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Understanding a Bone Density (DEXA) Test and Your T-score

Here is what a DEXA bone density scan measures, and how to make sense of T-score and Z-score results.

What a DEXA bone density scan is

Dual-energy X-ray absorptiometry (DEXA or DXA) measures the mineral density of bone using two X-ray energy levels, and is widely used as the standard method for evaluating osteoporosis.

T-score

T-score compares your bone density to the average peak bone density of a healthy young adult. Under World Health Organization criteria, a T-score of -1.0 or above is generally classified as normal, -1.0 to -2.5 as low bone density (osteopenia), and -2.5 or below as osteoporosis -- though a clinician makes the final interpretation alongside other factors, not the number alone.

Z-score

Z-score instead compares your bone density to the average for people of your own age and sex. It is used to flag bone density that is unusually low for someone's age, and is referenced particularly for younger patients or when looking for an underlying secondary cause.

Who testing is typically recommended for

Postmenopausal women, older adults, people with a prior fracture, and people on long-term medications known to affect bone, such as corticosteroids, are commonly cited as groups for whom testing is often recommended. The right timing for an individual should be discussed with a doctor.

The main areas measured

The scan usually measures the spine (lumbar vertebrae) and hip (femur), and sometimes the wrist or another site as well, depending on the clinical reason for testing.

How long it takes and how much radiation is involved

The scan itself usually takes about 10-20 minutes, and the radiation dose involved is reported to be lower than that of a standard chest X-ray.

How to prepare

It is generally advised to avoid clothing with metal fasteners or decorations on the day of the scan, and fasting beforehand usually is not required. Follow the specific instructions given by the facility performing your scan.

A note on interpreting your result

A bone density result is one of several inputs a clinician weighs when assessing fracture risk, alongside factors like age, body weight, and fracture history -- it is not meant to be read as a standalone verdict.

T-score and Z-score compare against different baselines

T-score is compared against young, healthy adults and is mainly used to assess osteoporosis, while Z-score is compared against your own age group and is mainly used to flag bone density that looks unusually low for your age. Because they serve different purposes, it is worth not confusing the two when reading a report.

Bone density is one input among several for assessing fracture risk

A T-score alone does not automatically determine whether treatment is needed. Clinicians commonly weigh it together with other risk factors -- age, weight, smoking, alcohol use, and prior fracture history -- sometimes using a fracture-risk calculation tool, to arrive at a management recommendation, rather than applying a single numeric cutoff on its own.

Frequently Asked Questions

If my T-score is below -2.5, do I automatically need medication?

Not necessarily from that number alone. Whether treatment is needed is a decision a clinician makes by weighing your age, fracture history, and other risk factors together, not just the T-score.

Is the radiation exposure from a DEXA scan something to worry about?

The radiation dose is reported to be lower than a standard chest X-ray, so it is not considered a major concern at typical testing frequencies. Still, let the facility know in advance if you are or might be pregnant.