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Osteoporosis: Know Your Bone Status

• Bone Mineral Density Tests
• Indications for a BMD test
• Types of BMD Tests
• What the Different Bone Mineral Density Test Numbers Mean

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Bone Mineral Density Tests

A Bone Mineral Density (BMD) test measures the density of your bones (bone mass). A BMD test is the only way to diagnose osteoporosis and can help determine your risk for fractures and monitor your response to an osteoporosis treatment. Since osteoporosis can develop undetected for decades until a fracture occurs, early diagnosis is important.

Your doctor can help you determine whether you should have a Bone Mineral Density (BMD) test. BMD tests cannot stand alone. They should always be a part of a complete medical workup supervised by your doctor.

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Indications for a BMD test

Your doctor may consider ordering a BMD test for you if you fall into one of the following categories:

• A postmenopausal woman under age 65 who has one or more additional risk factors for osteoporosis in addition to being postmenopausal and female.
• A woman age 65 and older regardless of additional risk factors.
• A postmenopausal woman who presents with fractures. (This is to confirm diagnosis and determine disease severity).
• A woman who is considering therapy for osteoporosis if BMD testing would facilitate the decision.
• A woman who has been on hormone replacement therapy (HRT/ERT) for a prolonged period of time.

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Types of BMD Tests

There are several different machines that measure bone density. Central machines measure density in the hip, spine and total body. Peripheral machines measure density in the finger, wrist, kneecap, shin bone and heel.

There are several ways to measure bone mineral density. All are painless, noninvasive and safe and are becoming more readily available.

The tests measure bone density in your spine, hip and/or wrist, the most common sites of fractures due to osteoporosis. Recently, the FDA approved bone density tests that measure bone density in the middle finger and the heel or shinbone.

Your bone density is compared to two standards, or norms, known as "age matched" and "young normal." The age-matched reading compares your bone density to what is expected in someone of your age, sex and size. The young normal reading compares your density to the optimal peak bone density of a healthy young adult of the same sex.

The information from a bone density test enables your doctor to identify where you stand within ranges of normal and to determine whether you are at risk for fracture. In general, the lower your bone density, the higher your risk for fracture.

With the information obtained from a BMD test, you and your doctor can decide what prevention or treatment steps are right for you.

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What the Different BMD Test Numbers Mean

Your BMD is compared to two norms, "young normal" and "age-matched." Young normal, known as your T-score, compares your BMD to optimal or peak density of a 30-year old healthy adult and determines your fracture risk, which increases as BMD falls below young-normal levels. Age-matched, known as your Z-score, compares your BMD to what is expected in someone your age and body size. Among older adults, however, low BMD is common, so comparison with age-matched norms can be misleading.

The difference between your BMD and that of a healthy young adult is referred to as a standard deviation (SD). As outlined in the World Health Organization's diagnostic categories, individuals whose T-score is within one standard deviation of the "norm" are considered to have normal bone density.

World Health Organization Definitions of Osteoporosis Based on Bone Density Levels:

• Normal Bone Mass
  Bone Density is within 1 SD (+1 or -1) of the young adult mean.
• Low Bone Mass or Osteopenia
  Bone density is 1 to 2.5 SD below the young adult mean (-1 to -2.5 SD). Scores below the "norm" are indicated in negative numbers.
• Osteoporosis
  Bone density is 2.5 SD or more below the young adult mean (> -2.5 SD). A score of more than -2.5 SD below the norm is considered a diagnosis of osteoporosis. For most BMD tests, -1 SD equals a 10-12 percent decrease in bone density.
• Severe (established) osteoporosis
  Bone density is more than 2.5 SD below the young adult mean and there has been one or more osteoporotic fractures.

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