DEXA Scans Explained — How They Work and What Your Results Mean

• What a DXA scan is and how it works
• What to expect during the procedure
• How T-scores and Z-scores work
• What your results mean for your bone health
• How often you should be scanned
• Insurance and Medicare coverage information

DXA stands for Dual-energy X-ray Absorptiometry (sometimes written as DEXA). It is the gold standard test for measuring bone mineral density (BMD) -- the amount of mineral packed into your bones.

The scan uses two low-energy X-ray beams that pass through your body at slightly different energy levels. Because bone, muscle, and fat absorb these beams differently, the machine can calculate precisely how dense your bones are.

Think of it like weighing something while it is still inside a box. The DXA machine can determine the "weight" (density) of your bones without opening you up -- just by measuring how much of the X-ray beam gets through.

A DXA scan is the most accurate and widely used test for diagnosing osteoporosis. It is quick, painless, and uses very low radiation.

Before the scan

• No special preparation is usually needed
• Wear comfortable clothing without metal zippers, buttons, or belts (you may be asked to change into a gown)
• Remove jewelry around the areas being scanned
• Tell the technician if you have had a barium study or CT scan with contrast recently, as residual contrast material can affect results
• Calcium supplements: Some facilities ask you to avoid taking calcium supplements for 24 hours before the scan -- check with your provider

During the scan

1. You lie on a padded table, fully clothed (or in a gown)
2. A scanning arm passes over your body -- it does not touch you
3. The machine measures bone density at two main sites:
   • Lumbar spine (lower back) -- vertebrae L1 through L4
   • Hip (proximal femur) -- the femoral neck and total hip area

4. You need to stay still for a few minutes while each site is scanned
5. The entire procedure typically takes 10-20 minutes

What it feels like

Nothing at all. There is no injection, no enclosed space, and no pain. The radiation dose is extremely low -- approximately 1-5 microsieverts (equivalent to about 1/10th of a chest X-ray, or roughly the same natural background radiation you receive in a single day).

Many patients are surprised by how simple and comfortable the scan is. It is easier than most routine medical tests.

Your DXA results are reported primarily as a T-score, which compares your bone density to that of a healthy young adult at peak bone mass (typically a 30-year-old of the same sex).

What the numbers mean

How to read the number

The T-score is measured in standard deviations from the average. Each whole number represents a significant difference:

• A T-score of -1.0 means your bone density is 1 standard deviation below the young-adult average
• A T-score of -2.0 means you are 2 standard deviations below
• A T-score of -3.0 means you are 3 standard deviations below

As a general rule, each standard deviation decrease in bone density roughly doubles the fracture risk.

An example

Imagine your DXA report shows:

• Lumbar spine T-score: -1.8
• Femoral neck T-score: -2.3

Your lowest T-score is -2.3, which falls in the osteopenia range. Your doctor will use the lowest score from any measured site to determine your overall classification. In this example, you would be classified as having osteopenia, and your doctor would likely discuss preventive strategies and possibly a FRAX assessment to estimate your fracture risk.

Your T-score is just one piece of the picture. Your doctor will consider it alongside your clinical risk factors to make treatment decisions.

In addition to the T-score, your DXA report may include a Z-score. While the T-score compares you to a young adult, the Z-score compares you to someone of your same age, sex, and ethnicity.

Z-scores are particularly useful for:

• Premenopausal women (under 50)
• Men under 50
• Children and adolescents

In these groups, T-scores are less meaningful because some bone loss with aging is expected. A Z-score of -2.0 or lower is considered "below the expected range for age" and may prompt your doctor to look for underlying causes of bone loss (such as vitamin D deficiency, celiac disease, or hyperparathyroidism).

A typical DXA report includes:

1. An image of the scanned area (spine or hip) showing which regions were measured
2. A table with your BMD values (in g/cm2), T-scores, and Z-scores for each measured region
3. A graph showing where your results fall relative to the reference population
4. Trend data if you have had previous scans, showing how your bone density has changed over time

When reviewing your results with your doctor, the most important numbers are:

• The T-scores at the lumbar spine and hip
• The change in BMD compared to any previous scan (measured as percentage change)

A change of less than 3-5% between scans may be within the precision error of the machine and may not represent a true change. Your doctor can explain what constitutes a significant change for your specific facility.

First scan (baseline)

The following guidelines represent general recommendations -- your doctor may adjust based on your individual circumstances:

United States (USPSTF / NOF):

• Women aged 65 and older
• Men aged 70 and older
• Postmenopausal women under 65 with risk factors
• Men 50-69 with risk factors
• Anyone who has had a fragility fracture

United Kingdom (NOGG):

• Adults with risk factors, typically assessed via FRAX first
• After a fragility fracture

Australia (RACGP):

• Women 65+, men 70+
• Earlier if risk factors are present

Follow-up scans

• If your initial scan is normal and you have few risk factors: repeat in 10-15 years
• If you have osteopenia: repeat in 2-5 years, depending on your T-score and risk factors
• If you have osteoporosis and are on treatment: typically repeat every 1-2 years to monitor response
• If you start or change medication: a follow-up scan after 1-2 years helps assess whether treatment is working

Consistency matters

For the most accurate comparison over time, try to have your follow-up scans:

• At the same facility (different machines can give slightly different readings)
• On the same machine if possible
• With the same positioning and measurement sites

United States

Medicare covers a DXA scan once every 24 months (every 2 years) for beneficiaries who meet at least one of the following criteria:

• A woman whose doctor determines she is estrogen-deficient and at risk
• An individual with vertebral abnormalities suggesting osteoporosis
• An individual receiving or planning to receive glucocorticoid therapy for more than 3 months
• An individual with primary hyperparathyroidism
• An individual being monitored for an approved osteoporosis drug therapy

Most private insurance plans also cover DXA scans for individuals who meet screening criteria. Check with your specific plan for details.

United Kingdom

DXA scans are available through the NHS when referred by a GP or specialist. Referral is typically based on clinical risk assessment (often using FRAX). Wait times vary by region.

Australia

DXA scans are covered by Medicare (Australia) when requested by a doctor for patients who meet specific criteria, including those over 70 or those with known risk factors. A rebate applies under the Medicare Benefits Schedule (MBS).

If you are concerned about cost, ask your doctor about coverage before scheduling. In many cases, DXA scans for people at risk are covered by public or private insurance.

Q. Is a DXA scan safe?

Yes. The radiation dose from a DXA scan is extremely low -- about 1-5 microsieverts, which is less than the natural background radiation you receive in a day, and far less than a standard chest X-ray (about 20 microsieverts) or a CT scan (several thousand microsieverts). It is safe to repeat at recommended intervals.

Q. Can I eat or drink normally before the scan?

Yes. No fasting is required. The only common instruction is to avoid calcium supplements for 24 hours beforehand at some facilities, as residue can temporarily affect readings.

Q. Will I need to undress?

Usually not. You can typically keep your clothes on, as long as they do not have metal components (zippers, buckles, underwire). If there is metal in your clothing, you may be asked to change into a gown.

Q. My T-score is in the osteopenia range. Does that mean I will definitely get osteoporosis?

Not necessarily. Osteopenia means your bone density is lower than normal but not yet at the osteoporosis threshold. Whether it progresses depends on many factors, including your age, risk factors, and the actions you take. Some people with osteopenia maintain stable bone density for years, while others progress to osteoporosis. This is why monitoring and lifestyle measures are important.

Q. Can a DXA scan tell me if I have had fractures?

Standard DXA scans are designed to measure bone density, not to detect fractures. However, some newer DXA machines include a feature called Vertebral Fracture Assessment (VFA), which can identify spinal compression fractures from the same scan. Ask your facility if VFA is available.

Q. I have had a hip replacement. Can I still get a DXA scan?

Yes, but the replaced hip cannot be measured. Your doctor will assess bone density at the opposite hip and the lumbar spine instead. Metal implants in the spine may also affect measurements at that site, so your doctor will determine the best approach for your situation.

• International Society for Clinical Densitometry (ISCD). Official Positions. https://iscd.org/learn/official-positions/
• Bone Health & Osteoporosis Foundation (BHOF). Bone Density Exam/Testing. https://www.bonehealthandosteoporosis.org/
• World Health Organization (WHO). Assessment of Fracture Risk and its Application to Screening for Postmenopausal Osteoporosis. Technical Report Series 843, 1994.
• US Preventive Services Task Force (USPSTF). Screening for Osteoporosis to Prevent Fractures. JAMA, 2018; 319(24): 2521-2531.
• Centers for Medicare & Medicaid Services (CMS). Bone Mass Measurements. https://www.medicare.gov/coverage/bone-mass-measurements
• Kanis JA et al. Interpretation and use of FRAX in clinical practice. Osteoporosis International, 2011; 22: 2395-2411.

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