The gold standard for diagnosing osteoporosis has long been dual-energy X-ray absorptiometry (DXA scan). It is often called a bone density test or bone mineral density (BMD) test. A DXA measures the quantity of bone in at least two locations on the body, usually the lower spine and one or both hips. The results of a DXA scan are converted to a T-score that compares an individual’s bone density to that of an average 30-year-old woman. DXA is inexpensive, it provides valuable results, and it delivers a very low dose of radiation.
Recently, however, a growing number of patients have been getting quantitative computed tomography (QCT) scans to test their bone mineral density (QCT is a technique for testing bone; it is distinct from other types of computed tomography that you may be familiar with.) A recent investigative report in MedPage Today (subscription required) reviewed Medicare claims for QCT studies of bone density. It found there were 32,694 such claims submitted by providers between 2015 and 2019. The number increased each of those years. The article quoted doctors who are concerned that QCT is less accurate, more expensive, and delivers a much higher dose of radiation than DXA.
American Bone Health shares those concerns. Let’s look at each of them in more detail.
Accuracy
DXA measures the bone mineral density of both the outer (cortical) shell and the inner (trabecular) structure of the bone. QCT, on the other hand, measures only the inner part. Because of this difference, QCT studies of the spine show a lower T-score than a DXA would. People could be diagnosed incorrectly with osteoporosis based on the lower T-score from a qCT scan. It overstates how much bone they have lost. They could be prescribed treatments that they don’t need.
For scans of the hip, QCT results are comparable to DXA.
There is another reason that QCT is less accurate than DXA. For DXA, individual bone scans are compared to a national standard that reflects changes over time in the U.S. population. That means there is more data for comparing an individual’s results with the average U.S. population. The accuracy of DXA is improved if you are scanned on the same machine as your prior scan.
Cost
Citing the American College of Radiology and Medicare physician resources, Medpage Today reported that Medicare pays almost three times more for QCT in a physician’s imaging center than for a DXA. After Medicare reimbursement rates for DXA were cut in 2007, many physicians stopped offering the service and removed the testing machines from their offices. This trend might have contributed to a reversal of the decline in hip fracture rates, according to a 2017 analysis of Medicare data.
Radiation
DXA uses a low-dose X-ray to scan the bones. The procedure delivers only about one-tenth the radiation of a standard chest X-ray. You might be exposed to more radiation on a cross-country airline flight. QCT, on the other hand, “delivers radiation that is approximately 1,000 to 3,000 times greater than a radiation from a DXA,” MedPage Today reported, citing the American College of Radiology.
What you should know
DXA remains the gold standard to test patients for osteoporosis. QCT is most often used to assess bone mineral density for research purposes. If your health care provider is recommending a QCT instead of a DXA to evaluate you for osteoporosis, be sure to ask them why they are making that recommendation.
The U.S. Preventive Services Task Force recommends DXA for all women 65 and older, and for women younger than 65 who have increased risk factors for osteoporosis. These risk factors include having certain medical conditions, like diabetes, and taking certain medicines, like corticosteroids. The Endocrine Society recommends DXA for all women who are at risk for osteoporosis or who have broken a bone due to a low impact. Some experts feel it is important to obtain a DXA at a younger age to establish your normal baseline bone density.
If you are a woman younger than 65, or a man, private insurance will cover a DXA if you have at least one risk factor. Your health care provider might use a tool called FRAX to assess your risk factors. The American Bone Health Fracture Risk Calculator™ is designed for consumers 45 and older to learn about their risk factors and their risk of breaking a bone in the next 10 years. Discuss your Fracture Risk Calculator results with your healthcare provider, and ask if a DXA is right for you.
Posted: 06/22/2022
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