A bone density scan can detect thinning bones at an early stage. If you already have osteoporosis, bone scans can also tell you how fast the disease is progressing.
But an abnormal bone scan can create as many questions as it answers. Who should get a bone density scan, and what do the results mean? If your bone density is below normal, what can you expect, and what should you do?
A Date With DEXA
Most bone scans use a technology called DEXA (for dual energy X-ray absorptiometry). In a DEXA scan, a person lies on a table while a technician aims a scanner mounted on a long arm. (Think of the machine that X-rays your teeth at the dentist; the difference is that this test uses very low energy radiation.)
"DEXA currently is the easiest, most standardized form of bone density testing, so that's what we use," says Mary Rhee, MD, MS, an endocrinologist and assistant professor of medicine at Emory University in Atlanta.
The DEXA scanner uses beams of very low-energy radiation to determine the density of the bone. The amount of radiation is tiny: about one-tenth of a chest X-ray. The test is painless, and considered completely safe. Pregnant women should not get DEXA scans because the developing baby shouldn’t be exposed to radiation, no matter how low the dose, if possible.
Measurements are usually taken at the hip, and sometimes the spine and other sites. Insurance or Medicare generally pays for the test in women considered at risk for osteoporosis, or those already diagnosed with osteoporosis or osteopenia.
Other less commonly used technologies can measure bone density. They include:
- Variations of DEXA, which measure bone density in the forearm, finger, or heel.
- Quantitative computed tomography (QCT). Essentially a CAT scan of the bones, QCT provides more detailed images than DEXA.
- Ultrasound of the bones in the heel, leg, kneecap, or other areas.
While all of these can determine bone density and osteoporosis risk, "DEXA is the most important test and is the gold standard," says Felicia Cosman, MD, clinical director for the National Osteoporosis Foundation.
Interpreting Your DEXA Bone Scan Results: T-Scores and Z-Scores
DEXA scores are reported as "T-scores" and "Z-scores."
- The T-score is a comparison of a person's bone density with that of a healthy 30-year-old of the same sex.
- The Z-score is a comparison of a person's bone density with that of an average person of the same age and sex.
Lower scores (more negative) mean lower bone density:
- A T-score of -2.5 or lower qualifies as osteoporosis.
- A T-score of -1.0 to -2.5 signifies osteopenia, meaning below-normal bone density without full osteoporosis.
Multiplying the T-score by 10% gives a rough estimate of how much bone density has been lost.
Z-scores are not used to formally diagnose osteoporosis. Low Z-scores can sometimes be a clue to look for a cause of osteoporosis.
DEXA Bone Scans: What Your T-Score Means
Being told your bones are thin is cause for concern, but not alarm. If your T-score is low, what can you expect?
First of all, unless you're a woman past menopause or a man older than 50, your risk of fracture is very low. In these groups, even with a T-score less than -2.5, bones are usually strong and treatment isn't recommended.
On the other hand, if you've been told you have osteoporosis, take it seriously. Feeling fine is no protection at all: fractures of the spine can be silent and painless. "Anyone with osteoporosis should be on some kind of treatment," according to Baker.
For those with osteopenia (T-score between -1.0 and -2.5), the picture gets confusing. It's harder to predict fracture risk in this group of people. Focusing too closely on the T-score can be a mistake. "The DEXA T-score is not a perfect predictor for bone health or fracture risk," says Rhee.
Actually, bone density (measured by T-score) is only one aspect of fracture risk. Your risk factors (see above) can be just as important. Using both the T-score and risk factors for fracture leads to better predictions.
The World Health Organization is developing a formula using risk factors in combination with the T-score to determine 10-year fracture risk. "We'll probably see this coming into use in the next few years," says Rhee.
Bone Scan T-Scores: When It's Time to Treat
The National Osteoporosis Foundation recommends treatment for:
- Postmenopausal women with T-scores less than -2.0, regardless of risk factors.
- Postmenopausal women with T-scores less than -1.5, with osteoporosis risk factors present.
In addition, anyone with a fragility fracture (a fracture from a minor injury) should be treated for osteoporosis. This is true regardless of the DEXA scan results.
Treatment generally begins with a bisphosphonate medicine (Actonel, Fosamax, Boniva, or Reclast). These drugs are proven to increase bone density and reduce the risk of fracture. Other options include:
When Should You Get a Bone Density Scan?
When, and how often, you should get a bone density scan depends on your age, risk factors, and whether you’ve already been diagnosed with thinning bones.
The general rule: anyone at risk for osteoporosis should get a bone density scan. Don’t wait for a fracture or a formal diagnosis.
Postmenopausal women are at highest risk, because estrogen (which falls after menopause) preserves bone strength. But men get osteoporosis, too. "They just get it later," says Mary Zoe Baker, MD, an endocrinologist and professor of medicine at the University of Oklahoma Health Sciences Center. Usually around age 70, "men start to catch up to women" in developing osteoporosis, according to Baker.
Major expert groups make the following recommendations for osteoporosis screening and bone scans:
Women over age 65: All women over the age of 65 should get a DEXA scan, according to the National Osteoporosis Foundation and the U.S. Preventive Services Task Force.
Postmenopausal women under age 65: For women under 65, a bone scan is not universally recommended. The National Osteoporosis Foundation recommends a bone scan for women with risk factors for osteoporosis:
Premenopausal women: Generally, premenopausal women should not get bone scans. Even with an abnormal DEXA scan, the risk of fracture is still very low, and treatment isn't recommended. "The No. 1 rule is, don't get the test unless you know you're going to treat" if the result is abnormal, says Baker.
Men: Experts differ in their recommendations for bone scans for men. The National Osteoporosis Foundation recommends all men over the age of 70 should get a bone scan. At that age, "many men are on their way to developing osteoporosis," says Cosman.
Bone Scans for Osteoporosis: How Often?
If you've been told you have thin bones, you'll want to know if they're improving or getting worse over time. How often should a bone scan be done?
Medicare and many insurance companies will pay for a bone scan every two years in women with osteoporosis or who are at high risk. Because the response to treatment occurs slowly, this is usually an acceptable time interval, according to Rhee.
"In cases with high bone turnover rates, like women taking high-dose steroids," checking bone density as often as every six months may be necessary, says Rhee.
For women with a normal bone scan, waiting a few years to retest is fine, adds Rhee.
Another thing to keep in mind: not all DEXA scanners are created equal. There are slight differences in the calibration of different manufacturers' machines. Ideally, you should get all your bone scans on the same DEXA scanner. Getting retested on a different manufacturer's scanner could give a false impression of bone loss (or gain).
Besides the Bone Scan: Other Tests for Osteoporosis
Are other tests needed besides a bone scan for osteoporosis? Certain medical conditions can cause thinning of the bones. These include:
- Kidney disease
- Hyperparathyroidism (overactive secretion of parathyroid hormone)
- Vitamin D deficiency
- Hyperthyroidism (overactive thyroid)
- Liver disease
- Intestinal disease
By taking your medical history and checking routine laboratory blood tests, your doctor can detect these and other causes for low bone density.
Since estrogen keeps bones strong, can getting your estrogen levels checked help? "Probably not," says Baker. Rarely, perimenopausal women with heavy periods might need hormone checks. But for the vast majority, "DEXA is the only test they need."