Basics
An advanced form of X-ray technology called bone densitometry---also known as bone density testing or dual-energy X-ray absorptiometry (DMX)---is used to measure the mineral density in bones. DMX is a noninvasive radiology test most often used to diagnose osteoporosis.
During menopause, many women experience bone loss that makes their bones more subject to breaking; older men experience it too, but less often. Doctors at Radiology Info, a medically supervised online resource, report that in addition to testing post-menopausal women, bone density tests are used to diagnose bone density issues in men and women with a family history of bone breakage, those with thyroid conditions and people who have broken bones after a minor accident. Certain medications, such as corticosteroids, barbiturates and anti-seizure medications also can cause bone loss.
Procedure
The bone density test measures bone loss by exposing parts of the body to ionizing radiation, which electrically charges atoms and molecules. Little or no preparation is needed. Patients typically are asked to avoid taking calcium supplements for 24 hours prior to the exam and to alert doctors about any tests using barium or other injected contrast X-rays that occurred recently. When the exam is being performed in a clinic or radiology room, the patient lies on a padded table that is lit from below, similar to a tanning booth, report Mayo Clinic doctors. A medical imager passes over the body without touching it. The large lit arm sends radiation through the body, equal to about one-tenth of the amount of radiation received in a typical chest X-ray. The entire procedure takes about 10 minutes. A smaller portable device can be used on the hand or foot to measure bone density. Portable bone scans often are provided by pharmacies, and though helpful, they're not as accurate as full-body or hip and leg bone scans.
Readings
Two numbers are gleaned from a bone density test, one to measure how much the bone density deviates from the norm and one that indicates what number is expected from the patient according to age, weight, sex, race and gender. A T-score compares the density to the average and the Z-score is the number that can help doctors diagnose the cause of the bone loss. A T-score above -1 is considered normal. The beginning signs of osteoporosis, also referred to as osteopenia, are identified with a score between -1and -2.5. Numbers on the T-score scale below -2.5 indicate osteoporosis.
If a Z-score is less than 1.5, then other conditions may be present that are causing the bone loss. Other factors that can influence bone density scores include tobacco use, breast cancer, eating disorders, depression and alcoholism.
