Osteoporosis and other disorders of low bone mass are common, and osteoporotic fractures can incur substantial morbidity and mortality. However, determining a patient’s level of risk for fracture and monitoring the effectiveness of treatment can be challenging for clinicians. One reason is that osteoporosis is essentially clinically silent until a fracture occurs.
Another is that the impact of the most common forms of osteoporosis on the skeleton and the response to therapies to treat osteoporosis are both very slow. Thus, a change in therapy or clinical risk factors can take months to translate into changes in bone parameters as measured by dual energy X-ray absorptiometry (DXA), a standard radiographic method to estimate bone mass and fracture risk.
This slow response of the skeleton can make optimizing medical management difficult, as it greatly delays empiric assessment of therapy response and switching to another agent if necessary. This issue is further exacerbated as clinicians increasingly consider sequential or combination therapies for osteoporosis management, escalating the overall complexity of management options.
In this context, bone turnover markers (BTMs) can help clinicians refine treatments to individual patients, and there is substantial interest in using BTMs to either assess fracture risk or to assess the response to therapy. However, preanalytical factors and proper utilization are key for effective BTM testing.
RESORPTIVE AND ANABOLIC BONE TURNOVER MARKERS
BTMs are categorized into either resorptive markers or anabolic markers that correlate with the rate of bone matrix removal by osteoclasts or deposition by osteoblasts. Many BTMs are fragments of collagen or other bone matrix proteins liberated during bone matrix synthesis or resorption. The best validated BTMs for bone resorption by osteoclasts are the Collagen type I C-terminal and Collagen type I N-terminal telopeptides (CTX and NTX, respectively) that are released during osteoclast-mediated removal of bone matrix.
In practice, CTX is often run on serum, and NTX is run on urine, as serum NTX is less responsive to treatment with drugs that block osteoclast activity, such as bisphosphonates. Both CTX and NTX generally show similar performance.
A separate group of markers, most notably including the N-terminal propeptide of type 1 collagen (PINP) and bone-specific ALP, correlate with bone formation by osteoblasts. The collagen propeptides are cut when bone matrix is produced and therefore correlate with rates of bone formation. PINP is initially present as a bundled trimer of three propeptides that is then converted to single monomer units in circulation.
Clinical assays can measure either a combination of the monomer and dimer, termed total PINP, or just the trimeric form termed intact PINP. Trimeric PINP is cleared via uptake in the liver, while monomeric PINP is mostly cleared renally. In patients with renal disease, testing of intact PINP is preferred to avoid biologic interference from impaired renal clearance of the PINP monomer.
Although alkaline phosphatase (ALP) is made at high levels by bone-forming osteoblasts and is an almost universally available clinical assay, total alkaline phosphatase activity in serum reflects the combined activity of 4 genes (ALPI, ALPL, ALPP, ALPP2). As a result, the ALP activity produced by bone-forming osteoblasts is only a small fraction of the total ALP present. Accordingly, only diseases with truly remarkable elevations in bone formation, such as Paget’s disease of bone, typically display elevations in total ALP levels.
To counter this issue, a number of methods have been developed to measure only bone-derived ALP, with immunoassays currently being the method of choice. Importantly, current bone ALP immunoassays have some degree of cross-reactivity with forms of ALP produced by the liver and must therefore be interpreted with caution in patients with liver disease.
However, despite this potential usefulness of BTMs, bone turnover marker testing poses a particular challenge for laboratory professionals. BTMs can display substantial analytic and biologic variation that often masks their ability to predict rates of bone turnover. Careful consideration of preanalytic testing conditions and clinical context of utilization can minimize these issues.
UTILITY OF BONE TURNOVER TESTING
Total bone mass reflects the balance between the activity of osteoblasts to build bone and osteoclasts to resorb bone. As a result, there has been great interest in using BTMs to predict both bone loss and responsiveness to osteoporosis treatment.
However, while there is a clear association between perimenopausal BTM levels and subsequent bone loss, the correlation between BTM levels and bone loss in elderly Caucasian women is modest.
Although there may be utility in measuring BTM levels, currently the cost-effectiveness and efficacy of BTM monitoring have yet to be demonstrated in prospective randomized clinical trials. Therefore, while BTM monitoring may have utility in addressing the risk of bone loss in individual patients when these caveats are kept in mind, the use of BTMs is not currently recommended as a systematic public health measure to identify patients at risk of rapid bone loss.
The use of BTMs to assess fracture risk should be considered separately from their ability to predict bone loss, as many other factors besides the total amount of bone—including bone architecture and materials properties—also contribute to fracture risk. Increased resorptive BTM levels do appear to correlate with subsequent fracture risk for up to 5 years after measurement. However, anabolic BTM levels do not appear to show this same ability to predict fracture, perhaps reflecting that postmenopausal osteoporosis is often a resorption-driven process.
The risk associated with elevated BTM levels can be further modified by other conditions, and BTMs are known to notably underestimate fracture risk in patients with diabetes.
Osteoporosis is a known risk factor for orthopaedic fixation failure; therefore, optimizing bone quality prior to surgery is imperative. The preoperative assessment is an opportune time to screen for osteopenia and osteoporosis and subsequently provide the appropriate intervention.
BTMs can be assessed preoperatively to initiate osteoporosis therapy if needed to improve postsurgical outcomes. Patients can be monitored for bone health improvement and optimized prior to undergoing surgery. Post-operatively, recent evidence suggests that BTMs may be useful for monitoring successful fusion.
In summary, while BTM levels can be linked to both future bone loss and fracture risk, these associations may not be clear or strong enough to recommend that all patients get BTM measurement to predict future bone loss or fracture risk. However, BTMs may still be highly useful for this purpose in individual patients, especially in patients with other secondary diseases driving bone loss, including hyperparathyroidism, hyperthyroidism, vitamin D deficiency, and certain blood cancers.
Interestingly, beyond just predicting future bone health, increases in bone turnover markers are associated with all-cause and cancer morality in older men, recent studies suggest. The underlying mechanism responsible for this link is unclear and must be uncovered before this observation can be used in clinical practice.
USE OF BTMS TO MONITOR OSTEOPOROSIS THERAPY
A clearer case can be made for the use of BTMs to monitor osteoporosis therapy, than for using them to monitor bone loss or fracture risk. Both baseline BTM levels and BTM levels shortly after initiating therapy generally predict changes in bone mass.
For example, PINP levels at baseline or after therapy initiation predict responses to the bone anabolic agent teriparatide. Similarly, suppression of bone resorption markers seen after treatment with antiresorptive drugs such as bisphosphonates or denosumab correlates with later changes in bone mass. Moreover, BTM levels correlate with the degree of reduction in fracture risk in response to these therapies. Therefore, assessing BTMs may aid clinical decision-making in patients for whom there is particular concern regarding the response to osteoporosis treatment.
STRATEGIES TO MINIMIZE VARIATION IN BONE TURNOVER MARKER TESTING
Nearly all BTMs display marked diurnal variation, peaking between midnight and 8 a.m. and reaching a trough in the early afternoon. BTMs also drop after meals due to the direct effect of gastrointestinal hormones on bone resorption.
There is also seasonal variation of BTMs, with BTMs typically peaking in the early winter months, especially in premenopausal women. For this reason, BTMs are strongly recommended to be measured in a morning fasting draw. BTMs can also vary with menstrual cycle, being highest in the mid- to late-follicular phase and lowest during the mid-luteal phase, leading to a preference to sample during the follicular phase in premenopausal women when feasible.
Many bone turnover markers are cleared renally, with the notable exception of tartrate resistant acid phophatase 5b(TRAP5b). They are therefore not recommended for testing in the setting of renal failure. There are also potentially concerns that collagen remodeling occurring in the setting of fibrotic or tissue remodeling diseases of other tissues, such as systemic sclerosis, congestive heart failure, or dilated cardiomyopathy may impact BTM levels and mask their ability to reflect bone metabolism.
In addition to sources of preanalytic variance, other demographic factors strongly affect BTMs.
BTMs are generally high in growing children, reflecting the active ongoing skeletal growth and modeling, that often reaches a peak during puberty. Men in their 20s and 30s generally display higher BTM levels than young women, likely due to the greater total skeletal mass present undergoing remodeling. However, this reverses later in life as postmenopausal women experience an increase in resorptive BTMs because of ongoing bone loss.
For most other analytes, laboratories typically construct reference ranges using patient groups closely matched for demographics, especially age and sex. In the case of BTMs, this poses a particular challenge because many patients tested are older, with a particular emphasis on postmenopausal women.
However, bone dynamics in older women, even those otherwise considered healthy, may not be conducive to maintenance of healthy bone mass and, therefore, the usual ideal practice of comparing patient results to an age-matched reference range may be falsely reassuring.
A VALUABLE TOOL
While bone turnover markers are subject to numerous preanalytic factors that complicate their interpretation, they can provide useful information to guide clinical decision-making for risk assessment or management of osteoporosis or other diseases of skeletal fragility. While there is not currently evidence supporting routine BTM testing for all postmenopausal women or osteoporosis patients, BTMs remain a valuable tool when testing is carefully applied.
Matthew B. Greenblatt, MD, PhD, is an associate professor of pathology and laboratory medicine at Weill Cornell Medical College and the Hospital for Special Surgery research division. Email: [email protected].
Kyle W. Morse, MD, is a resident in orthopaedic surgery at the Hospital for Special Surgery in New York City. Email: [email protected].
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