protein-a cooperation between the EFLM Working Group on Biological Variation and the International Osteoporosis Foundation-International Federation of Clinical Chemistry Committee on Bone Metabolism

We have calculated the biological variation (BV) of different bone metabolism biomarkers on a large, well-described cohort of subjects. BV is important to calculate reference change value (or least significant change) which allows evaluating if the difference observed between two consecutive measurements in a patient is biologically significant or not.

INTRODUCTION

Within-subject (CV_I) and between-subject (CV_G) biological variation (BV) estimates are essential in determining both analytical performance specifications (APS) and reference change values (RCV). Previously published estimates of BV for bone metabolism biomarkers are generally not compliant with the most up-to-date quality criteria for BV studies. We calculated the BV and RCV for different bone metabolism markers, namely β-isomerized C-terminal telopeptide of type I collagen (β-CTX), N-terminal propeptide of type I collagen (PINP), osteocalcin (OC), intact fibroblast growth factor 23 (iFGF-23), and uncarboxylated-unphosphorylated Matrix-Gla Protein (uCuP-MGP) using samples from the European Biological Variation Study (EuBIVAS).

METHODS

In the EuBIVAS, 91 subjects were recruited from six European laboratories. Fasting blood samples were obtained weekly for ten consecutive weeks. The samples were run in duplicate on IDS iSYS or DiaSorin Liaison instruments. The results were subjected to outlier and variance homogeneity analysis before CV-ANOVA was used to obtain the BV estimates.

RESULTS

We found no effect of gender upon the CV_I estimates. The following CV_I estimates with 95% confidence intervals (95% CI) were obtained: β-CTX 15.1% (14.4-16.0%), PINP 8.8% (8.4-9.3%), OC 8.9% (8.5-9.4%), iFGF23 13.9% (13.2-14.7%), and uCuP-MGP 6.9% (6.1-7.3%).

CONCLUSIONS

The EuBIVAS has provided updated BV estimates for bone markers, including iFGF23, which have not been previously published, facilitating the improved follow-up of patients being treated for metabolic bone disease.