When should you get another DXA scan after your first one? There are no cut and dried recommendations because the decision needs to be individualized. A lot depends on your expected rate of change.
In general, if you started an osteoporosis medicine, a follow-up bone density is recommended after two years of therapy. The improvement with treatment is expected to increase bone density, particularly at the lumbar spine site where the greatest changes are observed. These changes will exceed the small variability of the DXA machine.
Because most osteoporosis medications are so effective, there is some debate about whether a follow-up bone density is even needed after starting medication. However, I believe that it is important to know whether you are responding with favorable bone density changes or stable bone density instead of bone density loss.
If no treatment is indicated after your first DXA scan, the follow-up is dependent on the results of your first DXA and the expected change. For example, if you are early menopausal, the rate of change may be rapid, from two to five percent a year, on average. If your baseline DXA is in the normal range, you will not be losing bone at a rate significant enough to make a difference in your fracture risk. The follow-up period could be three or four years.
On the other hand, if you start out with a lower measured baseline DXA, you have less to lose. The same rate of bone loss from a smaller total amount of bone could make a difference and place you in a higher risk category. The follow-up period should be shorter and a follow-up scan would be appropriate in two years.
Also, all other risks need to be factored in. Are you taking a medicine that accelerates bone loss like aromatase inhibitors or steroids? If so, your physician may want to obtain bone density tests at one-year intervals to monitor the effects of an FDA-approved osteoporosis drug or to closely follow your bone density if you are not on medicine for your bones.
Your insurance coverage may have a say in the answer as well. Medicare coverage is set for two years. Most commercial insurers are in line with Medicare coverage. For a shorter interval, your doctor may need to file an appeal to get approval for the test.
Bottom line, there is no set answer. The follow-up interval depends on many factors. Talk with your doctor about the appropriate time to have your next DXA scan.
SAME PLACE, SAME MACHINE
Repeating your DXA scan is like using a scale to weigh yourself. You always try to use the same scale at the same time of day to monitor change. To know if there is a change in bone density, you need to use the same device. However, other factors may come into play:
- Your insurance coverage or health plan changes.
- You move to a different community.
- The DXA center is no longer in business.
- The DXA machine may be upgraded to a new machine or replaced with a different manufacturer's machine.
- Your doctor sends you to a different DXA center within the same system.
These barriers are not insurmountable. In the case of upgrade to new equipment from the same manufacturer, most centers do a special evaluation in order to have the new machine calibrated with the old one. Whenever possible, try to go to the same place for a repeat DXA, even if a new clinic location is more convenient.
If the same exact machine is not used, you cannot make a direct comparison and certainly cannot calculate the rate of change. Each manufacturer has proprietary acquisition of data and software. Even the region of interest may vary. For instance, measurement of the neck region of the hip is derived differently by each manufacturer. There are also differences in the placement of the femoral neck box, detection of the bone, and method of dual-energy production.
Nevertheless, if on the same day you had a hip scan on two different manufacturers' machines, each one would yield similar results. However, you cannot compare change in bone density over time with different machines.
DO NOT COMPARE T-SCORES
The most common error in interpreting results is comparing T-scores to monitor change. The correct approach is to monitor the absolute change in bone mineral density expressed as g/cm2. The physician who reads your DXA should look at the absolute change in BMD. The change is usually calculated for the lumbar spine and the total hip. If both hips are scanned (dual femur), sometimes the “mean” (or average) of both hips is used. Measurement of the small area of the neck region of the hip (femoral neck) is not as precise, so calculation of change usually is not based on the neck region.
Is this a significant change? The interpretation is based on the variability of the individual machine. Did your change exceed the variability of the machine, which is called the least significant change or LSC? The report should indicate if your measured change is “statistically significant” based on exceeding the LSC. If your bone density change is statistically significant, it means that a biologic change happened that is not by chance and is real. Unfortunately, the calculation of the LSC by DXA centers is commonly not performed. Therefore, a center that has not calculated this value cannot accurately tell if your bone density differences are real.
GAIN, LOSS, OR NO CHANGE?
Although everyone wants to see a big increase in his or her bone density, don't be discouraged if your bone density is stable. The response to therapy is defined as either no change (stable bone density) or a significant increase. Since loss is expected without treatment, no change is good. Small changes are usually not significant. This is also good because it means that your bone mass is stable. Here are several examples:
Increase in BMD
The total hip BMD increased significantly after two years of therapy.
Stable BMD
In both of these examples, the bone density is stable. The hip example uses the mean (or average) of both total hips, which shows a small increase that is not statistically different from the baseline scan.
The example of the lumbar spine shows a lower BMD. However, this is not statistically significant (the decrease of 0.014 g/cm2 did not exceed the variability of the machine, which is 0.018 g/cm2). Although the number is lower, it is not actually different.
Decrease
This hip example has two follow-up scans. On the first follow-up a significant decrease (0.077 g/cm2 or 10.8 percent loss) was observed over about three years. With treatment, a one-year follow-up showed stable bone density, no change.
If a significant loss occurs while on therapy, a further evaluation should be conducted to look for previously unrecognized problems. Something new that was not present initially may have occurred to cause bone loss. You should also review with your doctor that you are properly taking your prescribed medicines.
If you are not taking medicines and have significant bone loss, your risk of fracture increases. Higher levels of bone turnover increase risk of fracture independent of bone density. If losses exceed expected age-related changes, an evaluation should be done to look for other causes beyond age. If you are not taking osteoporosis medicine, a reassessment of risk using FRAX should also be done.
DYNAMIC USE OF FRAX
FRAX helps determine the risk of fracture if you have low bone density with a T-score between -1.0 and -2.5 and have not taken osteoporosis medicines. The more risk factors, the greater the risk. For example, Karen had her first DXA at age sixty. On her two-year follow-up, her bone density was stable but her risk factors had dramatically changed.
At age sixty, Karen had low bone density. The lowest T-score of -2.1 was at the femoral neck region. Using FRAX, she had no positive responses for any of the risk factors. Her ten-year probability of major osteoporotic fractures was 9.4 percent and for hip fracture it was 1.4 percent. Her only medicine was a blood pressure pill. Otherwise, she had no other problems. She focused on exercise, calcium, and vitamin D, then returned for a two-year follow-up DXA.
The good news: her bone density was stable. The bad news: her eighty-six-year-old mother had fallen and fractured her hip. Although her bone density did not change, her probability of fracture doubled as a result of her parent sustaining a hip fracture. Her ten-year probability of major osteoporotic fractures had increased to 20 percent. A parent breaking a hip becomes a very strong predictor of future fracture.
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The Bare Bones
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