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When is a Repeat Bone Density Test Needed?

 

 

A highly respected group of researchers reported on nearly 5,000 older women who did not take an osteoporosis drug. All these women were over age 65 and did not have osteoporosis to start. Over the years, the researchers routinely measured bone density. The report tells us that, in this age group, bone loss is really quite slow—it takes many years for bone loss to accumulate. The average older woman with normal bone density loses only about 5% of her bone density in 10 years.

Given the slow bone loss over time, the researchers found very few women moved from a normal or low bone density category to a level indicating osteoporosis. In fact, even after 15 years only 1 in 10 women had crossed this important threshold. However as expected, those women whose beginning bone density was already borderline osteoporotic were more likely to cross over to the category of osteoporosis.

Bone density is important, but it is not the whole story

The study looked at the progression of bone loss in women who began with relatively normal bone density scores— all had T-scores better than –2.5, a level indicating osteoporosis. The T-score level is important, but it is far from the whole story. Bone density is only one of several important risk factors that together determine the fracture risk in the future. Age is probably the most important risk factor for fracture. We know that every 7-8 years, the risk of having a fracture doubles.[1] There are other clinical risk factors that markedly affect future risk of fracture. Some common ones are: prior fracture with little trauma, smoking, heavy alcohol intake, a mother or father who had a hip fracture, and exposure to cortisone-like drugs. 

How can you use the information from the study

This study’s results do not apply to you if you:

  • Already have a high risk for fracture
  • Already have osteoporosis
  • Are taking an osteoporosis drug
  • Are taking cortisone-like drugs

The logic behind extending testing intervals

  1. One T-score unit (e.g. -1.0) is roughly 10% of one’s bone density. 
  2. Once you are a few years beyond the age of menopause, assuming you are generally healthy, your expected bone loss is 0.5% per year or 5% in 10 years.
  3. 5% loss is -0.5 T-score; thus, you can expect to drop one-half a T-score every 10 years.
  4. Use the level of bone density (T-score) you had on your last test to determine when a repeat test would be worthwhile. You should know your T-score—certainly your doctor knows it.  The table below shows the study’s recommendations.

According to the study, if nothing important changes in your fracture risk profile, when would it be reasonable to have a repeat bone density test?*

What category of bone density did your last test fit into?

Category of bone density results

T-score of this category

Years to next test *

Normal (even for a young woman)

-1.0 or higher

15 years

Mildly low

Between -1.0 and -1.5

15 years

Moderately low

Between -1.5 and -2.0

5 years

Borderline osteoporosis

Between -2.0 and -2.5

1 year

* based on a 10% chance of the next test showing osteoporosis (T-score –2.5 or lower)

*This table doesn’t apply to you if:

  • Your last T-score was -2.5 or lower, (already diagnosed as having osteoporosis).
  • If you are taking a drug for osteoporosis.

What if you have never had a bone density test?

The US Preventive Services Task Forcerecommends that all women over age 65 years have a bone density test. In addition, they recommend a test for women who are under age 65 but have clinical risk factors that predict a 9.3% or higher 10-year risk of having a major osteoporotic fracture. If fracture risk is less than 9.3%, a woman may safely wait a few years for that first test. 

[1] Ettinger et al. Osteoporosis International 2010; 21:25-3

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