cs53,
bone turnover markers measure particles in the serum that tell you if you are adding or resorbing bone. It tells you if a medication is working for you. Some suffer the side effects of a drug one or two years (until their next dexa only to find out that the medication wasn't effective.
CTX and P1NP each measure a specific fragment left in the blood during each process. It is best measured before the drug and two or three months after beginning to take the drug.
With drugs that work by preventing resorption of the bone CTX is the fragment that should be lowered. With drugs that work by building bone P1NP is the fragment that should rise while taking the anabolic.