Hi @elisabeth007 - A description of the CTX test is here https://www.mayocliniclabs.com/test-catalog/overview/83175#Clinical-and-Interpretive. The ranges that are listed are based on the measurements of healthy people, about the middle 90%. It's not an optimal range, just a range where most health people reside.

Like @normahorn said, you really need to have both CTX and P1NP to get a good picture of what's going on. They are linked.