In the case of osteoporosis, biochemical "markers" are chemical substances that indicate bone turnover. When osteoclasts (the cells of bone resorption; see Question 5) break down the collagen in bone, byproducts of this breakdown (for example, N-teleopeptide crosslinks [NTx]) are released into the bloodstream and excreted in the urine. When new bone is formed, byproducts such as osteocalcin and other substances also find their way to the bloodstream and get excreted in the urine.
By measuring the byproducts of bone breakdown (usually in the urine) and bone formation (usually in the blood), the rate of bone turnover can be determined. If bone turnover is very rapid, like it is in women following menopause, the quality of bone may be poor, thus increasing the risk for fracture.
Currently, blood and urinary biomarkers are not used to diagnose osteoporosis, but they can be helpful in assessing how fast bone is formed and broken down. BMD testing, while important for assigning fracture risk and measuring bone mass, does not provide information about bone turnover or bone quality. Tests for biomarkers do not give any information about bone mass; however, there is some evidence that the presence of bone breakdown biomarkers in urine is associated with an increased risk of hip fracture.
While the tests are relatively simple, there is not enough consistency between tests for them to be used widely, even for monitoring the course of treatment. Research has yet to confirm that biomarkers could be used routinely to monitor the effectiveness of treatment. There is some evidence based on urinary biomarkers, however, that individuals with high bone turnover have the best response to certain treatments and those with reduced amounts of the markers, which indicate bone resorption (break down) also had fewer vertebral fractures. Other research demonstrates that urine biomarkers can show bone's response to medications in as little as 6 weeks, which is much faster than the traditional follow-up BMD testing done after 1 to 2 years of treatment. The future hope is that medications for osteoporosis could be changed to different ones more quickly if bone weren't responding in the intended way, based on the urinary biomarkers.
If bone turnover is very rapid, like it is in women following menopause, the quality of bone may be poor, thus increasing the risk of fracture.
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