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).
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). Alignment of the choice of biomarkers across future bone health studies may allow for cross-study comparison or future meta-analyses.