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A bone density test—also referred to as a DXA scan or a bone mineral density (BMD) test—is the current way we diagnose osteoporosis. The test measures the QUANTITY of bone at specific locations in the body, namely the mid-spine bones (lumbar region) and the upper end of the thighbone where it connect to the pelvis (total hip and its subdivisions – neck, trochantor). These locations are used for two primary reasons: 1) they have a large quantity of trabecular bone – the spongy, lattice-like bone on the inside- that tends to lose density as we age; and 2) measurements taken at these locations can be easily replicated so that we can determine if there are changes in the quantity (density) of bone over time.
The quantitative results of the test are measured in grams and then converted to a T-score to compare your results with what we might expect them to be. (See “what does my score mean”). Your T-score, along with other risk factors, helps your doctor assess how likely it is that you will break a bone in the future. Armed with this information, your T-score and your 10-year fracture risk, you can have a conversation with your doctor about both treating and preventing bone loss.
It is very important to keep all of your BMD test results so you can track your bone health as you age.
The machine used to measure bone density consists of a twin-bed-size padded table with a half-moon arm that passes above your body as you lay face up. The scanner uses a very small x-ray beam to get an image of the bone. A technician or technologist will adjust your position on the bed to view the lumbar spine and the hip. The technician will then use a computer to analyze the image and determine the bone density. All completed scans are then read by a trained physician to ensure accuracy of the positioning and analysis before the final report is sent to you and your doctor.
Generally your doctor will ask for a DXA of the spine, left hip, right hip or both. It is the recommended protocol from the International Society for Clinical Densitometry (ISCD) to scan two different locations to confirm a diagnosis.
If your doctor suspects that you may have a spine fracture, she may also order an image of your spine from the side. This Vertebral Fracture Assessment, or VFA, helps your doctor confirm if you have a fracture and determine its severity. This information will help you and your doctor make decisions about how to treat the fracture and prevent other fractures.
There are times when the technician or technologist has challenges getting proper images of your bones at two locations. If you have a hip replacement, the technician will scan your opposite hip. If you have had both hips replaced, the technician will likely scan your forearm. On occasions when a patient has a diagnosis of hyperparathyroidism, the technician will scan the forearm because this condition can cause loss of the kind of bone that is found in the forearm. In all cases, the technician will make every effort to scan two locations on your body.
Since changes to bone happen relatively slowly, depending on your initial test results, you may not need another DXA for at least two years. On the other hand, if your doctor is concerned that you may be losing bone quickly or is putting you on a treatment that acts quickly, you may need to get retested sooner. If you are getting re-tested within two years of your last test, be sure to check with your insurance company about coverage.
For you and your doctor to follow potentially small changes in bone density is difficult. The best thing you can do is return to the same DXA machine where you were originally tested. Going back to the same machine increases the likelihood that your previous images are on the machine and that the technician can create a better replication of your previous scan. If returning to the previous machine is not possible, always keep your test results and ask your doctor for the images. Bring the images with you when you are retested. This will help the technician position you to measure the same location as your previous DXA.
Even when you return to the same machine and the same technician, there is bound to be slight variations in the repeat testing. We call this “least significant change”. At good DXA centers, the technicians test their ability to replicate the positioning of patients every few years. To do this, they get patients to volunteer to be scanned, then get up off of the table and then be scanned again. Least significant change will vary from DXA center to DXA center, but can run from about 3% to 5%. What this means for you is that an increase or decrease in bone density must exceed the least significant change at the DXA center. For example, if your results indicate that your bone density decreased by 3% since your last scan, and the least significant change for the DXA center is 4.3%, the physician reading your report would not say that you had lost bone density. You would need to lose more than 4.3% of your bone mass for it to be considered a significant loss.
You may have seen smaller machines at a health fair or drug store that measure the density of the bones in your wrist or heel. These machines are typically used for screening and can provide you with helpful information about what next steps to take. You should take the screening results to your next doctor’s appointment and talk about whether you should have a central DXA scan.
DXA scanners use a very low level of radiation—about 1/10 of the radiation in a chest x-ray or what you’d be exposed to on a airplane flight from California to New York. In our DXA center, for example, we have had a meter to detect radiation attached to the DXA machine for one year and it never registered any radiation. Bone density testing is a very safe test.
The DXA test takes 10-15 minutes and is painless. And as long as you are not wearing any metal on your clothing, like zippers and underwire bras, you can keep your clothes on! Your job is to relax on the padded DXA table. The technician will help adjust your legs to get the best image of your spine and hips. As with any x-ray, you will need to keep still for accurate results.
Bone density is measured in grams. To make test results both standard and understandable, DXA test results are usually reported using a statistical formula that compares your results to others in an enormous database of results - either using a “T-score” or “Z-score”. A T-score compares your bones to the bones of a 30 year old healthy person of your gender and ethnicity. A Z-score compares your bones to the bones of individuals of your own age, gender, and ethnicity. It is recommended by the ISCD that we report Z-scores for all premenopausal women and T-scores for all postmenopausal women.
The World Health Organization came up with a way to classify BMD results by giving a 30 year old a T-score of 0. If your T-score is -1.0, your bone density is one standard deviation (SD) below the 30-year old. If your T-score is lower than 1 SD below a 30-year old, you are considered to have low bone mass. And if you are 2.5 SD below a 30 year old, then you are classified as having osteoporosis.
T-score at -1.0 and above (therefore more bone)
Low Bone Mass
|T-score between -1.0 and -2.5|
T-score at or below -2.5
|Severe Osteoporosis||T-score at or below -2.5 and one or more adult fractures|
The higher the negative number,
the lower your bone density,
and the greater your risk of fracture.
KNOW YOUR T-SCORE!
We are now calling osteopenia low bone mass. It is not “pre-osteoporosis”. There are several things to think about if you are diagnosed with low bone mass. Just because you have a BMD score of low bone mass doesn’t mean that you are necessarily losing bone mass. Perhaps you never reached a peak bone mass that was any higher. For example, if you were not a milk drinker, or you were a young athlete who didn’t start her period until after 15, you may not have built bone into what we are now calling the “normal” range.
Now that doesn’t mean that you shouldn’t be paying attention. If you have low bone mass, you will want to preserve as much as you can as you age by having a calcium rich diet, vitamin D and good weight bearing exercise. For women with osteoporosis, this is where understanding fracture risk is so helpful. Go to http://www.americanbonehealth.org/tools-and-resources/risk-calculator, print the results, and take them with you to your next doctor’s visit.