What is your T-Score, Really?

Category: Metabolic Bone Disease Published on Wednesday, 25 June 2008 Written by Yong Tsai, MD

Linda, a 73-year-old woman whose recent BMD (bone mineral density) test revealed an L2-L4: 0.864 g/cm2 with a T-Score –2.8: 72% and a Femur: 0.577 g/cm2 with a T-Score of –3.5, 58% was totally confused about her results. And like many others in her situtation, Linda needed some clarification.

Our bones are composed of cells called osteoclasts, which break down bone creating microscopic pits, and osteoblasts, which form new bone to fill these holes. From birth to about age twenty (our peak bone mass age), more bone is made than is absorbed allowing a continual increase of bone density. From then on, bone formation and absorption continues at an equal rate, until menopause for women or about ten years later for men. When bone absorption surpasses formation, bone density decreases, possibly causing osteoporosis and fractures.

Today’s most accurate and sensitive test used to measure bone density, a major factor in determining bone strength, is called Dual Energy X-ray Absorptiometry (DEXA-bone density testing). While the bone density machine measures the size of the femur and vertebrae and calculates the amount of mineral in each square centimeter of bone, it computes an average of three vertebrae (L2-L4) and a total femur to determine bone density. Finally, comparisons are then made to produce further measurements.

The best measurement to predict a patient’s overall risk of fracture is the famous T-Score, or Young Adult Comparison. T-Scores, comparisons of BMD with the average 20-45 year olds’ T-Score of 0, are measured in standard deviations (SD), in approximate increments of 0.12 g/cm2. Think of class grades: A=90-100 or T-Score of 1 being excellent, B=80-90 or a T-Score of 0 being average, C=70-80 or a T-Score of -1 being satisfactory, D=60-70 or a T-Score of – 2.0 being unsatisfactory, and F= 50-55 or a T score of -2.5 as unacceptable. Essentially, the lower the T-Score, the greater the risk of fracture.

Furthermore, because everything has a limit, like a pass/fail grade of 50, BMD T-Scores are divided into two levels: –1.0 to –2.49 SD below the average defining osteopenia and greater than –2.5 for a diagnosis of osteoporosis. For example, Linda’s L2-L4 BMD of 0.864 g/cm2 has a T-Score of –2.8 or 72%, which suggests that she has lost 28 % of her peak bone density and that she has osteoporosis.

The National Osteoporosis Foundation recommends treating all postmenopausal women regardless of risk factors with a T-Score of –2.0 and those with a T-Score –1.5 with a previous fracture, weight of less than 127 pounds, current smoking and a family history of osteoporosis.

And even though bone loss due to osteoporosis is otherwise difficult to detect at an early stage, the new TV commercial is right about the power of T-Scores: definite diagnosis, precise assessment of severity, and accurate prediction of future fracture.

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