Bone formation markers include osteocalcin, bone specific alkaline phosphatase (BSALP), carboxyterminal propeptide of type I procollagen (PICP), and PINP. BSALP and osteocalcin are released by osteoblasts and play a major role in bone mineralization. PICP and PINP are cleaved from procollagen type I during collagen synthesis. In Korea, osteocalcin and BSALP are most commonly used bone formation markers.[11] Osteocalcin is measured by immune-radiometric assay (DIAsource Immunoassays S.A., Nivelles, Belgium) and an electrochemiluminescence assay (ECLIA; Roche Diagnostics, Mannheim, Germany). BSALP is measured by chemiluminescence assay (Beckman Coulter Inc., Sacramento, CA, USA), enzyme immunoassay (Quidel Corporation, San Diego, CA, USA), and electrophoresis assay (Helena Laboratories, Beaumont, TX, USA). Although PINP has not been widely used yet in Korea, it can be measured by the ECLIA (Roche Diagnostics) and covered by insurance in osteoporotic patients recently.

Bone resorption markers include CTX-I, N-terminal telopeptide of collagen type I (NTX-1), free and total pyridinoline (PYD), and free and total deoxyPYD.

In Korea, CTX-I is the most commonly used as a bone resorption marker and has been mainly measured by the ECLIA using β-CrossLaps kit (Roche Diagnostics).

The primary challenge to the adoption of many BTMs in routine practice has been the poor reproducibility within-subject and between-laboratory. For clinical utility of BTMs, the pre-analytical sources of variability, along with the underlying disease process, must be identified, minimized, and controlled through carefully standardized patient preparation and sample handling procedures.[12,13]

Because serum PINP and CTX-I have recently been recommended as a reference BTM,[14] we focused on the proper sample collection procedures for both CTX-I and PINP in this position statement.

Although either serum or plasma sample may be used for the measurements of CTX-I and PINP, ethylenediaminetetraacetic acid plasma has the advantage over the serum as it has superior sample stability.[15,16,17] The same sample type should be used consistently when monitoring a patient. CTX-I exhibits a circadian rhythm in blood. CTX-I level peaks during the early morning hours (2-5 a.m.) and reaches a nadir between 11 a.m. and 2 p.m. The only known modulator having a major effect on this circadian pattern is food intake. Overnight fasting markedly reduced the circadian variation of CTX-I. Therefore, blood samples for CTX-I measurement must be collected following an overnight fast during the morning between 7:30 a.m. and 10:00 a.m.[18,19,20] Other BTMs have minimal circadian rhythm and are minimally affected by food intake. Sample processing is described previously.[11]

The patient related factors can be divided into controllable and uncontrollable factors (Table 1). Controllable factors include the menstrual cycle, seasonal variation, and physical activities. The optimal time to collect samples in pre-menopausal women is the early-mid-follicular phase.[21] There is a minor but detectable seasonal variation for CTX-I in older adults and those with severe vitamin D deficit.[22,23] Intensive physical training (e.g., elite soccer players) moderately increases serum CTX-I and slightly decreases PINP. Vigorous exercise should be avoided the day prior to sampling.[24] Uncontrollable factors include age, sex, pregnancy, geography, renal function, and specific diseases and medications.[14]

Several BTMs were introduced as having an additive effect on fracture risk in women with a low BMD.[26] Two large population-based studies found that increased levels of serum BSALP, urinary and serum CTX-I are significantly associated with an increased risk of osteoporotic fracture (hip and vertebra) with a relative risk of 2.3 to 4.8.[5,27] The predictive value of BTMs for the risk of fracture was slightly weakened, but persisted after adjustment for BMD. Additionally, the patients with both increased bone resorption markers and decreased bone formation makers were at higher risk of fracture than the patient with either one of 2 predictors.[7,27,28,29] These studies suggested that the index of BTMs provides the information on the risk of fracture independently of BMD, therefore, the risk of fracture can be assessed and supplemented with BTMs if DXA is not available.[28]

Nonetheless, uncertainties over their clinical use still remains, and the routine use of BTMs to assess the fracture risk is not recommended. Despite with the association between BTMs and the risk of fracture, the previous studies have shown inconsistent findings due to their large variability.[30,31] The difference in the predictive value of the various BTMs were noted in many studies and the reasons for this discrepancy are not clear. Another critical flaw is that a common approach to statistically analyze the results of BTMs in the prediction of osteoporotic fracture is odds ratios of fractures per standard deviation of increase in BTMs (e.g., the risk of fracture in patients with higher values is compared with that with lower values).[28] The use of odds ratios is not ideal for clinical decision-making in predicting fractures, so it is not easy to apply the results to the general population.[32]

Therefore, the measurement of absolute risks, such as 10-year probabilities, should be evaluated in a further study with large population. Another possible avenue remains the development of international reference standards and standardization of their measurement that may help minimize or eliminate old problems.[33,34]

Monitoring of BTM at an individual level may improve the compliance of patients on anti-osteoporotic treatment.[47] The IOF has proposed that a BTM such as PINP or CTX-I measured within 3 months of starting therapy would help to identify those with poor adherence which occur commonly with osteoporosis therapy, especially oral BPs.[48] The absence of change in the BTMs concentration during anti-osteoporotic therapy might reflect poor compliance (e.g., BP not taken at all, or not taken in the fasting state, or taken with milk), poor adherence, inappropriate technique of teriparatide injection, a medical problem (e.g., recent vertebral fracture), secondary osteoporosis, or real absence of response.

According to recently published Consensus Statement in Asia-Pacific region,[49] in patients who are receiving antiresorptive therapies, serum CTX-I and/or PINP can be used to monitor compliance and drug response with measurements at baseline, 3, 6, and 12 months after starting treatment. In patients who are receiving anabolic therapies, serum PINP can be used to monitor compliance and drug response, with measurements at baseline, 1 to 3, 6, and 12 months after starting treatment.

However, further studies are necessary to define an adequate response through the change in BTM concentration that is associated with a greater reduction in fracture incidence. The reference range of BTMs concentration for the adequate response could be helpful when used as target for anti-osteoporotic treatment.

BPs are considered as a first-line treatment for osteoporosis. It is effective in reduction of fracture risk over 3 to 5 years of treatment. However, long-term use of BPs is associated with the rare but serious adverse effects such as osteonecrosis of the jaw (ONJ) and atypical femoral fractures (AFF). Therefore, the concept of a ‘drug holiday’ from BPs has emerged as being potentially beneficial in avoiding these adverse effects. The effect of BP such as anti-resorption and fracture prevention persist for a long period after cessation of drugs because of long-term deposition of BPs in bones. However, despite BP has long skeletal retention time, fracture risk will increase after discontinuation of it. Therefore, continuous monitoring during drug holiday will be needed for the assessment of fracture risk and to restart treatment if needed. There are limited clinical practice guidelines on BTM monitoring during drug holiday. There are few studies about an assessment of fracture risk using BTMs during drug holiday. A recent retrospective study showed that those in newly developed fracture group during drug holiday was older and had lower BMD than non-fracture group. Other factors associated with fracture such as BTMs, vitamin D level, body mass index, and PTH were not significantly different between 2 groups.[50] Another study presented that BMD by DXA, BTMs (CTX-I, PINP), and fracture-risk assessment tool score may help to guide when to resume treatment during drug holiday.[51] Some researchers suggested that empirical approaches such as monitoring BMD and BTMs are necessary during drug holiday.[52] Several meta-analyses presented that increased bone turnover is a significant determinant of fracture susceptibility.[53,54,55] In one guideline, they suggested that the increase in bone resorption markers compared to the levels before treatment might be a signal to stop the “holiday”.[56] During drug holiday, a significant decrease in BMD or a significant increase in BTMs suggests that the residual effect of BP therapy may be diminishing and that it may be time to resume active therapy.[52,57,58] Roberts and colleagues [51] recommended that if BTMs rise over 30% after drug holiday, treatment should be resumed.

Prolonged BP therapy has been related with rare adverse events such as ONJ and AFF. ONJ is a rare oral disease in which the jaw bone's healing power is impaired because of use of BP or other antiresorptive agents. Patients may be considered to have ONJ if they had current or previous treatment with antiresorptive or antiangiogenic agents and as if there is exposed bone in the jaw that has persisted for more than 8 weeks but without history of radiation therapy or obvious metastatic disease.[59] A number of case reports have suggested that long-term treatment with BPs may be related to the occurrence of fractures of the femur below the lesser trochanter, which was regarded as an atypical based on their unusual site and radiographic appearance in patients with osteoporosis.[60] These rare but serious side effects were supposed to be related to the severely suppression of bone turnover. Therefore, several biochemical markers have been used as effective evaluation methods during the bone-remodeling period. In one cross-sectional study, 4 BTMs including serum CTX-I, osteocalcin, PTH, and BSALP were compared between medication-related ONJ patients and control groups. Serum CTX-I, osteocalcin, and BSALP levels were lower and serum PTH level was higher in ONJ patients than in control groups.[59] Another study showed that high CTX-I level was associated with a low risk of ONJ in patients who was treated with BPs and had a tooth extraction.[61] BSALP level over 10 µg/L denoted faster healing and indicated a better prognosis in ONJ patients.[62] Some dentists have suggested CTX-I measurement for the prediction of ONJ. If patients' CTX-I level is lower than 0.100 to 0.150 ng/mL, the risk of ONJ is high, therefore, drug holiday or having surgery after recovery of CTX-I level could be recommended.[63,64] One prospective cross-sectional study suggested that the levels of BTMs including PINP, tartrate-resistant acid phosphatase 5b (TRACP-5b) and undercarboxylated osteocalcin were significantly lower in patients with AFF than in patients with typical osteoporotic femoral fracture. They suggested that severe suppression of bone turnover was associated with the pathogenesis of AFF.[65] Systematic review of case report and case series on AFF presented that bone formation marker was decreased in 14%, normal in 79%, and increased in 7.0% in AFF patients. And bone resorption marker was decreased 18.2%, normal 69.7%, and increased 12.1% in AFF patients.[60]

Because of urinary and some serum BTMs are excreted into urine by kidney, the serum level of those are affected by renal dysfunction. BSALP, intact PINP, and TRACP-5b are not affected by renal impairment.