A doctor may order a bone density test to determine your risk of osteoporosis, a disease that causes bones to become weak and brittle. Bones become weak when you have lower amounts of calcium or other minerals in your bones. Weak or brittle bones break or fracture more easily, even from seemingly minor problems such as a cough. The bone density test accurately measures bone density and consequently predicts whether someone is at risk for osteoporosis.
Risk Factors
Osteoporosis rarely causes symptoms until it is already at an advanced stage. Therefore, the best way to determine if you should have a bone density test is to have your doctor assess your risk factors. Some risk factors are being older than 65 if you are a woman, or older than 70 if you are a man; being older than 50 with a history of broken bones; being a woman with early menopause or taking certain medications, such as Prednisone or various anti-seizure medications.
Bone Density Test
If your doctor orders a bone density test, you can expect a fairly painless test called DXA, or dual energy X-ray absorptiometry. The test uses X-rays to estimate the density of calcium and other minerals in your bone -- usually in your spine, hip and wrist. The DXA machine has a central device used for measuring bone density in the hip and spine, and a peripheral device that measures bone density in the wrist, heel or finger. The DXA machine sends low-dose X-rays through bones and measures the absorption of energy peaks.
Procedure
Before the exam, you should not take calcium supplements for at least 24 hours but you may eat as you normally would. You should wear comfortable clothing that do not have metal parts, such as zippers, belts or metal buttons. The technician may ask you to remove jewelry, dentures and other metal objects and have you wear a gown. If you recently had a barium exam or have been injected with a contrast material, you may need to wait 10 to 14 days before having a DXA test since contrast interferes with this test. You will be asked to lie on a padded table and to hold still. The technician will then take X-rays of the relevant bones. The procedure should take between 10 and 30 minutes, depending on what needs to be scanned.
Results
A radiologist will interpret the results and send a report to your doctor. Your test will be composed of two scores: a T score and a Z score. The T score compares your bone mass with a same-gender young adult's bone mass. A score above -1 is considered normal, whereas a score between -1 and -2.5 would be classified as osteopenia, or low bone mass. Scores below -2.5 are defined as osteoporosis. The Z score, on the other hand, compares your bone mass to that of other people of your gender and age. Small fluctuations in this score are normal; large discrepancies may indicate the need for further testing. Your doctor may ask you to repeat this test every two years to see changes in your bone density over time.
Limitations
DXA machines cannot accurately measure hip and spine bone density in people who weigh more than 300 lbs. Some newer machines exist that can handle patients weighing up to 400 lbs. but these are not widely available. The DXA test has limited effectiveness in people who have a spinal deformity or who have had spinal surgery in the past. If you fall in either of these categories, your doctor may order different tests to predict your risk of fractures.
