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Young People Get Osteoporosis Too 

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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.