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My test results were reported as a T-score to my clinician. What is a T-score?

The results of your BMD tests will most likely be expressed as a T-score[1], which uses a mathematical formula and assigns your bone density results either a positive or negative number. Normally, density would be expressed as weight per volume, but because the bone images are only two dimensional, area must be used to calculate your results (grams per centimeter squared [g/cm2]). Because there are different types of machines for BMD testing, a standard way of expressing the bone density measurements was developed. This method uses a formula comparing your individual results of BMD testing with the mean (average) bone density of healthy young adults of your same sex. The formula is expressed in standard deviations[2] and looks like this:

The WHO came up with the following classification of BMD testing results:

• T-score above —1.0 indicates that your bone mass is normal for your age.

• T-score between —1.0 and —2.5 indicates that you have osteopenia (low bone mass).

T-score —2.5 or lower indicates that you have osteoporosis.

T-score —2.5 or lower plus the presence of one or more fragility fractures indicates that you have severe osteoporosis. A fragility fracture[3] is a fracture of a bone that happens with little or no force or trauma, such as falling from your standing height or less (see Question 74).

In 1994, when the WHO developed this method of classification of results, the researchers felt it was important to present results in a way that related them to the lifetime risk of fracture. After all, understanding your risk for breaking bones is a major reason for doing the BMD testing. Bone density is about 10% to 12% lower for each standard deviation below normal (or BMD of 0). Likewise, the risk for fracture doubles

As the T-score decreases, the risk for fracture increases. Courtesy of the National Association of Nurse Practitioners in Women's Health (NPWH).

Figure 9 As the T-score decreases, the risk for fracture increases. Courtesy of the National Association of Nurse Practitioners in Women's Health (NPWH).

Source: Meunier PG et al. Clin Ther 1999;21:1025-1044.

with each standard deviation below normal bone density. So if your T-score is —1.0, your risk of having a fracture is roughly twice that of a young adult. If your T-score is —2.0, your risk is about 4 times higher, and if your T-score is —3.0, your risk for fracture is about 8 times higher than that of a young adult! Figure 9 shows how much more at risk you are as your T-score decreases. But remember, your T-score is only one indicator of fracture risk; other factors—like thinness, age, prior fracture, diet, exercise, and more—also contribute to your risk for breaking a bone.

The initial development of this method was based on data from White postmenopausal women, making interpretation of results less appropriate for men, pre-menopausal women, or non-White postmenopausal women. However, data have been collected from men and persons of varied racial/ethnic groups over time. T-scores are now calculated against a mean score developed from young normal people of the same sex as the person being tested. This method is the standard used for all postmenopausal women and men over the age of 50.

The International Society for Clinical Densitometry (ISCD) recommends using sex-matched and race- or ethnic-adjusted Z-scores for interpreting BMD test results of premenopausal women, men, and children (see Questions 32—33).

During conventional DXA testing, hip and spine measurements are taken. Depending on the brand of DXA machine, the neck of the femur (upper leg bone) or the point between the neck of the femur and the long part of the upper leg bone (intertrochanteric region) are usually measured for the hip T-score. Sometimes both are measured and the scores are averaged. The lumbar vertebrae (lower back bones) L1 to L4 are measured for the spine T-score. The lower of the two T-scores (hip or spine) is used to classify your degree of bone loss using the WHO guidelines. So, if you have a lower T-score (more bone loss) in your hip than in your spine, your hip T-score will be used for interpreting your results and making the diagnosis. Table 2 shows an example of the results from a DXA test.

Delia's comment:

I'm used to receiving lab results that express a number that I can understand. For example, a white blood cell (WBC) count might come back as 6000 and I know that I have 6000 white blood cells in every little drop of blood, and that's pretty normal. But when I received the results of my BMD testing of my hip and spine, I was not expecting

Table 2 DXA Results

BMD

Young Adult

Age Matched

Region

(g/cm2)

(%)

T-score

(%)

Z-score

Femoral Neck

0.715

73

-2.2

90

-0.6

L2-L4 (spine)

1.605

134

3.4

154

4.7

Change Versus

Region

Measured Date

Age (years)

BMD

(g/cm2)

Baseline

(%)

Baseline

(%/yr)

Femoral Neck

02/22/2005

82.5

0.715

-0.3

-0.1

06/05/2002

79.8

0.718

Baseline

Baseline

L2-L4

02/22/2005

82.5

1.605

2.6*

1.2*

06/05/2002

79.8

1.549

Baseline

Baseline

T-score is based on the U.S. female reference population aged 20—40. Z-score is matched to others on age, weight, and ethnicity.

These results show that the patient (an 82-year-old White female) has osteopenia at the femoral neck of the hip (T-score of —2.2) and normal bone density at the lower spine (T-score of 3.4). Her bone density decreased at the femoral neck by a very small amount (0.3%) since her last test, but her bone density increased at the lower spine by 2.6%. The small change in bone density at the hip is not significant (e.g., it might be caused by error of the machine rather than real change), but the change at the spine is significant, meaning that she has successfully increased her bone density in that area. This report also gives the percent of change per year from current results compared to her baseline. Again, the change per year at the hip was not significant (decrease of 0.1% per year) but was significant (meaning it was not caused by measurement error, but is a real change) at the lower spine where she has had an increase of 1.2% per year since her last test. For a BMD change to be significant, it must be greater than a 2—4% change at the person's spine or a 3—6% change at the person's hip. Her Z-scores (—0.6 at the hip and 4.7 at the spine) indicate that she is close to the same bone density of others in her age group at the hip but has a higher density than others do in her age group at her spine. The Z-score results together with the T-score results suggest that she most likely has osteoporosis related to postmenopause and aging, and not due to secondary causes such as illnesses or medications.

*Indicates significant change based on 95% confidence interval

just a negative number. I was told that my spine was okay. However, I was also told that my T-score was —1.5 and therefore I had low bone mass in my hip bone. What's even more confusing is that a normal result can still be a negative number, so if I get my bone density to improve enough to be considered normal, I still might have a T-score of —0.9! That doesn't feel like very good feedback to me!

  • [1] A positive or negative number denoting bone density in comparison with that of healthy young adults.
  • [2] A mathematical measure that indicates how far or how near something is to the mean (average).
  • [3] Term used to describe a fracture that occurs with very little trauma or force and from a height that is usually not great enough to cause broken bones, usually indicating that the bone is weak. Also called an osteoporotic fracture.
 
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