A bone density test, formally known as dual-energy X-ray absorptiometry (DXA or DEXA), is a specialized, non-invasive imaging procedure. It uses a very low dose of X-ray energy to measure bone mineral density (BMD). The primary purpose of a DXA scan is to screen for osteoporosis, a condition characterized by fragile, weakened bones that increase fracture risk. The scan provides a quantitative measure of bone mass, helping healthcare providers assess current bone health and estimate future fracture risk.
Preparing for the Scan
You do not have to completely undress for a DXA scan, but preparation focuses heavily on clothing choices to ensure accurate results. Metallic objects interfere with the low-dose X-ray beams and distort the measurement. Therefore, patients should wear loose, comfortable clothing free of metal components, such as zippers, buttons, snaps, or buckles.
You must also remove all jewelry, including necklaces, body piercings, and watches, before the procedure begins. If your clothing contains metal, or if you are wearing an underwire bra, the technician will ask you to change into a hospital gown. Wearing metal-free apparel helps prevent the need for a change of clothes and keeps the scan on schedule.
Beyond clothing, dietary and medical considerations are necessary leading up to the test. Avoid taking any calcium supplements or multivitamins containing calcium for at least 24 hours prior to your appointment. Inform the technologist if you have recently had an imaging procedure involving contrast dye (e.g., a CT scan or barium study). These substances can temporarily interfere with the DXA results, potentially requiring a waiting period of several days.
What Happens During the Test
Once prepared, you will lie down on the DXA machine table. The technician positions your body to optimize the view of the target areas, typically the lower (lumbar) spine and one or both hips. For the hip scan, your foot may be placed in a holder to rotate the leg slightly inward. A foam block may be used under your legs to flatten the lower back for the spine scan.
The scan begins when a mechanical arm holding the X-ray tube slowly passes above your body. This arm emits two distinct X-ray energy beams absorbed differently by bone and soft tissue. A detector measures the X-ray energy passing through your body to calculate bone density. You must remain very still to prevent blurry images, but the entire process usually takes only 10 to 20 minutes, allowing you to return to normal activities immediately afterward.
Interpreting Your Results
The core result of a bone density test is the T-score, which compares your measured bone density to that of a healthy young adult of the same sex. This single number is the standard metric used by the World Health Organization to diagnose bone conditions. A T-score of -1.0 or higher is considered normal bone density.
A score between -1.0 and -2.5 indicates low bone mass, a condition called osteopenia. If the T-score is -2.5 or lower, a diagnosis of osteoporosis is made, signifying reduced bone strength and a high risk of fracture. Your healthcare provider uses this T-score and other clinical factors to determine the appropriate course of action, such as lifestyle adjustments, medication, or a follow-up scan.