The C-terminal telopeptide (CTX), also known as carboxy-terminal collagen crosslinks, is the C-terminal telopeptide of fibrillar collagens such as collagen type I and type II. It is used as a biomarker in the serum to measure the rate of bone turnover. It can be useful in assisting clinicians to determine a patients nonsurgical treatment response as well as evaluate a patients risk of developing complications during healing following surgical intervention.  The test used to detect the CTX marker is called the Serum CrossLaps, and it is more specific to bone resorption than any other test currently available.