Bone Density Testing: Specialists & Information

The test measures how much radiation is absorbed by the bones. This provides information about the mineral content of the bones.

Bone density testing allows doctors to determine the overall risk of bone fractures. It also plays an important role in the diagnosis and monitoring of osteoporosis. Osteoporosis (bone loss) is an age-related bone disease that increases the risk of fractures.

Clinically significant fractures can occur in the lumbar spine and the femoral neck. For this reason, bone density testing is usually performed in these areas to assess the risk of fractures.

The various methods that can be used for bone density measurement include:

• Dual-energy X-ray absorptiometry
• Quantitative computed tomography
• Quantitative ultrasound
• Peripheral densitometry

Monitoring is ideally performed through regular bone density measurements. These should be conducted by the same physician using the same device. This allows for the determination of how rapidly bone mass is being lost.

A bone density test for diagnostic purposes is recommended when osteoporosis is suspected. A bone density test should also be performed if certain risk factors for osteoporosis are present. These risk factors include:

• age over 65,
• a bone fracture after age 40,
• a femoral neck fracture in the mother,
• a family history of osteoporosis,
• female gender,
• early menopause,
• low body weight,
• smoking,
• alcohol consumption,
• low dietary calcium intake,
• vitamin D deficiency,
• low physical activity.

In addition, there are several high-risk factors for which regular bone density testing is recommended:

• long-term corticosteroid therapy,
• hyperthyroidism,
• Cushing’s syndrome (excessive levels of the hormone cortisol in the body),
• organ transplant recipients.

Dual-energy X-ray absorptiometry

This test is usually referred to as DXA or DEXA. These are abbreviations for the term “Dual Energy X-Ray Absorptiometry.” DXA is the most commonly used method for measuring bone density.

During the test, two X-ray beams of different intensities are passed through the body. The X-rays pass through the bone but are attenuated by it. Bone density is then calculated based on the extent to which the bone has affected the X-ray beam.

DXA is a very reliable and accurate measurement method. In addition, it uses only a small amount of ionizing (radioactive) radiation.

In principle, the test can be performed on the entire body, provided there are no metal implants in the areas being examined.

Quantitative Computed Tomography

This examination, also abbreviated as QCT, is performed on the lumbar spine. It involves using X-rays to create a cross-sectional image of the bones. Based on these images, the bone density of the outer bone layer and the inner so-called trabeculae can be determined separately.

Metabolic activity is higher in the trabeculae than in the outer layer. This allows QCT to also detect changes in bone metabolism. This is its advantage over DXA.

However, QCT involves higher radiation exposure and is less accurate than DXA. For this reason, QCT is used only for patients with very specific clinical questions.

In addition, a peripheral QCT (pQCT) can also be performed. In this procedure, the measurement is taken not at the lumbar spine but at the forearm. Radiation exposure is lower with this method than with standard QCT; however, unlike DXA, it is localized.

Quantitative Ultrasound

Unlike the other tests, this method—QUS for short—uses ultrasound waves instead of X-rays. There are two different procedures:

• In the first procedure, the extent to which the sound energy is attenuated by the bone is measured.
• In the second method, the speed at which the ultrasound travels through the bone is analyzed.

The major advantage of QUS is that it involves no radiation exposure whatsoever.

However, the method is still very new, which is why it is not yet considered fully developed. So far, it is not used as a standard procedure.

Peripheral Densitometry

Peripheral densitometry can be performed using DXA or pQCT. The measurement is usually taken on the forearm. Since clinically significant fractures do not occur in this area, the measurement has only limited predictive value regarding the risk of fractures.

After a bone density measurement, the determined bone density value is reported as a so-called T-score. This value is standardized across all examinations to ensure comparability between the various methods.

The T-score indicates how much the patient’s values deviate from the average values for a 30-year-old. The T-score can then be interpreted as follows:

• 0 to -1: normal bone density
• -1 to -2.5: low bone mass (osteopenia)
• less than -2.5: bone loss (osteoporosis)
• less than -2.5 and typical bone fractures: severe bone loss

In addition, a so-called Z-score can be calculated from the data. This indicates how much the patient’s results deviate from those of healthy individuals of the same age. The result thus shows whether the existing bone loss falls within the range of what is considered normal for this age. 

Based on this, the doctor can determine the next steps, such as whether medication is indicated.