Special remarks: Osteoprotegerin (OPG), also known as osteoclastogenesis inhibitory factor (OCIF), is a cytokine and a member of the tumor necrosis factor (TNF) receptor superfamily. It is a basic glycoprotein comprising 401 amino acid residues arranged into 7 structural domains. It is found as either a 60 kDa monomer or 120 kDa dimer linked by disulfide bonds. Osteoprotegerin inhibits the differentiation of macrophages into osteoclasts and also regulates the resorption of osteoclasts in vitro and in vivo. Osteoprotegerin is a RANK homolog, and works by binding to RANK ligand on osteoblast/stromal cells, thus blocking the RANK-RANK ligand interaction between osteoblast/stromal cells and osteoclast precursors. This has the effect of inhibiting the differentiation of the osteoclast precursor into a mature osteoclast. Recombinant human osteoprotegerin specifically acts on bone, increasing bone mineral density and bone volume. Osteoprotegerin has been used experimentally to decrease bone resorption in women with postmenopausal osteoporosis and in patients with lytic bone metastases.
Since OPG exhibits an inhibitory effect on osteoclasts, it acts as a soluble factor in the regulation of bone mass. Osteoclast formation activity may be monitored principally by determination of concentration ratio OPGL/OPG. Alteratio n of this ratio may be the cause of bone loss in many imbalances in bone metabolism such as osteoporosis, osteopetrosis, hypercalcemia, metastatic osteolytic lesions and rheumatic bone degradation.
Osteoprotegerin production is stimulated in vivo by the female sex hormone estrogen, as well as the experimental osteoporosis drug, strontium ranelate.

The molecula weight of OPG as a glycosylated dimer molecule is 120 kDa. Therefore the conversion factor is 1 pMol/L = 120 pg/mL.