CTX (beta-CrossLaps)

Ranges depend on the assay, sex, age and menopausal status (some labs print up to 573 for premenopausal women and up to 850 for men over 70), and a fasting morning sample is assumed; use the range printed.

CTX, often printed as beta-CrossLaps, is a small fragment of collagen released into the blood when old bone is broken down and removed. It is a marker of how quickly bone is being resorbed, which is one half of the constant rebuilding cycle of the skeleton. The test measures the fragment in a blood sample.

Doctors use it mainly to follow how well a medicine for thinning of the bones is working, usually a few months after starting, and sometimes when deciding about a break from treatment. It is also used in kidney disease and Paget's disease and before some dental procedures.

• The years after the menopause, when bone breakdown speeds up
• Overactive parathyroid or thyroid glands
• Low vitamin D
• Reduced kidney filtering, which clears the fragment more slowly
• Paget's disease and cancer that has spread to bone
• Being bedbound or long-term steroid tablets
• A non-fasting or afternoon sample

• Bisphosphonates, denosumab and hormone replacement, which slow bone breakdown
• Fasting and the morning, as part of its daily rhythm
• Older age in men, by a small amount

• It has a strong daily rhythm and falls after eating, so a non-fasting or afternoon sample can read well above range
• It is cleared by the kidneys, so it reads higher when kidney filtering is reduced
• Results from different assays and different units (ng/L versus ug/L) are not interchangeable

Take fasting, in the morning, ideally between 8 and 10 am, and at the same time on each occasion. Follow within one lab.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.