Quick and painless, bone scans are essential screening tests for assessing bone health. They can uncover the start of bone loss, called osteopenia, or the more advanced osteoporosis, letting you know when you might need treatment.

Dual-energy X-ray absorptiometry, often referred to as a DXA or dexa scan, has long been the gold standard screening test to measure bone mineral density (BMD). But, in recent years a different screening tool, called quantitative computed tomography (QCT), has replaced DXA in some states. Here’s what you need to know about these tests and which one is appropriate for you.

DXA details

DXA provides precise measurements of bone mineral density at important sites—the spine, the hip and, when needed, the forearm—with minimal radiation. Results are most often given as T-scores, which show how your bone density compares to a 30-year-old.

The lower your T-score, the greater your fracture risk.

  • Normal bone mineral density is a T-score between +1 and -1.
  • Low bone mass (osteopenia) is a T-score between -1.1 and -2.4. Having lower-than-normal bone density puts you at a higher risk of developing osteoporosis. Your doctor may suggest adding calcium and vitamin D to your diet and doing weight-bearing exercise.
  • Osteoporosis is a T-score lower than -2.5. Often this means you’ll be prescribed medication. Along with T-scores, your FRAX (Fracture Risk Assessment Tool) score will help determine your 10-year risk for a major fracture. The FRAX assesses 12 factors, like your age, weight, and whether you smoke, drink alcohol, take steroids, and had a fracture or a parent who had a hip fracture.

In some cases, people under age 50 may also be given a Z score that compares bone density to other people of the same age.

Questions about QCT

Quantitative computerized tomography is a type of CT scan that uses a 3D image that contrasts the outer cortical bone and the inner spongy trabecular bone (DXA is 2D).

It can be appropriate for people in very specific circumstances. For instance, it may offer more detail than DXA for people with a very low or very high BMI, with a severe degenerative disease of the spine, or when results of a DXA are inconsistent. That means it may be the right test for people with rheumatoid arthritis, scoliosis, or another disease that causes the body to put down more bone and artificially elevate the bone density calculation. (One clue this may be happening to you is if your spine shows an increase in calcium from a previous test, but your hip score is in the osteopenia range.)

According to the American College of Radiology, QCT can be used as a tool to diagnose and manage osteoporosis and other diseases that affect BMD because it can help assess how well you’re responding to treatment. While QCT has its place, it may not be the right BMD test for the general public for a few reasons:

Radiation exposure. QCT delivers between 1,000 and 3,000 times the radiation of DXA. Women having a scan of their hip are getting that radiation across the entire pelvis, including the ovaries and other sensitive structures. If you need to retest every two years, that extra radiation adds up.

Misinterpreted results. Endocrinologists must not use the T-score to interpret QCT results. The QCT measures more of the spongy area of the bone, whereas DXA focuses on cortical bone. Looking at results the same way is like making an apples-to-
orange comparison. If a T-score given by a DXA of the spine is -2.0, it’s likely that the QCT T-score on the same person would be -2.7, showing more bone density loss than actually exists. The American College of Radiology recommends using an absolute value of <80 mg/cm3, and not a T-score, when results are being evaluated.

If your doctor orders a QCT even though you’re not in a situation that warrants it, and your results indicate osteoporosis, consider getting a second opinion and a DXA before going on any bone medications.

Cost. QCT is much more expensive than DXA. That could be costly if you have a deductible or co-pay. QCTs can also be a strain on Medicare, which reimburses for the test at a much higher rate than it does for a DXA.  

Osteoporosis is Underdiagnosed

Osteoporosis is underdiagnosed in the United States because people are missing out on BMD screenings in general. With osteoporosis, you don’t have any symptoms until you break a bone, and that can have devastating consequences. Testing is the only way to know if you have bone loss before a break occurs.

Since the Deficit Reduction Act of 2007 cut reimbursement rates for DXA scans, there’s been a sharp drop in the number of procedures being done. “Increasing Access to Osteoporosis Testing for Medicare Beneficiaries Act of 2021,” a bill to improve DXA payments, has been in front of Congress for nearly two years. Its passage should make testing more accessible.

In the meantime, talk to your healthcare provider about BMD testing, especially if you have a family history of osteopenia or osteoporosis, have any health condition that can cause estrogen deficiency, are frail, underweight, or a smoker. The recommendation to start osteoporosis screening is age 65 for women and age 70 for men, but based on health factors and family history, your doctor may suggest it sooner.

IMPORTANT: Whenever you need a follow-up BMD study, try to do it at the same facility as the prior one. There are different models of DXA instruments, and the results are easier to compare if they have been taken on the same model.

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