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What do Bone Density Test Results Mean?

Understanding Dexa (DXA) Test T Scores for Osteoporosis

Sep 12, 2009 Melissa Murfin

Bone loss due to osteoporosis is common as people age. Find out what T scores mean and how they relate to fracture risk.

According to the National Osteoporosis Foundation, approximately 10 million Americans currently suffer from osteoporosis. Another 34 million have osteopenia or low bone mass which is not as severe as osteoporosis. Both conditions increase risk of fracture and are diagnosed using bone mineral density test results called T scores. Once a patient has been diagnosed with osteoporosis, she is often treated with medications such as alendronate (Fosamax), risedronate (Actonel) or teriparatide (Forteo).

DXA Bone Density Test

A DXA, or dual-energy xray absorbtiometry, scan tests a patient's bone mineral density. This allows a practitioner to predict the risk of bone fracture based on the density of the bones. The test is basically a type of X-ray and is painless to the patient. Usually one hip and four of the lower spine vertebra are measured as these are sites most likely to be at risk for fracture. One forearm may be scanned, particularly in a patient who has had a hip replacement and cannot have a DXA at that site due to an artificial hip joint.

Bone density screening is usually begun for women after menopause and men after age 70. The DXA test is performed every one to two years, depending on whether or not the patient is receiving treatment. Insurance companies and Medicare may refuse to pay for bone density testing if done more frequently than every two years.

T Scores for Osteoporosis

The DXA scan measures bone mineral density as a raw score that is converted to a T score. These T scores represent a standard deviation, which is how far the patient's bone density is from normal. In considering bone density, normal is a female 20 to 29 years of age, which is before the natural bone loss begins. The World Health Organization recognizes three categories of T scores for diagnosis:

Normal: T score greater than or equal to -1.
Osteopenia: T score between -1 and -2.5
Osteoporosis: T score less than or equal to -2.5

T Score Relation to Fracture Risk

The T score for osteopenia represents a decrease in standard deviation from normal bone mass of 1 to 2.5. With each decrease in standard deviation, fracture risk nearly doubles. For example, a female with a T score of -2 has a fracture risk nearly double that of a normal female age 20 to 29. A woman with osteoporosis whose hip T score is -3 has a risk of fracture nearly four times that of someone with normal bone density.

Improving T scores

Medications used to treat osteoporosis are designed to increase bone density, thereby decreasing a patient's risk of fracture. For patients with osteopenia, adequate calcium and vitamin D supplementation may be helpful to improve bone density. Weight-bearing exercise like walking also may help.

As bone density improves, T scores are lower. A patient with a T score of -3.5 who is started on medication may show an improvement with a T score of -3 on her next DXA test.

Surrogate Marker

Ultimately, T scores act as a surrogate marker, a convenient way to estimate fracture risk. This is similar to using blood pressure to estimate a patient's risk of stroke. An osteoporosis patient who knows her T scores and what they mean can be a more active participant in her care.

The copyright of the article What do Bone Density Test Results Mean? in General Medicine is owned by Melissa Murfin. Permission to republish What do Bone Density Test Results Mean? in print or online must be granted by the author in writing.

Hip DXA