Measuring Bone Density in Osteoporosis

The diagnosis and treatment of Osteoporosis relies on a diagnostic called a bone density scan, which gives a person a Z-score and a T-score. T-scores compare bone density with that of a healthy person, whereas Z-scores use the average bone density of people of the same age, sex, and size as a comparative index.

Most physicians prefer using Z-scores for children, teenagers, premenopausal females, and younger males. These scores are helpful for diagnosing secondary osteoporosis, which stems from underlying medical conditions, rather than primary osteoporosis, which usually results from aging.

Bone density scans are swift and require little prep by the patient, and the results can tell a doctor whether a person has osteopenia or osteoporosis or is at risk of developing either condition.

If you have a lower than normal bone density score — between -1 and -2.5 — you have osteopenia. If your score is lower than -2.5, you may be diagnosed with osteoporosis. Osteoporosis is the more serious progression of osteopenia.

These scores are in the form of standard deviations. This mathematical term measures how close a number is to the average. A low standard deviation means that the number is close to the average, and a high standard deviation means that it is further from the average. The Z-scores and associated meaning for each follows–

  • +1–2: Bone density is higher than in others of the same age, sex, and body size.
  • 0: Bone density is the same as in others of the same age, sex, and body size.
  • -1: Bone density is lower than in others of the same age, sex, and body size.
  • -2: Doctors consider scores higher than this to be normal.
  • -2.5: This score or lower indicatesTrusted Source secondary osteoporosis.

The standard diagnostic for scoring is the DEXA (Dual-Energy X-ray Absorptiometry) scan that uses a low dose of ionizing radiation to measure bone density, and involves the following steps:

  • The patient lies on his/her back on a padded table, either placing the legs out straight or resting them on a padded platform.
  • One scanning machine will pass over the lower spine and hips, and another scanning machine will pass underneath.
  • During the scanning, the patient will need to be very still, and the technician might ask them to hold their breath.
  • The images go to a computer where a healthcare professional can view (“read”) them.

Little preparation is necessary for the central DEXA scan, however, a doctor may ask a person to refrain from taking calcium supplements 24–48 hours prior to the test. They may also ask the individual to avoid wearing metal jewelry or clothes with metal parts, such as buttons or buckles.

Uses of central DEXA scans include:

  • diagnosing osteoporosis
  • assessing a person’s risk of fractures
  • evaluating the body’s response to an osteoporosis treatment.

Unlike central DEXA scans, peripheral scans (via a portable machine) usually play a role in screening. They identify people who may need further bone scans, and measure bone density in the heel, finger, wrist or forearm.