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What is a Z-score?

A Z-score[1] matches your bone mineral density with individuals of the same age, gender, and ethnicity. The following formula is used to determine your Z-score:

Z-scores are used to identify low bone mass in men under the age of 50, premenopausal women, and children. The International Society for Clinical Densitometry recommends classifying Z-scores of -2.0 and lower as "low bone mineral density for chronological age" or "below the expected range for age." They suggest classifying scores above -2.0 as "within the expected range for age." Z-scores are not helpful in identifying osteoporosis or osteopenia because your Z-score may remain constant throughout your life. Z-scores are more useful in evaluating bone density in children, who have not yet reached peak bone mass, and younger adults.

Very low Z-scores in postmenopausal women or men over 50, indicating a much lower bone mass than that of their peers, do require further investigation into the causes of bone loss related to secondary osteoporosis (the type of osteoporosis not associated with postmenopause or aging). If you have a Z-score lower than -1.5, your clinician should be looking for reasons why you have significant bone loss compared to your own age group. If you have a medical condition, for example, hyperthyroidism (one of the illnesses that can cause secondary osteoporosis) that has gone undetected, a low Z-score may point your clinician toward identifying and treating this reason for bone loss.

How will my clinician use my test results to determine whether I have osteoporosis?

A T-score, expressed in standard deviations, will be reported to your clinician, and your score will be evaluated using the WHO guidelines. If you are a post-menopausal woman or a man over 50, most machines are calibrated with special software to determine your scores. Based on the guidelines (see Question 31), your results will indicate normal bone density, low bone mass (osteopenia), or osteoporosis. If you have osteoporosis with a fragility fracture, you will have a diagnosis of severe osteoporosis[2]. Because the T-score not only reflects bone density but also your risk of fracturing a bone, your clinician should discuss specific ways of not only increasing your bone mass but also lowering your fracture risk (see Question 79).

A Z-score is usually not helpful in making the diagnosis of osteoporosis. However, if it is particularly low (lower than —1.5), it is important for your clinician to evaluate you for conditions and illnesses that may be causing your bone loss associated with secondary osteoporosis. Such causes of secondary osteoporosis might include thyroid or parathyroid disease, cigarette smoking, excessive alcohol intake, problems with absorption from your gastrointestinal tract, or the use of medications known to be harmful to bone.

If you had a peripheral bone density test (at the wrist or heel), you may still require central testing[3] (at the hip or spine) even if your test results are interpreted to be in the normal range, unless you weigh over 250 pounds and a central test cannot accommodate you. Central BMD testing more accurately reflects the status of large central bones. Questions 19 and 20 identify who should have BMD screening or testing at the hip and spine, and this applies even if the test at your wrist, heel, or ankle was normal.

Its important to remember that even if your T-score fits into the category of normal, you may still have osteoporosis at another site.

It's important to remember that even if your T-score fits into the category of normal, you may still have osteoporosis at another site. Even though central bones measured on a DXA are the most predictive for the rest of your body, other sites can still have lower mass. Peripheral measurements (heel, wrist) are not as predictive for the rest of your body. So, even if you have normal measures of your heel or wrist, your clinician may send you for additional testing with different machines or on other bones, especially if you have many risk factors or a fracture. If you have a frailty fracture or a low-trauma fracture (see Question 74), you likely have osteoporosis whether you are tested for it or not, although it's important to get a baseline measurement so that the effectiveness of treatment can be evaluated by your clinician.

If your T-score indicates osteoporosis according to the WHO classifications (see Question 31), the American Association of Clinical Endocrinologists (AACE) and the National Osteoporosis Foundation (NOF) recommend that your clinician complete a physical exam and laboratory analyses, such as a complete blood count (CBC) and other blood tests for minerals such as calcium and phosphorus and for liver function, kidney function, and electrolytes such as salt and potassium. Most clinicians also will test for your vitamin D level in the blood (serum 25-OH-D levels), fasting serum calcium level, and urinary calcium level over a 24-hour period. These additional tests will help rule out other causes for your low bone density. If other causes are identified, they can be treated so the effects on your bones are minimized (see Questions 15—16, and 47—51).

While you may be told that you have osteopenia based on your T-score, a T-score between —1.0 and —2.5 does not necessarily mean that bone loss has occurred. It could mean that you never reached peak bone mass in young adulthood to begin with. If you have osteopenia, your clinician likely will evaluate you for causes of secondary osteoporosis (see Questions 9, 15, and 16). Treatment may be indicated to prevent osteoporosis, whether the osteopenia is due to some bone loss or a low peak bone mass (Part 3 discusses treatment options). The WHO developed an algorithm to assist clinicians in determining who would benefit from initiating treatment for osteopenia, and the results from this algorithm will be included on most bone mineral density test reports by the end of 2009 (see Question 55).

  • [1] Matches your bone mineral density with individuals of the same age, gender, and ethnicity, and is more helpful in evaluating children and premenopausal women.
  • [2] A T-score lower than -2.5 plus the presence of one or more fragility fractures.
  • [3] Usually DXA or QCT bone density tests of the hip, upper thigh, or spine.
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