Alicia Algeciras-Schimnich, Ph.D.
Professor of Laboratory Medicine and Pathology
Division of Clinical Biochemistry
Mayo Clinic, Rochester, Minnesota
Hello. My name is Alicia Algeciras-Schimnich and I’m a professor of laboratory medicine and pathology in the Division of Clinical Biochemistry and Immunology in the Department of Laboratory Medicine and Pathology at Mayo Clinic in Rochester, Minnesota. Today I will be talking about the laboratory testing of bone turnover markers.
I have no disclosures relevant to this presentation.
The learning objectives for today include the discussion of the role of bone turnover markers in osteoporosis treatment, as well as to understand how to best interpret changes in bone turnover markers.
Bone turnover markers refer to a broader category of bone markers that includes bone formation markers, which are produced by osteoblasts, and bone resorption markers, which are produced by osteoclasts. The concentrations of these biomarkers in blood and urine reflect the bone remodeling process in the body and therefore makes them useful markers in the evaluation of various metabolic bone diseases. Today the most common markers for bone formation include P1NP, or procollagen type I N-propeptide; BAP, or bone-specific alkaline phosphatase; and osteocalcin. The most common bone resorption markers include the C-telopeptide cross-linked type I collagen (CTx), also known as Beta-CrossLaps, N-Telopeptide cross-linked type I collagen (NTx), which could be measurable in urine and serum.
While the use of BTM has been proposed in various clinical settings such as the ones listed on this table, most of these applications are not supported or endorsed by clinical guidelines due to the limited clinical evidence to justify their use.
The one application where there is sufficient evidence for the performance of BTM is for monitoring of response to therapy in postmenopausal osteoporosis. In this case, CTx and P1NP show the largest reduction in resorption and formation markers following antiresorptive therapy. Whereas in therapy with anabolic agents such as teriparatide, P1NP shows the most significant change after 3-6 months post- treatment.
There are various professional and clinical societies that have put forward recommendations for the use of BTM. For example, the International Osteoporosis Foundation (IOF), the European Calcified Tissue Society (ECTS), and the International Federation of Clinical Chemistry and Laboratory Medicine have endorsed the use of P1NP and CTx as the reference biomarkers in osteoporosis. These biomarkers have been selected based on the data available on their performance in clinical studies; their biological and analytical variability; current efforts for standardization of measuring methods; and the stability and specimen type (serum vs. urine), among others.
The Endocrine Society guidelines for the Pharmacological Management of Osteoporosis in Postmenopausal Women include the use of Serum CTx and P1NP as an alternative to bone mineral density (BMD) for identifying poor response or nonadherence to therapy.
One of the key studies evaluating the effectiveness of bone turnover markers to identify women who responded to bisphosphonate treatment for osteoporosis is referred as the TRIO study.
In this study, biochemical responses to three oral bisphosphonates were assessed in an open, controlled trial comprising 172 postmenopausal osteoporotic women. The women were randomized to one of these three therapies. The concentrations of bone formation and bone resorption markers were evaluated pre- and post-treatment. In this figure, the percent changes in serum CTx and NTx from baseline are plotted at various timepoints. In all treatments, a decline in CTx and NTx is observed within weeks of treatment, plateauing by 3-6 months. While both CTx and NTx show a decrease or reduction from the baseline, this change was most significant and pronounced for CTx.
For BFM, the study evaluated the concentration of bone alkaline phosphatase, osteocalcin, and P1NP. Similar to what was observed with the bone resorption markers, there was a decrease from baseline after three months, with P1NP showing the most pronounced change from baseline.
When performing longitudinal measurements of BTM, it is important to be aware of preanalytical factors that might influence the measured concentrations. For example, CTx is affected by diurnal variation and food intake. Therefore, sample collection is recommended to happen early in the morning after an overnight fast. And serial measurements should be collected at the same time of the day. One advantage of serum CTx is that it has a lower intra-individual variability compared to urine BTM. For P1NP, there is no effect of food intake and only minor diurnal variation inputs. Depending on whether the assay measures intact or total P1NP, there might be some effect from hepatic or renal function resulting in higher concentrations of P1NP. Mayo uses the intact P1NP assay, which is not expected to be affected by renal function.
Long turnover markers from baseline are interpreted based on the least significant change, also known as the critical difference or reference change value. This is the smallest difference between two measurements that is considered a true change. And it is the minimal change required following initiation of therapy to indicate a successful treatment effect. The least significant change takes into consideration that within-person biological variability as well as the laboratory analytical variability.
In terms of the within-person biological variability, you could see that both P1NP and serum CTx shows a lower within-person variability compared to the urine NTx concentrations, making both of these, P1NP and CTx measurements in serum, the preferred biomarkers monitoring response to therapy longitudinally.
An algorithm proposed by the International Osteoporosis Foundation (IOF) and the European Calcified Tissue Society when monitoring adherence to osteoporosis bisphosphonate treatment is the measurement of P1NP or CTx prior to initiation of treatment to obtain a baseline value. Then repeat testing at three months following treatment initiation and determine if the decrease from baseline is less or greater than the least significant change. If the decrease is greater than the least significant change, then the therapy should be continued, as the patient is responding well to the treatment. If the difference is less than the least significant change, then treatment should be reassessed and patient compliance with the treatment should be investigated.
Now remember that the least significant change will be influenced not only by the intra-individual variability, but it will be influenced by the assay variability, and the expected least significant change between assays will be laboratory-to-laboratory dependent. For the assays used here at Mayo Clinic Laboratories, the least significant change for serum CTx will be approximately 27%, whereas for P1NP in serum will be a decrease of approximately 20%.
So, to summarize, the measurement of BTMs offers a fast and minimally invasive way to monitor the adherence and response to osteoporosis treatment. Changes in BTM markers are observed sooner than changes in bone mineral density scans. When looking at the various BTMs, CTx and P1NP are considered the best biomarkers and are the ones that are being endorsed by professional societies.
Serial measurements should be performed using the same assay and ideally the same laboratory.
The effect of diurnal variation, food intake, and renal clearance also need to be considered when measuring these BTM.
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