My test results were reported as a Tscore to my clinician What is a Tscore

The results of your BMD tests will most likely be expressed as a T-score, 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. The formula is expressed in standard deviations and looks like this:

Patient's BMD - Young normal mean BMD Standard deviation of young normal mean

The World Health Organization (WHO) came up with the following classification of BMD testing results:

Standard deviation

A mathematical measure that indicates how far or how near something is to the mean (average).

• 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 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 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.

Because the development of this method was based on data from white postmenopausal women, interpreting results of BMD tests using this method is less

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.

appropriate for men, premenopausal women, or non-white postmenopausal women. However, it is the standard used for all adults having bone density testing. Recent research suggests that the results do also apply to black women, but the risk of fracture in black women is still 30% to 40% less than in white women. More study is needed for women of other racial/ethnic groups and for men of all groups.

A Canadian panel of the International Society for Clinical Densitometry (ISCD) recently recommended new guidelines for interpreting BMD test results of premenopausal women, men, and children. They recommended that Z-scores be used for children and premenopausal women (see Question 33), and that T-scores can be used for men with fragility fractures or major risks for secondary osteoporosis. The International Society for Clinical Densitometry recommends using a young male norm for calculating T-scores in men (rather than the usual young female).

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.

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

Region

Measured Date

Age (years)

(g/cm2)

Change Versus 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. 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

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 6,000 and I know that I have

6,000 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 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!

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