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Medical Author:

Mythili Seetharaman, MD

Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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Bone mineral density tests measure bone density in the spine, hip, and/or wrist, which are the most common sites of fractures due to osteoporosis. The results of the bone mineral density test are compared to two standards (norms):

• The age-matched reading, known as the Z-score, compares a person's bone density to what is expected in someone of equivalent age, sex, and size. However, among older and elderly adults, low bone mineral density is common, so comparison with age-matched norms can be misleading.
• The young-normal reading, known as the T-score, compares bone density to the optimal peak bone density of a healthy young adult (30 years old) of the same sex. The T-score determines fracture risk, which increases as bone mineral density falls below young-normal levels. The T-score, which is a comparison between the solidness (density) of the bones and the bones of the average young healthy population, is measured in standard deviations (SDs). SD is a statistical term that describes variation in a population. According to the World Health Organization's (WHO) diagnostic categories, individuals whose T-score is within 1 SD of the norm are considered to have normal bone density. Scores below the norm are indicated in negative numbers. For most bone mineral density tests, -1 SD equals a 10%-12% decrease in bone density. The risk for broken bones increases by 50%-100% for every SD below the young-normal standard.

• Normal: Bone density is within 1 SD (+1 or -1) of the young adult mean.
• Low bone mass (osteopenia): Bone density is 1 to 2.5 SDs below the young adult mean (-1 to -2.5 SD).
• Osteoporosis: Bone density is 2.5 SDs or more below the young adult mean (less than -2.5 SD).
• Severe (established) osteoporosis: Bone density is more than 2.5 SDs below the young adult mean and one or more broken bones (osteoporotic fractures) has occurred.

With the information from a bone density test, a doctor can identify the degree of bone loss and determine whether a person is at risk for fracture. In general, the lower the bone density (weaker bones), the higher the risk for fracture. Test results help determine which prevention or treatment options are right.

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