Diagnosis

• Bone mineral density (BMD) is measured, typically in the hipbone, using bone densitometry.
• Measurements of BMD are given as mg/cm². This is the average concentration of bone mineral in the areas that are being scanned. In general, bone is normal if results are greater than 833 mg/cm². Low bone density (osteopenia) is between 833 and 648 mg/cm². Osteoporosis is diagnosed with results below 648 mg/cm².

These measurements still do not always indicate the true risk for fracture. The doctor also assesses risk factors and other considerations. The next step is to compare the patient's BMD to normal bone density, which is defined as the average BMD in the hipbones of premenopausal Caucasian women. (This group is used as the basis for the norm because of their high risk and greater proportion in the American population.)

The health professional then uses this comparison to determine her standard deviation (SD) from this norm. SD results are given as Z and T scores:

• A T score gives the SD of the patient in relationship to the norm in young adults. Doctors often use the T-score and other risk factors to determine the risk for fracture.
• A Z score gives the SD of the patient in relationship to the norm in her own age group. Z scores may be used to monitor the effects of treatments in women who have been diagnosed with osteoporosis.

For example, the lifetime risks for a younger woman with a specific T-score would be higher than the same scores in an older woman because the younger woman would have a longer time to lose bone density. In general, the T scores in a 55-year-old woman suggest the following degrees of risk for hip fracture.

• One standard deviation (SDs) or less below the norm indicates normal BMD. (This carries a lifetime chance for a hip fracture of up to about 20%, depending on age and other risk factors.)
• Between 1 and 2.5 SDs below normal defines osteopenia, which is low bone density. This carries between a 20 - 50% lifetime risk for fracture.
• More than 2.5 SDs predicts osteoporosis and over a 60% chance for hip fracture. Additional risk factors increase the risk. They include low weight, smoking, risks for falling, and especially a history of previous fractures. For example, in women 65 years old with low bone density but no adverse factors, the risk for fracture is 4.3% in 1 year and 28.6% over 5 years. In similar women with a previous fracture, the probability of fracture at 1 year is 11% and at 5 years is 71.8%.