A T-score measures deviations from normal. When used to check for osteoporosis, your T-score compares your bone density to that of other adults. Your T-score determines your degree of bone loss and helps you predict your risk for bone fracture. Knowing your risk allows you to weigh different options for increasing bone density, such as taking medication. A DEXA or DXA scan, which stands for dual energy X-ray absorptiometry, computes your T-score. This non-invasive test takes just a few minutes to complete; it differs from an X-ray and requires specialized equipment.
What a T-score Shows
A T-score in bone density testing takes the thickness of your bones and compares it to that of a healthy adult. The sites normally tested include the anteroposterior projection of the spine and the hip. The lateral projection of the spine can also be tested and might give a better indication for future risk of vertebral fracture, according to the Cleveland Clinic. A Z-score goes one step further and compares your bone density to that of a person in your age group.
If your bones are as dense as those of a healthy adult, you have a T-score of 0, which means that your bone density does not deviate from that of a healthy adult. Any score between 1.0 and -1.0 is considered a normal reading. The DXA scan doesn't give an accurate picture of fracture risk for pre-menopausal women and generally isn't used for routine screening under age 50, according to the National Osteoporosis Foundation.
A T-score of -1.0 to -2.5 indicates that you have osteopenia. A T-score of -2.5 or less indicates osteoporosis. Each standard deviation equals a bone loss of around 10 to 12 percent, according to the American Bone Health website. A T-score of -2.5 plus the occurrence of one or more osteoporotic fractures indicates severe osteoporosis. Each standard deviation in the spine increases the risk of vertebral fractures by 2.0 to 2.4, the Cleveland Clinic explains. Around 54 percent of postmenopausal women have osteopenia, or low bone mass, and another 30 percent have osteoporosis, according to the Cleveland Clinic. Osteoporosis in men tends to develop five to 10 years later than in women.
A T-score cannot tell how much bone mass you've lost. Even though you might have a T-score of -2 or more, indicating a drop of 20 to 30 percent in bone mass, it doesn't mean that you've lost that much bone. You might have always had lower bone mass than normal. Doing sequential bone mass density testing every one to two years gives an indication of whether or not you're continuing to lose bone. Blood tests can also help determine if you're currently losing bone. Your T-score in different areas of bone might also vary.