BONE MASS DENSITY

Bone Mass Measurement: What the Numbers Mean

• What Is a Bone Density Test?

  A bone mineral density (BMD) test is the best way to determine your bone health. The test can identify osteoporosis, determine your risk for fractures (broken bones), and measure your response to osteoporosis treatment. The most widely recognized BMD test is called a dual-energy x-ray absorptiometry, or DXA test. It is painless—a bit like having an x-ray. The test can measure bone density at your hip and spine.

  • What Does the Test Do?

  A DXA test measures your bone mineral density and compares it to that of an established norm or standard to give you a score. Although no bone density test is 100-percent accurate, the DXA test is the single most important predictor of whether a person will have a fracture in the future.

  T-Score

  Most commonly, your DXA test results are compared to the ideal or peak bone mineral density of a healthy 30-year-old adult, and you are given a T-score. A score of 0 means your BMD is equal to the norm for a healthy young adult. Differences between your BMD and that of the healthy young adult norm are measured in units called standard deviations (SDs). The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.

  As shown in the table below, a T-score between +1 and −1 is considered normal or healthy. A T-score between −1 and −2.5 indicates that you have low bone mass, although not low enough to be diagnosed with osteoporosis. A T-score of −2.5 or lower indicates that you have osteoporosis. The greater the negative number, the more severe the osteoporosis.

  
  

  World Health Organization Definitions Based on Bone Density Levels

  Level	Definition
  Normal	Bone density is within 1 SD (+1 or −1) of the young adult mean.
  Low bone mass	Bone density is between 1 and 2.5 SD below the young adult mean (−1 to −2.5 SD).
  Osteoporosis	Bone density is 2.5 SD or more below the young adult mean (−2.5 SD or lower).
  Severe (established) osteoporosis	Bone density is more than 2.5 SD below the young adult mean, and there have been one or more osteoporotic fractures.