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?
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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.