What is the normal T score in a Bone Mineral Density (BMD) scan and which score is a concern?

From the Guidelines

A normal T-score in a BMD scan is defined as a T-score ≥ -1.0, while a T-score between -1.0 and -2.5 indicates osteopenia, and a T-score ≤ -2.5 indicates osteoporosis, which is a significant concern requiring medical attention. The T-score represents the number of standard deviations above or below the average bone density of a healthy 30-year-old adult. According to the World Health Organization criteria, as mentioned in the study by 1, a T-score value less than or equal to -2.5 at the lumbar spine, one-third radius, femoral neck, or total hip is consistent with osteoporosis.

Key Points to Consider

• The World Health Organization established diagnostic thresholds to help identify individuals who would benefit from intervention to prevent fractures, which can include calcium and vitamin D supplementation, weight-bearing exercise, and in some cases, prescription medications like bisphosphonates.
• BMD scans typically measure density at the hip, spine, and sometimes forearm, as these are common fracture sites.
• A T-score between − 2.5 and − 1.0 is defined as “osteopenia”, “low bone mass” or “low bone density”, as stated in the study by 1.
• The study by 1 also emphasizes the importance of accurate BMD measurements, highlighting that DXA is a clinically proven method of measuring BMD in the lumbar spine, proximal femur, forearm, and whole body.

Clinical Implications

• It is crucial to identify individuals with low bone mass or osteoporosis to provide appropriate treatment and prevent fractures, as fracture risk is determined when BMD as measured by DXA is compared with a gender-matched asymptomatic reference population, as mentioned in the study by 1.
• The FRAX tool, which factors include hip BMD, age, sex, height, weight, family history of hip fracture, smoking, steroid use >3 months, rheumatoid arthritis, and alcohol use, can be used to assess fracture risk in patients with low bone mass, as stated in the study by 1.

From the Research

Normal T Score in a BMD Scan

• A normal T score in a BMD scan is typically considered to be above -1, as it compares the patient's bone density with healthy, young individuals of the same sex 2.
• The World Health Organization has established criteria for classifying BMD in postmenopausal women as normal, osteopenia, or osteoporosis based on T-scores 3.
• A T-score of -2.5 or less at the femoral neck is generally considered to define osteoporosis 2.

Concerns with T Scores

• A negative T-score of -2.5 or less at the femoral neck defines osteoporosis, which is a concern as it increases the risk of fractures 2.
• A negative Z-score of -2.5 or less should raise suspicion of a secondary cause of osteoporosis, which is also a concern as it may require further investigation and treatment 2.
• The diagnosis of osteoporosis based on a single measurement of BMD and point estimates of changes in BMD may be inappropriate and unreliable, which is a concern as it may lead to misdiagnosis or inappropriate treatment 4.

Interpretation of BMD Results

• BMD results should be interpreted in conjunction with clinical risk factors for fracture, such as age, sex, and family history 3.
• The use of clinical risk calculators can help predict the 10-year probability of a hip or major osteoporotic fracture, which can inform treatment decisions 2.
• Serial BMD measurements can be used to monitor the effectiveness of treatment, but require special attention to instrument calibration, acquisition technique, analysis, and precision assessment 3.