| Bone density is
the best single predictor of future fractures.
Density accounts for about 80 percent of the
strength of your bones. Several different tests
are used to measure bone density, which is also
referred to as bone mineral density or BMD. All
of these tests are safe, painless, quick, and
precise.
Depending on the kind of test
you take, as well as your medical history,
different bones will be measured. Though
density scores of bones in different parts of
the skeleton are usually closely correlated,
some of your bones may be stronger than
others.
Dual X-ray absorptiometry (DXA)
The current test of
choice is dual X-ray absorptiometry (called DXA
or DEXA) DXA is widely available and relatively
inexpensive; it can measure bone density in the
hip, the spine, and the forearm. The hip and
spine measurements are the ones usually done
as these are the main weight-bearing
areas of the body and are the most accurate
accessible areas to assess..
They are particularly significant because that's
where fractures have the most serious
consequences.
A smaller version of the DXA
machine can measure bone density in the forearm
and the heel. While this isn't quite as good as
measuring the hip and spine directly the results
are very closely correlated. The test only
takes two minutes, and the smaller machine can
be used in community settings or doctors'
offices.
Ultrasound
densitometry
Another commonly
available testing option is ultrasound. This
technique uses a device no bigger than a
suitcase; a test costs only $30 to $100 and
involves no radiation. Unfortunately,
ultrasound can't check bone density in the spine
and hip. The site most commonly measured is the
heel; other bones in the lower leg and hand can
be checked too. The results reflect not only
density but also properties of collagen in the
bone. While this information is helpful because
the results are strongly correlated with
fracture risk, it's not a substitute for direct
measurement of hip or spine bone density with
DXA
Single X-ray
absorptiometry (SXA)
Now that DXA is available, SXA --which can't
check spine and hip bones as been phased out.
SXA measures bone density in the fingers, wrist
and heel. Those results correlate strongly with
hip and spine density, so the test remains a
good general indicator of bone health.
Radiographic
absorptiometry (RA)
Radiographic
absorptiometry is a special type of X-ray. It
measures bone density in the hand, which is
closely correlated with hip and spine density.
The chief advantage of RA is low cost. Also,
nearly any X-ray machine can be adapted for RA.
This makes it a valuable screening tool for
women without easy access to DXA, such as those
who live in small rural areas.
Computerized axial tomography (CT or CAT scan)
CT scans are used
mainly in research. But they can be helpful
when other tests aren't available, or in special
situations, DXA, SXA, X-ray and RA all produce
a two-dimensional image of the bone. CT also
uses an X-ray beam but it can create a
three-dimensional image. That can be important
when a woman appears to be losing significantly
more trabecular than cortical bone. In such a
case, a CT scan would allow separate examination
of the trabecular bone in the center of her
spine
How the tests work
DXA uses a technique
called densitometry or X-ray absorptiometry; the
machine passes an X-ray beam through an area of
the bone. Ultrasound testing uses sound waves
instead of X-rays. Radiation (or sound waves)
are absorbed by the bone-- the denser the bone,
the more it absorbs. The machine's detectors
translate absorption information into a measure
of bone density.
The "dual" in dual X-ray
absorptiometry (DXA) refers to the use of two
different X-ray beams, which enables the machine
to distinguish between bone and the soft tissue
(e.g., muscle, fat) covering it. That's why DXA
can measure density of the hip and spine bones,
even though they lie deep inside the body.
Tests that use just a single beam can only
measure bones that are just under the skin, such
as the bones in the hand, wrist, and heel
Bone density tests are the
best way to determine if you have osteopenia or
osteoporosis. But these tests can't tell you
why you're losing bone or how fast it's
happening. Nor can they evaluate suspected
spine fractures or broken bones. So your doctor
may suggest one or more additional tests. Here
are some possibilities.
X-ray
X-rays are the most
accurate way to detect and assess bone
fractures. If you break your hip or wrist, or
if you have symptoms of spinal fractures, your
doctor probably will suggest a diagnostic X-ray.
Bone Markers
When you're being treated for osteoporosis, your doctor orders a blood or urine test. This reveals several markers -- levels of different enzymes, proteins, and other substances circulating in the body -- that provide clues about your disease and the progress of your treatment.
Some of these measures include:
- Bone-specific alkaline phosphatase (Bone ALP or BALP). This is an estimate of the rate of bone formation over your entire skeleton. Bone formation may sound like a good thing, but depending on the circumstances, too much can be bad. People with osteoporosis generally have BALP levels that are up to three times normal.
- Osteocalcin. This is another marker of bone formation.
- Urinary N-telopeptide of type I collagen, or uNTX. This is a marker of bone resorption, or loss of bone.
- Vitamin D levels. This measure assesses whether you have a deficiency of vitamin D, which is essential for your body's absorption of calcium. You can be taking plenty of calcium, but if you don't have enough vitamin D, it won't be efficiently absorbed by your body.
Hormone Tests
Blood tests can check
levels of hormones important to bone, including:
Estradiol
This is the most
potent form of estrogen. If you are under age
45 and experiencing menstrual irregularities,
your doctor may check your estradiol and other
estrogens. If levels are abnormally low, birth
control pills mught be suggested to boost your
supply of estrogen, thereby protecting your
bones.
Follicle-stimulating hormone (FSH)
FSH is a pituitary
hormone that stimulates the ovaries, indirectly
affecting estrogen supplies. As a woman
approaches menopause, her FSH levels normally
rise. Checking FSH helps your doctor determine
if you're entering menopause. This narrows down
possible causes of amenorrhea and other
menstrual irregularities.
Thyroid and parathyroid
Problems with the
thyroid and parathyroid glands can lead to bone
loss. Thyroid or parathyroid hormone tests are
the first diagnostic step when these issues are
suspected.
Calcium
metabolism tests
Abnormalities in blood
calcium levels don't necessarily mean that you
have osteoporosis, but they can help clarify
your medical situation. For example, some
parathyroid problems cause an increase in blood
calcium.
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