Dual-energy x-ray absorptiometry, or DXA, is a low-dose x-ray procedure that measures the amount of calcium in the bone and how tightly it is packed in the spine and hip. The results are based on standard deviations — unit decreases from the norm — for healthy young adults. In other words, "normal" is the average peak bone mass of a healthy young white woman at age 30, when bone mass peaks for most people. Any difference between a woman being measured and the established standard is reported as a T-score. A score between +1 and -1 is considered within the range of normal for women at any age. A rating between -1 and -2.5 is defined as osteopenia. Osteoporosis is defined as a T-score reading of -2.5 or lower, which translates into bones that are 32 percent less dense than those of the average 30-year-old woman. DXA is typically used to make an initial diagnosis and treatment decision. DXA can also be used to monitor patients long-term, whether or not they choose to go on medication. Because different manufacturers use different calibration methods, it is crucial that you have subsequent tests on the same machine.
In addition to DXA, you may want to consider the following specialized tests.
Bone Turnover Markers
These urine tests measure how fast bone is remodeling, and so are often used along with DXA to determine the rate of structural loss. Turnover tests are also used to assess whether medical treatment is working to slow bone loss.
Vitamin D Level
The 25-hydroxy-vitamin-D test measures the level of the vitamin in your blood. "More than 50 percent of adult women are vitamin D insufficient," says Ruth Freeman, MD, director of the bone densitometry unit at Montefiore Medical Center in New York. "That means they’re not absorbing calcium." The fix: vitamin D supplements. Check labels — the most potent, effective form is vitamin D3 (cholecalciferol).
Parathyroid Hormone Measurement
Parathyroid hormone is responsible for maintaining normal levels of calcium in the blood. Excessive levels can result in bone loss as well as elevated blood levels of calcium. The cause is usually a benign tumor on the parathyroid gland, which can be corrected by surgery.
Urine Calcium Level
This 24-hour-urine-collection test reveals whether you’re excreting more calcium than normal. Medications such as diuretics and additional calcium supplements can even out this imbalance.
Other underlying conditions, such as multiple myeloma, celiac disease, or any disease that infiltrates the bone, will affect bone remodeling. Treating the underlying cause may help stabilize your skeleton.
Originally published in MORE magazine, June 2006.