Editor's note: This is Part 1 of a 2-part series on Osteoporosis. Part 2 is Medication-Induced Osteoporosis: Patient impact and prescribing implications.
More than 53 million Americans either have osteoporosis or are at high risk due to low bone mass. Debilitating acute and chronic pain in the elderly is often attributed to fractures from osteoporosis and can lead to further disability and early mortality. The risk of dying from a hip fracture in the elderly (aged >65 years) was estimated at up to 36% compared to age-matched controls, which is 3.6 times more deadly than the risk of death from breast cancer. And for those elderly patients with osteoporosis who survive a fracture, 40% to 60% don’t regain their prefracture level of mobility, and 55% to 60% who were previously living independently don’t regain that independence and require long-term care. This review evaluates currently available diagnostic and screening technologies and the most common laboratory tests for their ability to predict fractures and provide crucial information for treatment and monitoring of fracture risk. It also provides clinicians with recommended tests for patients at risk of bone loss.
Osteoporosis is a disease characterized by decreased bone quantity and quality and increased fracture risk. According to the National Institutes of Health (NIH), more than 53 million Americans either already have osteoporosis or are at high risk due to low bone mass.1 Since osteoporosis primarily affects older adults and the US population is aging, the number of people with osteoporosis is expected to grow rapidly over the next decade. From 2010 to 2019, the 65-and-older population grew by over a third, and by 2050 it’s estimated that there will be 83.7 million Americans over the age of 65.2,3
Debilitating acute and chronic pain in the elderly is often attributed to fractures from osteoporosis and can lead to further disability and dying early.4,5 In a systematic review of studies, excess mortality during the first year following a hip fracture in the elderly (aged >65 years) was estimated to be 36% higher than for age-matched controls.6 In a study looking at long-term mortality risk following all types of osteoporotic fractures (up to 18 years afterward) in women and men (aged >60 years), researchers concluded that there was an increased risk for dying in both sexes for 5 years after any fracture and for 10 years after a hip fracture.7 Compared to age-matched controls, mortality was elevated in women and men, with standardized mortality ratios (SMRs) of 1.38–2.53 for women and 1.64–3.52 for men. The highest SMR was observed after a hip fracture, and the risk for death from hip fracture was elevated for 10 years, SMR 1.52 (95% CI, 1.01–2.29) for women and 2.58 (95% CI, 1.29–5.17) for men. And for those elderly patients with osteoporosis who survive a fracture, 40% to 60% don’t regain their prefracture level of mobility, and 55% to 60% who were previously living independently don’t regain that independence and require long-term care.8
In 2018 the United States Preventive Services Task Force (USPSTF) updated its guidelines for osteoporosis screening.9 The task force recommends screening for all women aged 65 years or more and for postmenopausal women aged less than 65 years at increased risk for osteoporosis. Although it’s been estimated that 30% of hip fractures occur in men,10 the task force explicitly does not recommend screening men for osteoporosis.
Unfortunately, osteoporosis screening rates are low. A longitudinal study that looked at trends over several years estimated that only 30% of eligible women and 4% of eligible men are being tested.
However, other organizations take a more nuanced approach to screening recommendations. For example, the National Osteoporosis Foundation (NOF) position paper, Clinician’s Guide to Prevention and Treatment of Osteoporosis, stratifies testing recommendations based on sex, age, and additional risk factors.11 The NOF recommends all women aged 65 years or more and men aged 70 years or more get tested with a bone density scan. The organization also recommends bone density tests when risk factors (eg, medications and comorbidities that cause osteoporosis) are present in postmenopausal women aged less than 65 years or men aged 50 to 69 years. Finally, the NOF recommends that any adult aged >50 years who breaks a bone get a bone density test.
Screening of eligible patients for osteoporosis rests mainly with primary care providers. Unfortunately, osteoporosis screening rates are low. A longitudinal study that looked at trends over several years estimated that only 30% of eligible women and 4% of eligible men are being tested.12 Additionally, only approximately 5.5% of the sex-pooled Medicare population is getting bone density scans.13-15
Clinicians can help improve these screening numbers by recommending testing. However, knowing which test or combination of tests to order is crucial for increasing screening, ensuring an accurate diagnosis, and for informing intervention strategies. This review article evaluates the most common osteoporosis testing options. Tests include medical devices like the dual-energy X-ray absorptiometry (DEXA) test, biomechanical computed tomography (BCT), and quantitative ultrasound (QUS); and laboratory bone turnover markers (BTM). Since fractures are the most dangerous risk with osteoporosis, each test is reviewed for its ability to predict fragility fractures and times when clinicians may want to consider ordering 1 instead of another.
The DEXA test is the gold standard for osteoporosis screening and diagnosis and is the test against which all other medical devices are evaluated. There are 2 types of DEXA tests—central and peripheral. A central DEXA test measures bone mineral density (BMD) of the hip and lumbar spine (L1–4) while a peripheral DEXA test measures BMD at the wrist, fingers, leg, or heel.16 Central and peripheral DEXA tests expose patients to about 0.004 millisieverts (mSv) and 0.01 mSv of radiation, respectively.17
While the peripheral DEXA test has the advantage of being lower cost and portable, it is not diagnostic for osteoporosis and not appropriate for monitoring response to therapy. Despite that limitation, peripheral DEXA machines can fill an important clinical need by conveniently increasing osteoporosis screening at health fairs, smaller clinics, and pharmacies for which the larger and more expensive central DEXA machines are not cost-effective. If a peripheral DEXA test shows bone loss, clinicians should consider ordering a central DEXA text for diagnosis and treatment monitoring.
DEXA tests measure trabecular and cortical BMD, which makes up approximately 60% of bone mass. The remaining 40% of bone mass is composed of water and bone matrix, which the DEXA test does not detect. DEXA test results are expressed as grams per centimeter squared (g/cm2), which are then translated into T- and Z-scores.
A T-score is the number of standard deviations (SD) from peak bone mass compared to healthy individuals in a young population aged 20 to 29 years matched for race and gender. Diagnosis of osteoporosis was defined in 1994 by the World Health Organization (WHO) as a T-score of –2.5 SD and osteopenia as a T-score of –1 to –2.5 SD.18 In contrast, a Z-score compares a patient’s BMD to the average bone density of age- and sex-matched controls.
According to the International Society for Clinical Densitometry (ISCD), the T-score is not applicable to premenopausal women, men aged less than 50 years, or children. Instead, in these populations the ISCD recommends using the Z-score.19 A Z-score of –2.0 or less means a patient’s BMD is “below the expected range for age” while a Z-score greater than –2.0 means their BMD is “within the expected range for age.”20
Regardless of age, patients with a Z-score of –2.5 or less should be evaluated for secondary causes of bone loss, such as medication-induced osteoporosis or other disorders that damage bone, including autoimmune diseases, inflammatory bowel disease (IBD), endocrine abnormalities (eg, hyperthyroidism, diabetes), malabsorption, and liver and kidney disease.21
Although the DEXA test provides accurate and precise measures of BMD at specific sites, including the lumbar spine (L1–4), proximal femur (femoral neck and total hip), and distal forearm, the results are surrogate markers, which are only as useful as they are predictive for the most important clinical outcome, which for osteoporosis is fractures. Because T-scores from central DEXA tests are diagnostic for osteoporosis and osteopenia and most DEXA tests are run on postmenopausal women, the subsequent analysis focuses on the ability of central DEXA tests to predict absolute fracture risk.
The Rotterdam Study was a prospective, population-based cohort of Swedish adults 55 years and over that were recruited from 1990-1993 to participate. Overall analysis included 7,806 participants, with baseline BMD data available for 5,794 subjects (2,437 men and 3,357 women). The study tracked fracture incidence through general physician and hospital-based records. The results, published in Bone in 2004, showed that a diagnosis of not just osteoporosis but that of osteopenia correlates to incidence of nonvertebral fractures.22
In the Rotterdam study, overall (n=7806) incidence of non-vertebral fracture was 12% (n=939) during the follow-up period (mean 6.8 years). The age-adjusted incidence for non-vertebral fracture was 2.3 times higher in women than men. In the subgroup of subjects with BMD data (n=5794) available, 11.1% had at least one non-vertebral fracture during the study period.
While vertebral fractures correlated with BMD as expected, nonvertebral fractures were just as likely to happen in those with osteopenia as in those with osteoporosis. There were 499 women who had at least 1 nonvertebral fracture; 44% had osteoporosis, 43% had osteopenia, and 12.6% had normal BMD. For men, only 20.7% of those who sustained a nonvertebral fracture had osteoporosis, more than 61% had osteopenia, and nearly 18% had normal BMD. This data suggests that interventions to prevent fractures should be initiated as soon as the BMD reflects osteopenia for both men and women.
An additional explanation for the discrepancy between reported BMD and fracture risk might be partially explained by technician errors. According to a review article in the Canadian Association of Radiologists Journal, the majority of DEXA reports (up to 90% in 1 study) contained an error.25 Most issues were errors related to data analysis or reporting, incorrect patient positioning, artifacts, and demographic errors.
Artifacts that can bias DEXA results include degenerative changes in the hip or spine, vascular calcifications, oral contrast agents, and consuming foods or dietary supplements that provide high amounts of calcium or other heavier minerals or elements.26-28 According to the North American Menopause Society (NAMS) 2010 position statement, if anatomic factors such as obesity or arthritis invalidate measurements, the distal one-third of the radius bone density may be used as a diagnostic site.29 However, if patients are taking strontium, which is a popular bone health dietary supplement that is heavier than calcium and incorporates into bone, clinicians should be aware that strontium creates false bone-density test results.30
While DEXA errors can lead to undertreatment, they may also lead to overtreatment with medications inappropriate for many patients. A large, retrospective cohort study was conducted using electronic health records (EHR) at UC Davis health system from January 1, 2006, to December 31, 2011.31 The study involved patients who received a DEXA scan and were newly prescribed an osteoporosis drug. Researchers included EHR of 6,150 women aged 40 to 85 years in the analysis. The researchers found that “two-thirds of new osteoporosis drug prescriptions were potentially inappropriate because the osteoporosis diagnosis was based on [DEXA] abnormalities considered nondiagnostic by international guidelines. Of these potentially inappropriate prescriptions, half were provided to younger women without osteoporosis risk factors who may not have merited screening.”
Clinically, it’s important to understand that even though the diagnosis of osteoporosis requires a T-score, improving the T-score test number does not fully protect patients from fracture risk. Based on a review of the research, the NAMS position statement on osteoporosis states that fracture risk “depends on factors largely other than bone density.”32 A bone density test is only one piece of the fracture risk puzzle, and by no means the most important one.
DEXA Test Clinical Highlights
Central DEXA test
- Radiation: 0.004 mSv*
- Sites: Hip, spine
- Advantages: Precise, reproducible
- Disadvantages: Affected by artifacts; of limited use in people with spinal deformity or with a history of spinal surgeries; low predictive value for absolute fracture risk; low sensitivity and specificity; sensitive to technician errors; relatively insensitive to small changes in bone, so repeat DEXA tests are typically ordered only every 18-24 months.
- Clinical use: Diagnosis of osteopenia and osteoporosis; used to monitor effect of treatment.
Peripheral DEXA test
- Radiation: 0.01 mSv
- Sites: wrist, fingers, leg, or heel
- Advantages: Portable; can increase patient access to bone density screening.
- Disadvantages: Not diagnostic for osteoporosis; not appropriate for monitoring response to therapy.
- Clinical use: Can be used as a tool to increase access to bone density screening, provide an opportunity to educate the general public and patients about the importance of bone density testing, and potentially increase the number of patients getting screened with central DEXA tests by ordering a central DEXA test if a peripheral DEXA test result is positive.
Biomechanical Computed Tomography (BCT)
BCT is an alternative osteoporosis screening option that can be used to diagnose osteoporosis and track treatment response. BCT measures volumetric trabecular bone density. It does not superimpose cortical bone and other tissues and is expressed in milligrams per cubic centimeter (mg/cm3) of calcium hydroxyapatite.
In 2008 the American College of Radiology published conversion guidelines for comparing BCT with DEXA scans. For spinal BMD, the thresholds were <120 mg/cm3 for osteopenia (equivalent to a DEXA T-score of −1.0 SD) and <80 mg/cm3 for osteoporosis (equivalent to a DEXA T-score of −2.5 SD or less).
BCT has some advantages over DEXA. BCT is not affected by artifacts like the DEXA test. Additionally, and also unlike DEXA scans, BCT is accurate in patients with extremely high or low body mass index.33 However, BCT exposes patients to higher doses of radiation. Pelvic CT and abdominal CT scans expose patients to about 6 mSv and 8 mSv of radiation, respectively.34
One emerging advantage of BCT is that existing CT scans can be repurposed to measure DEXA-equivalent BMD without having to subject patients to a new CT test and additional radiation exposure. Approximately 10% of Medicare patients (adults aged ≥ 65 years) received an abdominal/pelvic CT scan in 2007. In that 1 year alone, 72 million CT scans were performed, with more than 20 million of those being lumbar spine or abdomen/pelvic CT scans.35 The ability to take existing CT scan data and repurpose them to predict BMD and bone strength—so-called opportunistic use—can provide clinically useful information without needing to expose patients to more radiation or the inconvenience of more testing. Clinicians would need to contact the radiology department that has the CT scan data and ask them if they are capable of repurposing the data and running a BCT evaluation.
BCT uses information that already exists within CT scans to provide an estimate of the breaking strength of bone and predict fracture risk. BCT is based on fine element analysis (FEA) of a patient’s CT scan. FEA converts CT data into geometric shapes to create a new data layer that can be manipulated and analyzed. Material properties are assigned to the elements by converting volumetric BMD (vBMD) from the CT scan to material properties using empirically derived relationships. Bone stiffness is then estimated using the reaction force and displacement data, and the failure load is estimated from the selected failure criteria.36
For detecting osteoporosis, BCT appears to be more sensitive than the DEXA test. A study of 140 postmenopausal women, mean age 63.2 years + 8.1 years, compared BCT and DEXA test results. The intragroup detection rates for BCT versus DEXA were significantly different (P<0.01), with BCT detecting osteoporosis more frequently than spinal and hip DEXA. Osteoporosis detection rates were 17.1% for lumbar DEXA versus 46.4% for lumbar BCT.37
The hope, however, that BCT would be superior to DEXA exams for predicting fractures has so far not been borne out by the research. Thirteen studies have looked at BCT’s ability to predict incident hip fractures. Eleven showed predictive value equivalent to BMD studies. Two studies showed superior predictive value, but both studies enrolled only men.36
The largest study to date is a retrospective case-cohort study using preexisting, anonymized clinical data from electronic health records to predict primary hip fractures. This study evaluated health records from 111,694 patients (64,992 women and 46,702 men) who had an abdominal or pelvic CT scan between January 1, 2006, and December 31, 2014, a DEXA scan within 3 years of the CT exam, and no hip fractures before either the CT or DEXA scans. The researchers took the original CT results and reinterpreted them as BCT scans. The researchers found no significant differences between CT tests and DEXA tests at predicting primary hip fractures. They concluded, “These results demonstrate that BCT analysis of previously taken routine clinical abdominal or pelvic CT scans is at least as effective as DEXA testing for identifying patients at high risk of hip fracture.”38 Therefore, while BCT may be superior to DEXA at detecting osteoporosis, it does not currently offer advantages over DEXA for fracture prediction.
While a strong advantage of BCT is that it can increase osteoporosis screening rates by using existing data and avoiding additional radiation exposure for patients, there are several limitations of the technology that clinicians should understand. An accurate analysis of the spine is not possible when contrast is used, which occurs in about 60% to 70% of abdominal CT scans.39,40 However, contrast does not interfere with hip BMD analysis. Additionally, the biomechanical properties evaluated by BCT miss molecular-level defects that may exist, such as decreased collagen that can affect bone strength independent of BMD.40
With the use of opportunistic BCT (scans that were not acquired primarily for bone assessment), data integrity can pose interpretation challenges. The problems that can arise include (1) inadequate image quality and quantitative calibration and (2) inconsistency of results across patients, especially when using different scanners.41 There currently is only 1 FDA BCT device, the VirtuOst BCT test.
If patients are undergoing CT scans for other underlying conditions that also warrant an evaluation for bone density testing, doctors can request a BCT be added onto the test. This could include, for example, patients undergoing CT enterography for IBD,42 CT colonography for colorectal cancer screening,43 spine CT before spinal fusion surgery,44 or a positron emission tomography (PET)/CT for prostate cancer staging.45
BCT Clinical Highlights
- Radiation: 6–8 mSv
- Sites: lumbar spine, hip
- Advantages: Interpretation not affected by deformities, degenerative changes, aortic calcifications, or obesity; can be run on existing CT scans without subjecting the patient to additional testing and radiation; correlates well with DEXA-test BMD.
- Limitations: Does not capture molecular-level defects (eg, patients with collagen or mineral deficiencies) that can change the mechanical properties independent of BMD; spinal CT cannot be evaluated for bone density if contrast is used.
- Clinical use: Diagnosis of osteoporosis, monitoring of treatment (hip BCT only).
Quantitative Ultrasound (QUS)
QUS uses radio waves to assess bone microarchitecture by evaluating bone stiffness (stiffness index [SI]), speed of sound (SOS), and broadband ultrasound attenuation (BUA). The calcaneus is the only clinically validated site for QUS.46-48 QUS offers a low-cost alternative to assessing bone health, is portable, is more easily accessible to primary care providers than DEXA and CT scans, and is radiation-free.
Similar to quantitative computed tomography (QCT), the QUS does not measure BMD. Therefore, the WHO definition of osteoporosis does not apply.49 Despite that fact, QUS provides practical benefits. Since QUS is portable and can be done in pharmacies, health fairs and other places, it can increase patient access to bone health screening. If QUS results are positive, it provides an opportunity for further discussion about bone health with the patient and a referral for diagnostic bone-density testing.
Validation studies of QUS devices compared to DEXA scans have been conducted, but each device has its own cutoff values, sensitivity, and specificity.50 There is no consensus as to which device, measured variable, and cutoffs are best.50 Therefore, if clinicians use QUS, they need to understand the manufacturer-recommended cutoffs associated with osteoporosis and verify that the device has been suitably validated against the DEXA scan.51
In addition to being a useful tool for bone health screening, studies conclude that QUS results are associated with predicting fracture risk,52,53 including fractures of the proximal femur,53-56 the vertebrae,51,57,58 and other sites.58-62 These studies demonstrated that the fracture risk associated with QUS results was at least comparable with other peripheral measurement approaches and, in some studies, even similar to central DEXA scans.
QUS Clinical Highlights
- Radiation: None
- Sites: Calcaneus
- Advantages: Lower cost than DEXA and QCT; can increase patient access to bone health screening since the units are portable.
- Limitations: Not diagnostic for osteoporosis; does not measure BMD; no consensus as to which device, measured variable, or cutoff values are best. Clinicians must understand the manufacturer-recommended cutoffs associated with osteoporosis and verify that the device has been suitably validated against the DEXA scan.
- Clinical use: Can be used as a tool to increase access to bone health screening, provide an opportunity to educate the general public and patients about osteoporosis, and potentially increase the number of patients getting screened with central DEXA tests by ordering a central DEXA test if a QUS test result is positive.
Osteocalcin and Undercarboxylated Osteocalcin
Osteocalcin (OC), also referred to as bone γ-carboxyglutamic acid (Gla) protein or BGP, is a 49-peptide, noncollagenous protein (NCP). OC is 1 of 180 to 200 NCPs in bone, including osteopontin (OP), osteonectin (ON), bone sialoprotein (BSP), bone morphogenetic protein (BMP), decoran, biglycan, and thrombospondin-2.63,64 These proteins are vital to bone extracellular matrix (ECM) integrity and exhibit multifunctional roles. While some NCPs contribute to ECM structure, their importance in bone morphology and strength is largely unknown.65 Given the sheer number of NCPs in bone, reducing fracture risk down to 1 analyte has yielded inconsistent results.
OC exists in 2 states: fully carboxylated osteocalcin (OC) and undercarboxylated osteocalcin (ucOC). In bone, serum OC and ucOC are thought to be markers of bone turnover. OC is produced primarily by osteoblasts, although smaller amounts are also produced by odontoblasts in teeth and hypertrophic chondrocytes.66 Its synthesis is stimulated by 1,25-dihydroxyvitamin D (vitamin D3, cholecalciferol) followed by post-translational carboxylation requiring vitamin K.67 OC is decarboxylated in the low-pH environment of osteoclasts to create ucOC.68 OC is metabolized by the liver and kidneys. Due to its renal clearance, higher serum concentrations are observed in patients with renal failure.67
Clinical trials have shown that in postmenopausal women, serum OC is inversely associated with BMD, and serum ucOC is inversely associated with both BMD and fractures. These correlations have not been shown in younger, healthy women or in men.
A 2015 case-control study in India enrolled 82 postmenopausal women aged 40 to 70 years (mean age 54.51 + 10.4 years).69 Serum OC was detected by enzyme-linked immunosorbent assay (ELISA). In this study, there was a significant inverse correlation between serum OC and femoral neck (P<0.001) and lumbar spine (P<0.001) BMD. Serum osteocalcin levels in women with osteopenia were a mean of 19.25 ± 5.09 ng/mL and 22.03 ng/mL in women with osteoporosis. The serum OC levels in the control group of women with normal BMD were significantly different than in those women with osteopenia (P=0.005) or osteoporosis (P<0.001).
A second Indian study evaluated 80 postmenopausal women and showed similar results between serum OC and BMD.70 The researchers also evaluated OC by ELISA and found a significant inverse correlation between the serum OC and total femoral BMD (P=0.019). They did not measure lumbar BMD, and neither study evaluated fracture risk.
One of the earliest prospective cohort studies to evaluate the association of serum ucOC with BMD and fracture risk was the Epidémiologie de l'Ostéoporose (EPIDOS) study.71 The researchers found an inverse association between serum ucOC and fracture risk. This 2-year, prospective cohort study took place from January 1992 through December 1993 with 7,598 healthy French women aged 75 years or more. The researchers found that serum ucOC was inversely associated with hip fracture risk. Laboratory assay methods can affect test results and interpretation; therefore, it’s important to note that this study utilized ELISA to determine serum ucOC.
A baseline serum ucOC ELISA test result in the highest quartile was associated with an odds ratio (OR) for hip fracture of 2.0 (CI, 1.2–3.2; P<0.008). Serum ucOC was also inversely correlated with BMD (P<0.0002). Even after researchers adjusted for BMD, the association between serum ucOC and hip fractures persisted.
When serum ucOC and BMD measurements were combined, the predictive value for fractures increased. Women who had serum ucOC in the highest quartile and BMD in the lowest quartile had a hip fracture OR of 5.5 (CI, 2.7–11.2; P value not reported) compared to women with only 1 of these independent risk factors (eg, with either high serum ucOC or low BMD).
A 2010 study in the Journal of the Korean Medical Society evaluated the association between serum ucOC and BMD in 337 healthy Korean women aged 20 to 80 years.72 They determined serum ucOC by ELISA. Serum ucOC declined until age 49 and then increased again. Unfortunately, the researchers didn’t analyze the association between serum ucOC and BMD in different age groupings (eg, 20–29, 30–39). Instead, they aggregated the data across all groups.
The aggregated data showed serum ucOC was inversely associated with lumbar BMD (P<0.001). There was a trend toward lower femoral neck BMD with elevated serum ucOC, but this was not statistically significant (P=0.052). Interestingly, serum ucOC was highest (2.59 + 1.98) in the group of women from 20 to 29 years old, but their BMD was also among the highest of any group. This calls into question the validity of serum ucOC in younger women. Current research has not determined at what age ucOC starts to become inversely associated with BMD or why it may be relevant only for older patients.
In a study published in 2020 in the Journal of Physiological Anthropology, serum ucOC and QUS were measured in 858 community-dwelling volunteers (503 women, 358 men) in Japan.73 Serum ucOC was measured by electrochemiluminescence immunoassay (ECLIA). Volunteers were stratified into 10-year age groupings (40–49 years, 50–59 years, 60–69 years, 79–79 years) and then 80 years and more.
Mean serum ucOC peaked in women aged 50 to 59 years and then declined in each subsequent age group to a mean low of 4.7 ng/mL in volunteers aged >80 years. In women aged 40 to 49 years, the mean serum ucOC was 3.5 + 1.7 ng/mL; in women aged 50 to 59 years, mean ucOC was 6.0 + 3.4 ng/mL; in women aged 60 to 69 years, mean serum ucOC was 5.9 + 3.4 ng/mL; in women aged 70 to 79 years, mean serum ucOC was 5.3 + 3.7 ng/mL; and in women aged 80 years and more, mean ucOC was 4.7 + 3.7 ng/mL. Mean serum ucOC was significantly different in women aged 50 to 59 years (P<0.05) and 60 to 69 years (P<0.05) compared to women aged 40 to 49 years.
In contrast to the trend of declining mean serum ucOC observed in women as they age, serum ucOC increased as men aged. The highest serum ucOC in men was observed in volunteers aged 80 years (5.9 + 6.1), which was significantly higher than in all other male age groups (P<0.05).
For the stiffness index, speed of sound (SOS) and broadband ultrasound attenuation (BUA), serum ucOC concentrations were negatively associated with all parameters in women (P=0.019, 0.034, and 0.045, respectively), but not in men (P=0.229, 0.149, and 0.421, respectively). Why QUS parameters were not inversely correlated with serum ucOC equally in women and men is not known.
This study and the Korean study raise important questions about how well serum ucOC relates to bone health. If serum ucOC is inversely associated with BMD and fracture risk, healthy young women would be expected to have low serum ucOC since peak bone mass occurs at age 30 to 35 years, and both men and women lose bone at approximately 0.5% to 2.0% per year after their early 30s.74-76 Additionally, since fracture risk doubles every 7 to 8 years after age 50, the median age for hip fractures is approximately 82 years, and the median age for vertebral fractures occurs in women in their 70s,29 it seems counterintuitive that serum ucOC in those aged 50 to 59 years would be higher than those aged more than 80 years.
One study published in 2012 in the journal Annals of Laboratory Medicine evaluated the associations of serum OC, serum ucOC, and the ucOC/OC ratio (%) to BMD in 88 Turkish women aged 27 to 74 years.77 Based on their data, the researchers also calculated the estimated positive predictive value of these markers for osteopenia and osteoporosis.
In this study, women were divided into 2 categories and 4 groups. One category included all premenopausal women (n=40), who were equally divided into 2 control groups. Group I included women aged 25 to 34 years (mean 30 + 4 years). Group II included women aged 35 to 45 years (mean age 40 + 3 years). The second category contained 2 comparison groups of 44 postmenopausal women stratified based on the number of years since menopause. Group III included women 1 to 5 years after menopause (mean age of 53 + 7 years) and Group IV included women >5 years after menopause (mean age 63 + 7 years).
Researchers took fasting blood samples from all participants between 8:00 am and 9:00 am. They measured osteocalcin by ECLISA and subsequently determined the amount of ucOC by titrating the samples with barium sulfate (BaSO4).
Taken together, there was a significant difference in overall mean serum OC in postmenopausal women compared to premenopausal women (P=0.012). Within those categories, there are interesting patterns that merit further analysis. The mean serum OC was significantly higher in Group III compared to Group II (31.8 + 12.0 ng/mL vs. 21.7 + 9.5 ng/mL, P=0.001). Interestingly, while not statistically significant, mean serum OC was lower in Group IV than in Group III (28.3 + 10.4 ng/mL vs. 31.8 + 12.0 ng/mL), and there was no significant difference between mean serum OC in Group IV compared to Group II (28.3 + 10.4 ng/mL vs 21.7 + 9.5 ng/mL).
A similar trend was seen with mean serum ucOC within and between groups. Mean serum ucOC was significantly higher in Group III compared to Group II (2.25 + 1.16 ng/mL vs. 1.21 + 0.72 ng/mL, P=0.002). However, unlike serum OC, mean serum ucOC was also significantly different between Group III and Group I (2.25 + 1.16 ng/mL vs. 1.48 + 0.52 ng/mL, P=0.038).
There were no significant differences in the ucOC/OC ratio between any groups; however, serum OC and ucOC also inversely correlated with most, but not all, BMD measurements. Serum OC was inversely associated with femoral neck BMD (P=0.016), L1-L4 BMD (P<0.001), and L2-L4 BMD (P<0.001). There was no association between femoral BMD serum ucOC, but inverse associations were detected between serum ucOC and L1-L4 BMD (P<0.001) and L2-L4 BMD (P<0.001).
Unlike previous studies, the researchers next evaluated serum OC and ucOC as predictors for osteoporosis. When they controlled the data for biochemical parameters (eg, total acid phosphatase [TACP], alkaline phosphatase [ALP], calcium, phosphorus), age, body mass index (BMI), and menopausal status, serum OC had the highest predictive value for osteoporosis (P=0.029 for femoral neck, P<0.001 for L1-L4 spine osteoporosis, and P=0.002 for L2-L4 spine osteoporosis).
Serum OC and ucOC may be good biochemical markers for osteoporosis risk that, when used in combination with DEXA testing for BMD, may make diagnosing osteoporosis, risk evaluation, and osteoporosis therapy more effective. Both the ELISA and ECLISA testing methods have produced clinically relevant results. Serum OC and ucOC can be used to screen for high bone turnover. If results are elevated, clinicians could then order a diagnostic BMD test for further evaluation. Since blood draws can be done in the clinic, these tests could increase the number of patients being screened for potential bone loss. Additionally, since biochemical markers can change more rapidly than BMD, serum OC and ucOC may be useful clinically at monitoring response to therapy.29
OC and ucOC Clinical Highlights
- Test specimen: Serum
- Advantages: Since blood draws can be done in the clinic, these tests could increase the number of patients being screened for potential bone loss.
- Limitations: Not diagnostic for osteoporosis.
- Clinical use: Screening for elevated bone turnover. If results are elevated, clinicians could then order a diagnostic BMD test for further evaluation.
Cross-Linked Telopeptides of Collagen I
Degradation products of collagen released during bone resorption include N-terminal telopeptide (NTX) and C-terminal telopeptide (CTX). Both can be measured in 24-hour urine, early morning spot urine, and serum samples. Both of these analytes exhibit significant circadian variation; therefore, time of day is crucial for sample collection.67 NTX and CTX peak at about 5:30 am (range 1:30 am to 07:30 am), reaching a minimum concentration at about 2:00 pm to 4:00 pm.78-80
A 2000 study published in the Journal of Bone and Mineral Research evaluated the association of serum and urinary NTX and CTX with fracture risk in postmenopausal women.81 The prospective cohort study enrolled 435 “healthy women” aged 31 to 89 years and had a mean follow-up of 5.0 + 1.3 years. Researchers took baseline fasting blood draws for hormones and bone turnover markers between 7:30 am and 9:30 am, and they also collected 24-hour urine specimens at baseline to measure urinary bone resorption markers.
Adjusted for age, prevalent osteoporotic fractures, and physical activity, the highest quartile of urinary and serum CTX were both associated with increased fracture risk. There was no significant association between urinary NTX and fractures.
Urinary CTX in the highest quartile was significantly associated with an increased risk of fracture with a relative risk (RR) of 2.4 for nonvertebral and symptomatic vertebral fractures (95% CI, 1.3–4.6, P=0.008) and 2.4 for only nonvertebral fractures (95% CI, 1.1–5.0, P=0.025). For women with serum CTX in the highest quartile, the RR was 2.3 (95% CI, 1.2–4.4, P=0.01) for nonvertebral fractures and symptomatic vertebral fractures and 2.5 (95% CI, 1.2–5.1, P=0.018) for only nonvertebral fractures.
Importantly, an additive effect was shown when multiple variables were combined in the evaluation. Women with serum CTX in the highest quartile plus serum estradiol 11 pg/mL had an RR of fracture of 3.0 (95% CI, 1.3–6.7, P value not reported). Women with both serum CTX in the highest quartile and serum dehydroepiandrosterone sulfate (DHEA-S) in the lowest quartile had an RR of fracture of 3.3 (95% CI, 1.6–6.9, P value not reported). The RR of fractures in women who had only serum DHEA-S in the lowest quartile was 2.1 (95% CI, 1.1–4.1, P=0.035).
The combination of urinary CTX in the highest quartile plus estradiol in the lowest quartile was associated with increased fracture risk, with an OR of 2.0 (95% CI, 1.1–3.6, P value not reported) and 2.2 (95% CI, 1.2–4.1, P value not reported), respectively. Women with both low hip BMD (T-score < -2.5) and the highest quartile of urinary CTX were at a higher risk of fracture than women with either low BMD or high urinary CTX, with an RR of 4.2 (95% CI, 1.9–9.3, P value not reported).
While this study collected fasting samples in the morning, and literature and labs report that fasting morning samples are required for optimal clinical use,82 a 2000 study in Bone found that afternoon serum CTX, with the blood drawn after lunch between 1:00 pm and 2:00 pm, is also a predictor for fractures in elderly women, with an RR of 1.9 (95% CI, 1.01–3.76, P value not reported).83
The International Osteoporosis Foundation (IOF) and International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) have recommended serum CTX as a resorption reference marker for use in fracture risk prediction and osteoporosis treatment monitoring.84 One benefit of bone turnover markers like CTX is that they show changes much faster than can be detected on a DEXA scan and may, therefore, be useful in monitoring response to treatment.29
CTX Clinical Highlights
- Test specimen: Serum
- Advantages: Since blood draws can be done in the clinic, these tests could increase the number of patients being screened for potential bone loss.
- Limitations: Not diagnostic for osteoporosis.
- Clinical use: Screening for elevated bone turnover. If results are elevated, clinicians could then order a diagnostic BMD test for further evaluation.
Despite the fact that the number of people at risk for osteoporosis is growing each year as the population ages, osteoporosis screening remains low, with less than 10% of the eligible population getting tested. There are 3 generally accepted options for osteoporosis screening. Despite the fact that the DEXA test predicts less than half of patients who will fracture, it is the gold standard and the technology on which the WHO definition of osteoporosis is based. The BCT test is a validated alternative that may allow clinicians to use existing scans to estimate bone density. This could increase the number of patients who are getting screened. And the QUS is a portable, low-cost device that does not expose patients to ionizing radiation.
BTMs may be clinically useful tests for fracture-risk screening and treatment monitoring. They exhibit significant circadian variation and provide different results depending on the test methodology and specimen handling. BTMs appear to provide the strongest predictive value when combined with other tests such as a bone density test, serum estradiol, or DHEA-S. Thus, clinicians may want to offer several tests to at-risk patients in order to better assess their bone health.
*A standard chest X-ray exposes patients to 0.02–0.06 mSv
Abbreviations & Acronyms
ALP: alkaline phosphatase
BaSO4: barium sulfate
BCT: biomechanical computed tomography
BMD: bone mineral density
BMI: body mass index
BMP: bone morphogenic protein
BSP: bone sialoprotein
BTM: bone turnover markers
BUA: broadband ultrasound attenuation
CI: confidence interval
DEXA: dual-energy X-ray absorptiometry
DHEA-S: dehydroepiandrosterone sulfate
ECLIA: electrochemiluminescence immunoassay
ELISA: enzyme-linked immunosorbent assay
FEA: fine element analysis
Gla: γ-carboxyglutamic acid
IBD: inflammatory bowel disease
NAMS: North American Menopause Society
NCP: noncollagenous protein
NOF: National Osteoporosis Foundation
NTX: N-terminal telopeptide
OR: odds ratio
Positron emission tomography (PET)
QCT: quantitative computed tomography
QUS: quantitative ultrasound
RR: relative risk
SI: stiffness index
SMR: standardized mortality ratio
SOS: speed of sound
TACP: total acid phosphatase
ucOC: undercarboxylated osteocalcin
USPSTF: United States Preventive Services Task Force
WHO: World Health Organization