CTX is a test of the activity of your Osteoclasts, which are bone cells. Osteoclasts chew up old bone, and Osteoblasts are new bone builders. Osteoblasts are commonly tested with a P1NP. Both are blood tests. Few physicians seem interested in these bone marker tests, in my personal experience, and if you do convince them to run the tests, they do not know how to interpret them. But really, no one should be taking ANY bone drug without knowing and monitoring those numbers, before, after and during taking the drug. All this is my own humble opinion.