Test Overview

A bone density test is a kind of X-ray test. It measures the
density of minerals (such as
calcium) in your bones. This information helps your doctor estimate the
strength of your bones.

We all lose some bone mass as we age.
Bones naturally become thinner (called
osteopenia) as you grow older. This is because existing bone tissue is
broken down faster than new bone is made. As this occurs, our bones lose
calcium and other minerals. They also become lighter and less dense.
This makes the bones weaker and makes them more likely to break
(fracture).

With further bone loss,
osteopenia can lead to
osteoporosis. So the thicker your bones are, the
longer it takes to get osteoporosis. Although osteoporosis can occur in men, it
is most common in women older than age 65.

If your bone
density is lower than normal, you can increase it and your strength. You can do things like exercising, lifting weights or
using weight machines. You can also make sure to get enough calcium and vitamin D. And you may need to take certain medicines.

There are several different ways to measure
bone density.

  • Dual-energy X-ray absorptiometry (DXA). This is the most accurate way to measure bone density. It
    uses two different X-ray beams to estimate bone density in your spine and hip.
    Strong, dense bones allow less of the X-ray beam to pass through them. The
    amounts of each X-ray beam that are blocked by bone and soft tissue are
    compared to each other. DXA can measure as little as 2% of bone loss per year.
    It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to
    measure bone density in the heel and forearm. But SXA is not used as often as DXA.
  • Peripheral dual-energy X-ray absorptiometry (P-DXA). P-DXA is a type of DXA test. It measures the
    density of bones in the arms or legs, such as the wrist. It can’t measure the
    density of the bones most likely to break, such as the hip and spine. P-DXA
    machines are portable units that can be used in a doctor’s office. P-DXA also
    uses very low doses of radiation. The results are ready faster than
    standard DXA measurements. P-DXA is not as useful as DXA for finding out how
    well medicine used to treat osteoporosis is working.
  • Dual photon absorptiometry (DPA). This test uses a radioactive
    substance to measure bone density. It can measure bone density in your hip and spine.
    DPA also uses very low doses of radiation. But the scan takes longer than the
    other methods.

Ultrasound is a screening test that is
sometimes offered at events such as health fairs. If results from an ultrasound test find
low bone density, DXA is advised to confirm the results. Ultrasound uses
sound waves to measure bone density, usually in your heel. It is quick and painless. And it does not use potentially harmful radiation like X-rays. One
downside of ultrasound is that it can’t measure the density of the bones in the hip and spine. These are the bones most
likely to fracture from osteoporosis. Ultrasound is not used to
keep track of how well medicine for osteoporosis is working.

Before you are screened for osteoporosis, you may want to
think about what you will do if the tests show that you have a high chance of
getting osteoporosis.

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Why It Is Done

A bone density test is
suggested for:

  • All women who are age
    65 or older. It’s also recommended for younger women who are at higher risk for broken bones caused by osteoporosis.footnote 1
  • Men with risk factors for
    osteoporosis, such as being older than 70.
  • Men and women who have
    hyperparathyroidism.
  • Men and women who
    have been taking
    corticosteroids, such as prednisone, for a long
    time.
  • Follow-up of how well treatment for osteoporosis is working
    for men and women being treated for 2 years or longer.

How To Prepare

Avoid wearing clothes with metal buttons
or buckles. You also may want to remove any jewelry that might
cause a problem with the scan. For instance, don’t wear a bracelet if you are having the scan done on
your wrist.

How It Is Done

In most cases, a bone density scan is
done in a radiology department or clinic by a technologist.
Peripheral dual-energy X-ray absorptiometry (P-DXA) machines are portable
units that can be used in a doctor’s office.

You will need to lie
on your back on a padded table. You probably can leave your clothes on. You may
need to lie with your legs straight. Or you may lie with your lower legs resting on a
platform built into the table.

The machine will scan your bones
and measure the amount of radiation they absorb. The DXA scan, which
scans the hip and lower spine, takes about 20 minutes. Other
kinds of scans may take 30 to 45 minutes.

Portable machines (P-DXA)
can measure bone density in the wrist or forearm.

Testing at least two different bones
each time is the most reliable way of measuring
bone density. The hip and spine bones are preferred. It is best to test the same bones each time. The same type of scan and bone density equipment should also be used.

How It Feels

A bone density test does not cause
pain. But if you have back pain, it may bother you to lie still on a table
during the scan.

Risks

During a bone density scan, you are
exposed to a very low dose of radiation. A bone density scan is not advised for
pregnant women because it exposes the unborn baby to radiation.

Results

A bone density test is a kind of X-ray test. It measures
the density of minerals (such as
calcium) in your bones. Results are usually ready in 2 to 3
days.

Results of bone density tests can be reported in
several ways.

T-score

Your T-score is your bone density compared to the
average score of a healthy 30-year-old. (This is called the young adult
reference range). It is expressed as a standard deviation
(SD).

  • A negative (-) value means that you have
    thinner bones (lower bone density) than an average 30-year-old. The more
    negative the number is, the less bone density you have compared with an average
    30-year-old.
  • A positive (+) value means that your bones are
    thicker and stronger than an average 30-year-old’s.

The following table contains the World Health
Organization’s definitions of osteoporosis based on
bone density T-scores.

Bone density footnote 2
 

T-score

Normal:

Less than 1 standard deviation (SD) below the young
adult reference range (more than -1)

Low bone mass (osteopenia):

1 to 2.5 SDs below the young adult reference range (-1
to -2.5)

Osteoporosis:

More than 2.5 SDs below the young adult reference range
(-2.5 or less)

If your bone density test
result is low:

  • You may have
    osteoporosis. Doctors usually use the lowest T-score
    to diagnose osteoporosis. For example, if your T-score at your spine is -3 and
    your T-score at your hip is -2, the spine T-score would be used to diagnosis
    osteoporosis.
  • Your chance of breaking a bone is higher than average. The more negative your T-score, the greater your chances of breaking a
    bone during a fall or from a minor injury. Every change of 1 SD means you have double the chance of a break at that site. For example, if you have a
    T-score of -1, your chances of having a broken bone are 2 times greater than if
    your T-score was 0.

Low bone density values may be caused by other problems,
such as:

Z-score

Your bone density value may also be compared to
other people of your age, sex, and race. This is called your Z-score. It is
given in standard deviations (SD) from the average value for your age
group.

  • A negative (-) value means that your bones
    are thinner (lower bone density) and weaker than most people in your age group.
    The more negative the number is, the less bone density you have compared with
    others in your age group.
  • A positive (+) value means that your
    bones are thicker and stronger than most people in your age group.

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

  • You aren’t able to be put in the correct position during the
    test.
  • You had a broken bone in the past. This can cause
    falsely high bone density results.
  • You have arthritis of your spine. The changes
    caused by arthritis in the spine may not make the spine the best place to
    measure for osteoporosis.
  • You have metal implants from hip
    replacement surgery or hip fracture.
  • You had an X-ray test
    that uses
    barium within 10 days of the bone density test.

What To Think About

  • Experts disagree about which bones are best to
    use for bone density measurements. Bones in the
    lower spine and
    hip are tested most often. These bones generally have
    the most bone loss and are more likely to break. Sometimes bones in the
    wrist are measured. Ultrasound screening is done on the bone in the
    heel.
  • A bone density measurement should be done only when the results of the test will affect treatment decisions. Bone density does not need to be
    measured more often than every 2 years to find out how well treatment is
    working.
  • Using DXA to measure bone density is replacing
    older methods, such as dual photon absorptiometry (DPA).
  • Regular
    X-rays can’t detect mild bone loss. A bone must lose at least a quarter of its
    weight before a regular X-ray can detect the problem.
  • If your bone
    density is lower than normal, you can increase it and your strength. You can do things like
    exercising, lifting weights or
    using weight machines. You also need to make sure you are getting enough calcium and vitamin D. And you may need to take some medicines. To learn more about
    how you can increase your bone strength and density, see the topic
    Osteoporosis.

References

Citations

  1. U.S. Preventive Services Task Force (2011). Screening for Osteoporosis: Recommendation Statement. Available online: https://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.
  2. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Liu H, et al. (2008). Screening for osteoporosis in men: A systematic review for an American College of Physicians guideline. Annals of Internal Medicine, 148(9): 685-701.
  • Nayak, S, et al. (2006). Meta-analysis: Accuracy of quantitative ultrasound for identifying patients with osteoporosis. Annals of Internal Medicine, 144 (11): 832-841.
  • Qaseem A, et al. (2008). Screening for osteoporosis in men: A clinical practice guideline from the American College of Physicians. Annals of Internal Medicine, 148(9): 680-684. Also available online: https://www.acponline.org/clinical_information/guidelines/guidelines.
  • U.S. Preventive Services Task Force (2011). Screening for Osteoporosis: Recommendation Statement. Available online: https://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.

Credits

ByHealthwise Staff
Primary Medical Reviewer Anne C. Poinier, MD – Internal Medicine
Specialist Medical Reviewer Carla J. Herman, MD, MPH – Geriatric Medicine

Current as ofMay 4, 2017