The Problem with the Bone Density Test: DXA Scans, Bone Density and Fracture Risk
Currently the best available evaluation of one's bone density is performed with a DXA scan. This is a radiological study that uses minimal radiation in evaluating the hip and the spine. This machine will give you the quantity of bone; however it does not give an evaluation of the patient's true bone quality. Fracture risk is made up of quality and quantity. The information about fracture risk obtained from a DXA is only ~20% of the patient's total risk.
Bone density results have to be evaluated with caution. This is due to the fact that many factors can cause the results to be significantly, and often falsely, changed. These include: artifacts, anatomy, machinery, location, varying technicians among others.
The importance of having someone experienced reading your bone density (DXA) is very important and cannot be stressed enough. The reader has to be experienced and be able to evaluate the scan for many of the above abnormalities. To achieve accurate results, direct observance of the scan is crucial. Without direct observance, the results can be riddled with many errors.
Possible problems that may influence results
- Artifacts: this applies to anything seen in the scanners field of vision. This can occur due to digesting calcium pills (often seen in the stomach or intestines on the scan), back arthritis, vertebroplasty as well as with zippers on dresses.
- Anatomy: It is extremely important that the areas in the scan are the same as what were done previously as small shifts can greatly change the measured mineralization.
- Different machine: Results that are performed on different manufacture's machines or different machines from the same manufacturer can not be compared accurately due to the significant variations in the technology.
- Different location of the same machine: Machines that were moved into a different location can alter the results and make the comparisons inaccurate.
- Different technician: Different people performing the scan on the same machine can affect the results.
- Different positioning: Shift in the patient's position on the examination table can considerably change the results.
It is evident that it is close to impossible to control all factors involved and therefore there are significant limitations to the current "gold standard" DXA test. One should be careful to understand this when they are scanned and not be disappointed when the results on follow-up evaluation are not as favorable as they had hoped. Often, multiple tests over the years will give one a better idea of the true results.
There is considerable research ongoing to find a better study to replace the DXA. Any new technology must be reproducible and be able to assist in the evaluation of the quantity as well as the quality and be a good judge of someone's risk of fracture. Currently studied tests include three dimensional bone density, micro CAT scan as well as MRI's.
We hope to have a better test available within the next few years. This will hopefully allow us to better decide which people with bone loss should be treated and for how long.