Bone density testing is used to diagnose Osteoporosis. It is also called Bone Densitometry or Dual-Energy X-ray Absorptiometry (DXA).
What is bone density testing?
Bone density scanning is a special form of X-ray which is used to measure bone loss. It can be used to diagnose Osteoporosis, but also to measure the effectiveness of subsequent treatments in delaying the progression of the disease.
Bone density tests are usually carried out on the lower spine, pelvis or thigh bones. It is recommended that women over the age of 50 (who are not taking Oestrogen) go for a bone density scan. This is especially important if you have a family history of Osteoporosis. Other medical conditions or medications may also increase your risk:
- Diabetes (especially type 1 - juvenile / insulin dependant diabetes)
- Liver or kidney disease.
- Thyroid conditions.
- Prolonged use of Corticosteroids.
- Anti-seizure medication.
How does bone density testing work?
The DXA machine transmits an invisible low-dose X-ray beam with two clear energy peaks through the bones being tested. One of the peaks is absorbed by soft tissues whilst the other is absorbed by bone.
The amount absorbed by soft tissue can be subtracted from the total, with the remainder being the patient's bone mineral density. The measurements are computed and displayed on a monitor.
DXA machines can either be large stationary objects in a hospital, or can be small, portable boxes. Those in a hospital consist of a couch with a moveable arm which scans the body area. The portable versions are used for smaller body parts which can be placed inside the device.
The results are interpreted by a radiologist who will usually send a report to your Doctor.
The readings are given 2 scores, a T score and a Z score. The T score compares your readings to that of someone of the same sex with optimal bone density. If your score is -1 or less then this is considered normal. Scores between -1 & -2.5 are classed as Ostopenia (low bone mass). A diagnosis of Osteoporosis is given if the results are greater than -2.5.
The Z score compares your results to those of a person of the same sex and age who has normal bone density. A very high or low score may require further investigation.