Test Overview
A bone mineral density (BMD) test measures the
density of minerals (such as
calcium) in your bones using a special
X-ray or
computed tomography (CT) scan. This information is used to 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 because existing bone is
broken down faster than new bone is made. As this occurs, our bones lose
calcium and other minerals and become lighter, less dense, and more porous.
This makes the bones weaker and increases the chance that they might break
(fracture).
With further bone loss,
osteopenia leads 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 take steps to increase your bone strength and reduce
your chances of having a fracture. Some ways to increase bone density and
strength include combining calcium and vitamin D supplements with
weight-bearing exercise (such as walking), weight training (such as lifting
weights or using weight machines), and using medicines such as calcitonin
(Miacalcin), alendronate (Fosamax), risedronate (Actonel), or ibandronate (Boniva).
There are several different ways to measure
BMD.
- Dual-energy X-ray absorptiometry (DEXA). This is the most accurate way to measure BMD. 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. DEXA 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 heel and forearm bone density, but SXA is not used as often as DEXA.
See a picture of a
DEXA X-ray of the hips or a
DEXA X-ray of the spine.
- Peripheral dual-energy X-ray absorptiometry (P-DEXA). P-DEXA is a type of DEXA test. It measures the
density of bones in the arms or legs, such as the wrist—it cannot measure the
density of the bones most likely to break, such as the hip and spine. P-DEXA
machines are portable units that can be used in a doctor's office. P-DEXA also
uses very low doses of radiation, and the results are ready faster than
standard DEXA measurements. P-DEXA is not as useful as DEXA 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 BMD in your hip and spine.
DPA also uses very low doses of radiation but has a slower scan time than the
other methods.
- Quantitative computed tomography (QCT). This is a type of CT scan that measures the density of
a bone in the spine (vertebra). A form of QCT called peripheral QCT (pQCT)
measures the density of bones in your arms or legs, usually your wrist. QCT is
not usually used because it is expensive, uses higher radiation doses, and is
less accurate than DEXA, P-DEXA, or DPA.
Ultrasound is a screening test that is
sometimes offered at events such as health fairs. It is only used to look for problems. If results from an ultrasound test find
low bone density, DEXA is recommended to confirm the results. Ultrasound uses
sound waves to measure BMD, usually in your heel. Ultrasound is quick,
painless, and does not use potentially harmful radiation like X-rays. One
disadvantage of ultrasound is it can't measure the density of the bones most
likely to fracture from osteoporosis (the hip and spine). Ultrasound is not used to
keep track of how well medicine for osteoporosis is working.
Before being screened for osteoporosis, you may want to
think about what you will do if the tests show you have a high chance of
getting osteoporosis. For more information, see:
Osteoporosis: Should I Have a Dual-Energy X-Ray Absorptiometry (DEXA) Test?
Why It Is Done
A bone mineral density (BMD) test is
suggested for:
- All women who are age
65 or older, and younger women who are at increased risk for broken bones caused by osteoporosis.1
- Older men, and men with other risk factors for
osteoporosis.2
- 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 for the test. You also may want to remove any jewelry that might
interfere with the scan, such as a bracelet if you are having the scan done on
your wrist.
How It Is Done
A bone mineral density (BMD) scan is usually
done in the special radiology department or clinic by a technologist.
Peripheral dual-energy X-ray absorptiometry (P-DEXA) 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 can usually leave your clothes on. You may
need to lie with your legs straight or 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 DEXA technique, which
scans the hip and lower spine, takes about 20 minutes to perform. Other
techniques may take 30 to 45 minutes.
Portable machines (P-DEXA)
can measure bone density in the wrist or forearm.
Testing at least two different bones
(preferably the hip and spine) each time is the most reliable way of measuring
BMD. It is best to test the same bones and to use the same measurement
technique and BMD equipment each time.
How It Feels
A bone mineral density test does not cause
pain. If you have back pain, it may be uncomfortable to lie still on a table
during the scan.
Risks
During a bone mineral density (BMD) scan, you are
exposed to a very low dose of radiation. A BMD scan is not recommended for
pregnant women because of the radiation exposure to the unborn baby.
Results
A bone mineral density (BMD) test measures
the density of minerals (such as
calcium) in your bones using a special
X-ray or
computed tomography (CT) scan. Results are usually available in 2 to 3
days.
Results of bone mineral density tests can be reported in
several ways.
T-score
Your T-score is your BMD compared to the
average score of a healthy 30-year-old. It is expressed as a standard deviation
(SD), which is a statistical measure of how closely each person in a group is
to the average (mean) of the group. The average BMD is determined by measuring
the bone density of a large group of healthy 30-year-olds (young adult
reference range). BMD values are then reported as a standard deviation from the
mean of this reference group. Almost all 30-year-old people have a BMD value
within 2 standard deviations of this mean.
- A negative (–) value indicates 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 indicates that your bones are
thicker and stronger than an average 30-year-old.
The following table contains the World Health
Organization's definitions of osteoporosis based on
bone mineral density T-scores.
Bone mineral density| | 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 mineral 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.
- You have a higher-than-average chance of breaking a
bone. 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 a twofold
increase in the risk of fracture 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 BMD values may be caused by other problems,
including:
Z-score
Your BMD 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
Reasons you may not be able to
have the bone mineral density (BMD) test or why the results may not be helpful include:
- Inability to be correctly positioned during the
test.
- Having a broken bone in the past. This can cause
falsely high BMD results.
- Arthritis of your spine. In this case, the changes
caused by arthritis in the spine may not make the spine the best place to
measure for osteoporosis.
- Metal implants from hip
replacement surgery or hip fracture.
- Having an X-ray test
that uses
barium within 10 days of the BMD test.
What To Think About
- Experts disagree about which bones are best to
use for BMD 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 fracture. Sometimes bones in the
wrist are measured. Ultrasound screening is done on the bone in the
heel.
- A BMD measurement should be done only when the information
provided by the test will affect treatment decisions. BMD does not need to be
measured more often than every 2 years to find out how well treatment is
working.
- Using DEXA to measure bone mineral density is replacing
older methods, such as dual photon absorptiometry (DPA).
- Regular
X-rays cannot 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 bone density and strength by
taking calcium and
vitamin D supplements, exercising, lifting weights or
using weight machines, and taking some medicines. For more information about
how you can increase your bone strength and density, see the topic
Osteoporosis.
- In the United States, legislation (called the
Bone Mass Measurement Coverage Standardization Act) requires Medicare to pay
for bone mineral density testing for people who have Medicare benefits and are
at risk for losing bone mass. This includes:
- Women who have gone through menopause and
are at high risk for a bone fracture.
- People who have increased
bone loss (osteopenia) or have broken a bone because they have
osteoporosis.
- People using long-term doses of
corticosteroids.
- People using medicine to
treat osteoporosis for 2 years or longer.
- People who have
hyperparathyroidism.
References
Citations
- U.S. Preventive Services Task Force (2011). Screening
for Osteoporosis: Recommendation Statement.
Available online: http://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.
- 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.
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.
- MacLean C, et al. (2008). Systematic review:
Comparative effectiveness of treatments to prevent fractures in men and women
with low bone density or osteoporosis. Annals of Internal Medicine, 148(3): 197–213.
- 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.
- U.S. Preventive Services Task Force (2011). Screening
for Osteoporosis: Recommendation Statement.
Available online: http://www.uspreventiveservicestaskforce.org/uspstf10/osteoporosis/osteors.htm.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | Anne C. Poinier, MD - Internal Medicine |
|---|
| Specialist Medical Reviewer | Carla J. Herman, MD, MD, MPH - Geriatric Medicine |
|---|
| Last Revised | February 23, 2011 |
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