Biochemical Markers
Biochemical markers of bone turnover can be used as an adjunct to DXA in the assessment of bone health among patients with SCI. Serum and urine markers provide useful insight into bone metabolism at specific time points after injury and are an effective tool for selecting patients who would benefit from therapy and monitoring response to therapy. The current therapeutic utility of bone turnover markers is limited by day-to-day, diurnal, inter-individual, and inter-assay variability. For urine markers, results need to be corrected for creatinine (Reiter et al. 2007).
Markers of bone formation include bone-specific alkaline phosphatase (BALP), osteocalcin (OC), N-terminal propeptide of type I collagen (PINP), and C-terminal propeptide of type I collagen (PICP). Markers of bone resorption include urinary free and total pyridinoline (Pyr) and deoxypyridinoline (DPD) crosslinks, type 1 collagen C-telopeptide (CTX), and N-telopeptide (NTX). Pyr and DPD are molecules that provide stability to collagen and, along with CTX and NTX, are released when collagen is degraded during bone resorption (Brown et al. 2009).
Biochemical markers of bone turnover can be used as an adjunct to DXA in the assessment of bone health among patients with SCI. Serum and urine markers provide useful insight into bone metabolism at specific time points after injury and are an effective tool for selecting patients who would benefit from therapy and monitoring response to therapy. The current therapeutic utility of bone turnover markers is limited by day-to-day, diurnal, inter-individual, and inter-assay variability. For urine markers, results need to be corrected for creatinine (Reiter et al. 2007).
Markers of bone formation include alkaline phosphatase (ALP), bone-specific alkaline phosphatase (BALP), osteocalcin (OC), N-terminal propeptide of type I collagen (P1NP), and C-terminal propeptide of type I collagen (CINP). Markers of bone resorption include urinary free and total pyridinoline (PYD) and deoxypyridinoline (DPD) crosslinks, type 1 collagen C-telopeptide (CTX), and N-telopeptide (NTX). PYD and DPD are molecules that provide stability to collagen and, along with CTX and NTX, are released when collagen is degraded during bone resorption (Brown et al. 2009) (Table 5).
For a bone marker to be useful in assessing the rate of bone turnover and/or monitoring therapy effectiveness, the difference in the rate of bone turnover before and after SCI, as well as the early period versus the late period after SCI, needs to be discernible. Consensus regarding which biomarkers are best to monitor bone turnover is needed in the SCI community. Several authors have suggested candidate biomarkers including sclerostin (Morse et al. 2013) and adiponectin (Doherty et al. 2014). However, due to analytical discordance between the different assay kits) and biological variability (type 2 diabetes, estrogen level, parathyroid hormone etc.), diagnostic performance of these biomarkers has to be yet validated (Wheater et al. 2013, Liu et al. 2013, Durosier et al. 2013, Morris et al. 2017) (Tables 6-8).
The International Osteoporosis Foundation and the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC-IOF) Working Group for Standardization of Bone Marker Assays, and the National Bone Health Alliance (NBHA) recommend CTX and P1NP as reference bone markers to inform on fracture risk and efficacy of osteoporosis treatment (Vasikaran et al. 2011, Bauer et al. 2012, Johansson et al. 2014, Morris et al. 2017). This is due in part to their low inter-individual variability, relatively stable nature in serum at room temperature and current availability of reference intervals for these biomarkers for geographic regions and individual assays (Morris et al. 2017). In some clinical studies, however, the urine NTX marker could be preferred due to its lower sensitivity to circadian changes and food intake (Wheater et al. 2013).
Consensus regarding the choice of biomarker and the associated assay techniques are needed to cross-study comparison and future meta-analysis.
Alignment of the choice of biomarkers across future bone health studies is critical as it may allow for cross-study comparison, future meta-analyses, and to inform the development of SCI-specific normative datasets. Therefore, we suggest the use of CTX and P1NP bone turnover markers as a future minimum data set and harmonization of units for reporting CTX (ng/L) and P1NP (μg/L) as recommended by IFCC-IOF Bone Marker Standards Working Group (Morris et al. 2017). In addition, adipokines showed promising results; however, more studies are needed to determine their feasibility as primary biomarkers for bone health and osteoporotic fracture risk among individuals with SCI.
The selection of an appropriate analytic assay remains the main limitation of data harmonization as there is an apparent lack of comparability between particular assays, automated (Roche Elecsys/Cobas and IDSiSYS) or manual (Orion Diagnostica). To overcome this issue, the US Foundation of the National Institutes of Health is currently collecting data from all clinical trials in osteoporosis to perform an individual meta-analysis (http://www.fnih.org/what-we-do/current-research-programs/biomarkers-consortium-bone-quality-project) that would overcome the criticisms of inconsistent statistical methodology and small sample size (Morris et al. 2017).
