The T-score is a standardized measurement used to assess bone mineral density (BMD). This score is obtained through a Dual-energy X-ray Absorptiometry (DEXA) scan, a low-radiation X-ray procedure. The primary purpose of the T-score is to quantify how a person’s bone density compares to the optimal bone mass achieved by healthy young adults. It is the principal diagnostic tool for identifying low bone mass and osteoporosis in postmenopausal women and men aged 50 and older.
Understanding the Statistical Basis
The T-score is a statistical measure expressing the difference between a person’s measured bone mineral density and the peak bone mass of a reference population. This reference group is composed of healthy young adults, typically around 30 years old, who have achieved maximum bone strength. The comparison is calculated in units of standard deviation (SD), measuring the variability from the mean of that population. A T-score of 0 indicates the person’s bone density is exactly the same as the average peak bone mass. A negative T-score signifies the individual’s BMD is lower than the young adult average; for example, a score of -1.0 is one standard deviation below the mean.
Interpreting the Diagnostic Ranges
Normal Bone Density
The World Health Organization (WHO) established specific T-score ranges to classify bone health in postmenopausal women and older men. A T-score of -1.0 or higher represents normal bone density, associated with the lowest risk of fragility fractures.
Osteopenia
A T-score that falls between -1.0 and -2.5 is classified as osteopenia, indicating low bone mass. This suggests an increased risk of bone fracture compared to individuals with normal BMD. For every one standard deviation decrease below the normal range, the fracture risk approximately doubles.
Osteoporosis
The diagnosis of osteoporosis is made when the T-score is -2.5 or lower. This result signifies that bone density is significantly reduced, leading to a substantial increase in bone fragility and the highest risk for fracture from a minor fall or trauma. A prior fragility fracture can also lead to an osteoporosis diagnosis even if the T-score is in the osteopenia range.
T-score versus Z-score
The T-score is used to diagnose low bone mass in older adults by comparing density to peak bone strength. The Z-score compares a person’s bone density to the average density of people who are the same age, sex, and ethnicity. The Z-score is the preferred measure for children, premenopausal women, and men under the age of 50. A low Z-score, typically defined as -2.0 or lower, suggests the person’s bone density is unusually low for their age group. This result prompts a medical investigation to look for secondary causes of bone loss, rather than just age-related bone thinning. While the T-score is a diagnostic tool for osteoporosis, the Z-score acts as a screening tool to detect unexpected bone health issues in younger populations.
Factors Affecting Bone Density Scores
Bone density scores are influenced by factors affecting the body’s ability to build and maintain bone mass. These factors are generally divided into non-modifiable and modifiable categories.
Non-Modifiable Factors
Genetics accounts for 60 to 80% of the variation in bone mineral density. Age and sex are also significant, as bone mass naturally declines over time. Women typically experience accelerated bone loss following menopause due to hormonal changes.
Modifiable Factors
Several modifiable lifestyle factors contribute to a lower T-score:
- A diet lacking sufficient calcium and Vitamin D.
- A sedentary lifestyle or low physical activity, which limits the mechanical stress necessary to stimulate bone strengthening.
- Chronic tobacco use and excessive alcohol consumption.
- Certain medications, such as long-term corticosteroid use.