A bone density test, or DEXA scan, measures the mineral content of your bones to assess their strength. This non-invasive procedure quantifies your bone mineral density (BMD) and evaluates your risk of fracturing a bone. The measurement is a key indicator for diagnosing osteoporosis, a condition characterized by weakened bones prone to fracture.
Understanding the DEXA Scan
The DEXA scan utilizes a very low dose of two different X-ray beams to distinguish between bone and soft tissue. The machine calculates the bone’s density by measuring how much of each beam is blocked. The radiation exposure is minimal, significantly less than a standard chest X-ray.
The procedure is painless, requiring the patient to lie still while a mechanical arm passes over the body. The central DEXA scan, considered the gold standard, focuses on measuring the density of the hip and the lumbar spine. These sites are chosen because they are the most common locations for serious osteoporotic fractures.
Standard Screening Guidelines
For individuals at average risk, major health organizations provide clear age benchmarks for screening. The U.S. Preventive Services Task Force (USPSTF) recommends screening for all women aged 65 years and older. Organizations also recommend screening for men beginning at age 70.
These guidelines apply to those without a history of fragility fractures or other significant risk factors. Postmenopausal women younger than 65 should be screened if their fracture risk is equal to or greater than that of an average-risk 65-year-old woman. This risk can be determined using formal clinical assessment tools, such as the FRAX calculator. For men under age 70, screening is recommended only if they have specific risk factors for bone loss.
Factors Requiring Earlier Testing
Certain medical conditions and medications can accelerate bone loss, necessitating a DEXA scan before standard age recommendations. A history of a fragility fracture—a bone broken from a fall from standing height or less—after age 50 is a strong indicator for immediate testing.
Specific Risk Factors
- Conditions that interfere with nutrient absorption, such as celiac disease.
- Long-term use of high-dose glucocorticoid medications (e.g., prednisone) for three months or more.
- Use of certain anti-seizure medications, such as phenytoin and carbamazepine.
- Chronic diseases like rheumatoid arthritis or hormonal conditions like hypogonadism.
- Low body weight, defined by a Body Mass Index (BMI) below 20 or a body weight less than 127 pounds.
- Current smoking and excessive alcohol consumption (three or more units per day).
Interpreting Test Results and Follow-Up
The primary result of the DEXA scan is the T-score, which compares your measured bone density to the average bone density of a healthy young adult of the same sex. A T-score of -1.0 or higher is considered normal. Scores between -1.0 and -2.5 are classified as osteopenia, meaning low bone mass.
A T-score of -2.5 or lower signifies a diagnosis of osteoporosis, indicating significantly weakened bones and a high risk of fracture. For osteopenia, follow-up involves monitoring BMD at regular intervals, alongside lifestyle modifications like increasing calcium and vitamin D intake and engaging in weight-bearing exercise.
A diagnosis of osteoporosis generally leads to pharmacological treatment to slow bone loss or rebuild bone mass, in addition to dietary changes and exercise. Follow-up scans monitor treatment effectiveness. The frequency of repeat testing depends on the initial T-score and the treatment plan, often ranging from one to two years for those on medication.