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What is a bone mineral density test and do I need one?

Posted January 21, 2021 by Charles A. Zonfa, MD, FACOG | Chief Medical Officer

It’s estimated about 54 million Americans are living with osteoporosis and low bone mass, according to the National Osteoporosis Foundation. More alarming, studies suggest about one in two women and up to one in four men age 50 and older will break a bone due to osteoporosis.

As we age, we tend to lose more bone mass than our bodies form — and the rate at which we lose bone mass speeds up once we hit middle age. With osteoporosis, your bones become porous, weak and brittle, putting you at risk for fractures or broken bones.

That’s where a bone mineral density (BMD) test can help. It’s a great way to diagnose and monitor osteopenia (decreased bone mass) and osteoporosis (thin, weak bones). Doctors use a BMD test, which is painless and requires no medication, to:

  • Confirm a diagnosis of osteopenia or osteoporosis
  • Determine the rate of bone loss
  • Predict the likelihood of future bone fractures
  • Monitor osteoporosis treatment

The most common and accurate BMD test uses a unique X-ray — called a dual-energy X-ray absorptiometry, or DEXA — to measure how much calcium and other bone minerals are packed into an area of bone. Usually, the spine, lower arm and hip are measured. The higher the mineral content, the denser (and stronger) your bones are and less likely to break.

BMD tests are more precise than standard X-rays and can find problems much earlier so treatment can begin sooner, hopefully before complications arise.

Bone Mineral Density test results

BMD test results are compared to two norms: healthy young adults (T-score) and age-matched adults (Z-score).

For your T-score, results are measured against a healthy 25 - 35 year old of your same sex and ethnicity. Positive T-scores indicate your bones are stronger than normal, and negative T-scores indicate they are weaker than normal. A T-score of 0 is considered ideal.

The National Institute of Health offers the following guidelines for bone density scores:

  • Normal is a T-score of 1 and -1
  • Low bone mass is -1 to -2.5
  • Osteoporosis is -2.5 or lower

In addition, your BMD result is measured against someone of your own age, sex, race, height and weight. This is called your Z-score, which is calculated in the same way as a T-score.

Who needs a bone mineral density test?

Osteoporosis is more common in women, but men can develop the condition, too. Your doctor may order a BMD test if there is a suspicion your bones are becoming weaker, you show symptoms of osteoporosis or you’ve reached the age when preventative screening is necessary.

You should discuss a BMD test with your doctor beginning at age 65 for women and 70 for men. Most people under the age of 65 don’t need a BMD test unless they are at an increased risk for osteoporosis or fractures. Common risk factors include:

  • Women who are post-menopausal and don’t take estrogen
  • Smoking and excessive use of alcohol
  • Family history of osteoporosis
  • Use of certain medications, such as steroids
  • Low body mass index or poor nutrition
  • Certain chronic diseases, such as rheumatoid arthritis, Type 1 diabetes, liver or kidney disease, or hyperthyroidism

Early treatment is key to help prevent bone fractures. The complications of broken bones due to osteoporosis are often severe, particularly in the elderly. So the earlier osteoporosis is diagnosed, the sooner you can begin treatment to improve your condition or keep it from getting worse.