Reader Question: My doctor who has been treating me for osteoporosis told me that I should stop taking my medicine because my ‘bone markers' were low. What does this mean? I am scared to go off my medicines. What is this test?
After the diagnosis of osteoporosis is made and one begins therapy, it is often difficult to determine if the treatment is working. This is due to fact that the current testing with a bone density, Dual Energy X-ray Absorptiometry (DXA) is often imprecise, (due to a multitude of technical factors - positioning, equipment, artifact, technicians etc). This limits the ability of the scans to show changes upon follow up bone density studies.
The Bone Remodeling Process
Another way that we can possibly follow how effectively osteoporosis medication is working is to try to measure the bone remodeling process. Remodeling is the normal physiologic process where cells called osteoblasts add bone and other cells called osteoclasts remove it. Up until age 20-30, osteoblasts add more bone than is removed, once again stating the importance of good dietary intake of calcium in the younger years.
Following this, up until menopause, there is a relatively equal amount of activity of the osteoblasts and osteoclasts, leading to a stabilization of bone mineralization. After one's menstrual cycles stop, the osteoclasts begin to remove bone significantly faster than bone is added, leading to increased bone turnover and loss of bone in the first 10 years after menopause. The most common osteoporosis medication class, the bisphosphonates (Fosamax, Actonel, Boniva and Reclast) inhibit the function of osteoclasts. This works to slow the remodeling process.
Practically speaking, bone biochemical markers can be checked to measure this bone remodeling process. There are bone markers that measure if an increase of bone activity is occurring and others that measure suppression of bone activity. For example, if one checks bone markers while on a bisphosphonate, it would be expected to show a significant decrease in these specific bone markers. This would show that the medication was successfully interacting with the osteoclasts to lower these bone cells activity level. This would make us believe that the medication was likely working.
These markers can be checked more frequently than a bone density, allowing us to follow the effectiveness of these agents more frequently than every 2 years.
The most common bone markers that are checked are N-terminal telopeptides - NTx levels, C-terminal telopeptides - CTx levels, and bone alkaline phosphotase. These tests are obtained from blood and urine.
Unfortunately, although these tests seem promising, there are some difficulties with them. First of all, the ability of these markers to predict fractures has not been fully established. Therefore, these bone markers cannot be used as a substitute for DXA testing. Also, there is some variability in the results of these tests as they have to be performed under specific conditions. This entails performing the testing after fasting and early in the morning. Testing should also take place prior to starting therapy in order to monitor for changes in the future.
In conclusion, bone marker testing is a promising tool and may be used to assist in treatment decisions, however, care must be placed on ensuring that the test is done properly and the results are used in conjunction with bone density testing.
Published On: November 21, 2008