The osteoporosis T score is a measurement used to evaluate the bone mineral density of people, most typically post-menopausal women and women over 50. It compares the bone mineral density (BMD) of an individual to the BMD of someone with peak bone density - namely, an average thirty-year-old Caucasian. The osteoporosis T score is simply a comparison of an individual's bone mineral density to that of someone with ideal bone mineral density. Because osteoporosis is a silent disease (there are few obvious symptoms of osteoporosis), osteoporosis tests such as the T score are important tools.
In order to understand the osteoporosis T score, you first need to understand the concept of bone mineral density and how it relates to T score. Bone mineral density is how many grams of mineral (and trabeculae - spongy marrow-filled bone) are in a square-centimeter of bone. Standard x-rays can only give a very general estimation of your bone mineral density, so other more accurate osteoporosis tests have been developed. DEXA (dual-energy X-ray absorptiometry) scanning, also referred to as DXA scanning or bone densitometry, is the state-of-the-art method for determining bone mineral density. It can quickly measure the bone mineral density in almost every part of the body. Depending on the machine, it can take anywhere from 30 seconds to a few minutes. It is completely painless and non-invasive. The x-ray exposure is only one-tenth that of a chest x-ray. It is highly accurate, able to measure changes in bone mineral density of as little as 1 to 4 percent. The most frequently scanned regions are the lumbar spine, total hip, and femoral neck. If these areas can't be scanned in a patient, then the forearm (especially the wrist) may be scanned.
The issue is that there are three different manufacturers of DEXA scanners: Hologic, Norland, and Lunar. Unfortunately, each one measures bone mineral density differently (due to differences in dual energy methods, means of calibration and detectors) and the results from one machine can't be reliably compared to the results of the other two. As a result, the BMD results are machine dependent. Although efforts have been made to try and correlate the different manufacturers' results, they have not been successful. The osteoporosis T score was derived as the solution to this problem.
The basic equation for T Score is:
T Score = (Patient's BMD minus the reference BMD)/1 SD (standard deviation) of the reference BMD
The T score is highly dependent on the reference BMD in the equation. The reference BMD is calculated based on the particular brand of DEXA scanner, the specific site of the bone being scanned, and the race and sex of the individual. The WHO (World Health Organization) uses healthy, young, Caucasian women in the BMD reference value. However, bone density machines in the United States use the race and gender of the person being scanned in the BMD reference value. So, there are a number of different equations depending upon what variables are used in the reference BMD value. Because the equations are also dependent on the skeletal site being examined, a T score for the femoral neck would be different from a T score for the total hip region. An example is as follows:
The subject is a white female who had a scan of her femoral neck region by a Hologic machine. Her BMD is 0.53 g/cm squared. She would have an osteoporosis T score of -2.72 standard deviations.
This result is calculated as follows:
So, the T Score for this patient = (her BMD - reference BMD)/SD = (0.53 - 0.858)/0.12 = -2.72.
A result of -2.72 would indicate a diagnosis of osteoporosis. Yes, it's complicated, but now you have a general idea of how the T score is calculated. For a more detailed explanation, please visit this site on bone densiometry.
The table below shows what an osteoporosis T score means in terms of whether or not an individual is considered to have osteoporosis (or osteopenia, the pre-cursor to osteoporosis).
|World Health Organization Standards|
for Diagnosing Osteoporosis
|Normal Bone Mass||1.0 SD|
|Mild Osteopenia||1.0 to -1.0 SD|
|Moderate Osteopenia||-1.0 to -2.5 SD|
|Osteoporosis||-2.5 SD or lower|
One standard deviation represents a 10-12% change in bone mineral density. So someone with a standard deviation of -2.0 has 20 to 25% less BMD than someone in the reference group (a healthy person with peak bone mass). Does this mean that this person has lost that much BMD? Not necessarily. In order to make that determination, we would need to know what this individual's peak bone mineral density was. My peak BMD may be more or less than yours. We are all individuals. Unless this person had had a scan done at the age of peak bone mineral density, it would be impossible to know for sure how much bone he or she had actually lost. So, what does the T score really tell us? Statistics show that the risk of fracture doubles with every 1 point decrease in standard deviation.
The Z score is a measurement used to evaluate bone mineral density, typically in healthy pre-menopausal women, men under age 50 and children (young women with multiple osteoporosis risk factors may want to be scanned for pre-menopausal osteoporosis). An osteoporosis Z score is similar to an osteoporosis T score, in that it is measured in units of standard deviation. But in the case of a Z score, the reference BMD is based on someone of the same sex, age and race of the patient. So a Z score compares your bone density to that of other like individuals. As with the T score, the resulting Z score is dependent on the DEXA machine manufacturer and the site at which the bone is scanned. A Z score above -2.0 means an individual's bone density is within the range expected for someone his or her age, while a Z score below -2.0 means that person's bone density is below normal for someone his or her age.
Although the Z score is reported as part of the standard bone mineral density testing results, it is not considered as valuable to medical professionals as the T score. The T score is a more reliable indicator of fracture risk.
A bone mineral density test and subsequent osteoporosis T score is recommended for the following people:
To read specifics on osteoporosis risk factors, and to determine if you are at risk, please go to my page on osteoporosis. And, if you've received a low osteoporosis T score and have been diagnosed with osteoporosis, don't miss all I've written about natural treatments for osteoporosis.. And finally, remember to check back often - as I find more information about new osteoporosis tests, I'll let you know!
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