How osteoporosis is diagnosed

A bone density test can help you determine if you have osteoporosis or are at risk.

How do you know if you have osteoporosis or if you are at risk? This is a common question, and one to get answered sooner rather than later.

Because bone loss typically happens gradually and painlessly, the first sign of osteoporosis can be breaking a bone, often more easily than you'd expect. But it is possible to determine if you have osteoporosis, even before a bone is broken, by getting a bone density test. The test can also detect if your bone density is lower than normal for a person of your age and sex. Bone loss that has not reached the stage of an osteoporosis diagnosis is called osteopenia.

Bone density testing

A bone density test is as close as your doctor can come to predicting your future bone health. The test results will show if you have osteopenia or osteoporosis, and how susceptible your bones are to fracture. A bone density test is the best way to predict fracture risk.

The test uses X-rays to measure how many grams of calcium and other bone minerals are in a square centimeter of bone. Generally, the higher the mineral content, the denser the bone is. And the denser bones are, the less likely they are to fracture.

Bone density testing uses a device called a bone densitometer. Most densitometers measure how much of a low-energy X-ray beam is absorbed as it passes through bone, in comparison to the absorption as the beam passes through the soft tissues next to the bone. The amount of X-ray energy that enters the bone is also compared with the amount of energy that leaves the bone. Denser bone absorbs more of the X-ray beam.

Central densitometers

These machines, typically found in hospitals and medical centers, are used to measure the density of the central, stabilizing parts of the skeleton, such as the spine and hip. This type of densitometer provides the most accurate bone density testing and can predict your potential risk of fracture.

Dual energy X-ray absorptiometry (DXA). A DXA machine uses two different X-ray beams to increase the precision of what it's measuring. As you lie on a padded platform, two mechanical arms containing an X-ray source and detector above and below your body are aligned. DXA is most often performed on the narrow neck of the upper leg bone (femur), just below the hip joint, as well as the lumbar vertebrae, which form the lower part of the spine. DXA testing is painless and takes only a few minutes.

Quantitative computerized tomography (QCT). This instrument measures bone density using computerized tomography (CT). Similar to having a CT scan, you lie on a movable padded table that slides into a large cylinder, where X-ray images are obtained from all angles. QCT is most often used to measure density in the vertebrae and the part of the femur below the hip.

Peripheral densitometers

Smaller devices are used to measure bone density on the periphery of the skeleton, such as in the wrist and heel bone. Although they are more portable, these densitometers are less accurate at predicting fracture risk. If your test on a peripheral device is positive for osteopenia or osteoporosis, then your doctor might recommend a follow-up scan of your spine or hip to confirm the diagnosis.

Quantitative ultrasound (QUS). This procedure is often called heel ultrasound because it typically measures bone density in the heel bone. Instead of X-ray radiation, QUS sends high-frequency sound waves through your heel while you rest your bare foot on the instrument. This type of densitometer measures the reflection of sound waves. Denser bone reflects sound waves back to the device sooner.

Peripheral dual energy X-ray absorptiometry (pDXA). This device is a compact, portable DXA scanner. Using X-rays, pDXA measures bone density in the wrist or heel. Although it's quick and accurate, this testing can be expensive.

Peripheral quantitative computerized tomography (pQCT). This portable version of QCT measures the bone density of the wrist or hand. This test is also expensive, and it produces a higher radiation exposure than do other tests.

Other types of testing, including 3-D imaging and high-resolution magnetic resonance imaging (MRI), are also under investigation.

Which test is right for you?

The U.S. Preventive Services Task Force recommends that all women over age 65 should have a bone density test. If you are younger than 65 and at high risk for fractures, then you should have the test. The type of bone density test that's best for you depends on your age and the reason you're being tested.

If you don't have any risk factors for osteopenia or osteoporosis and you haven't broken a bone, then a peripheral screening test may be all that's needed to make sure your bones are healthy. If the results indicate low bone density, however, follow-up testing with a central densitometer, such as DXA, likely will be recommended. For someone with multiple risk factors or who already has a broken bone, DXA testing is typically the next step.

If you have osteopenia or osteoporosis, your doctor will likely schedule periodic DXA tests of the primary places where bone fractures occur most often — the hip, spine, wrist or a combination of these.

If you're taking medication for osteopenia or osteoporosis, central densitometry of your spine is best for monitoring your treatment.

Test results

For older women, bone density test results are most commonly reported as a number called a T-score. Another number, the Z-score, is more appropriate for younger individuals.

  • T-score. This is bone density compared with what is normally expected in a healthy young adult of the same sex. The T-score is the number of units — called standard deviations — that an individual's bone density is above or below the average.

Bone density levels

T-score

What it means

-1 and above

Normal bone density

-1 to -2.5

Low bone mass (osteopenia): Bone density is below normal and can lead to osteoporosis

-2.5 and below

Osteoporosis

  • Z-score. This is the number of standard deviations above or below what's normally expected for a person's age, sex, weight, and ethnic or racial origin. A Z-score of -2 or lower in a premenopausal woman is considered to be low bone density.

History and physical exam

If your bone density is lower than normal, your doctor will likely want to look for possible causes. To determine if something about your general health or lifestyle is damaging your bones, you will need a complete medical evaluation, including a medical history and physical exam.

A medical history involves answering questions about your personal health history, as well as the medical history of your close relatives. You also will likely be asked about medications you're taking, what you eat, how much you exercise, and how much tobacco and alcohol you use. These factors help determine the risk of osteoporosis and identify other conditions that can cause low bone density.

As part of the physical exam, you will also have your blood pressure and heart rate checked. Your doctor can also order blood and urine tests. Combined with your medical history, a physical evaluation helps your doctor interpret the results of your bone density test.

March 10, 2017 See more In-depth