TNFRSF11A and chronic obstructive pulmonary disease: No significant difference existed in the level of OPG and RANK among the three groups, but the level of RANKL (Figure 3) and the ratio of RANKL/OPG (R/O) (Figure 4) were remarkably higher in COPD patients with low BMD compared to those with normal BMD and the normal controls (p < 0.05).