Bone Density Tests
How can you protect against osteoporosis and its precursor, osteopenia? The answer: By having routine bone density tests as recommended by your physician, knowing what the results mean, and treating bone density loss before a bone fracture occurs.
The primary test of bone density is called a dual-energy x-ray absorptiometry test, but everyone refers to it as a DEXA scan. A DEXA scan is very similar to a plain X-ray except that it uses far less radiation. Because of this low amount of radiation, it is associated with even less risk than the risk that is associated with a standard X-ray (which is very small). One advantage of having a low radiation test to monitor bone health is that the results can be followed over time and compared to one another. Comparing DEXA scan results allows your doctor to determine if osteoporosis treatment or prevention is directly improving bone density or if your disease is getting worse.
The DEXA takes a measurement of the density of your bone. More precisely, it determines the amount of mineralization or calcium content in bone. Osteopenia and osteoporosis are conditions in which the bones contain less calcium than they should. Less calcium means less mineralization, less mineralization means lower bone density, and lower bone density means weaker bones that are more prone to catastrophic fracture. The bone density loss in osteopenia is less severe than it is in osteoporosis but osteopenia can lead to osteoporosis if it is not effectively treated.
A DEXA scan is as fast and painless as taking a picture because, in many ways, that is precisely what is happening. A DEXA scan requires no advance preparation but notify staff if you have recently had another radiological test. You will either report to the radiology department of a hospital or clinic or you may have the DEXA test at an independent radiology center. The DEXA test can be performed with a central or peripheral device. A central device is essentially a table on which you will lay while the DEXA scanning arm hovers above you (without touching you) and takes low energy X-rays of your spine or hip. A peripheral DEXA scanner is a smaller machine that is used to test the bone density in your wrist.
If you have ever had a DEXA, you will have probably heard the terms T-score or Z-score. A T-score compares your bone density to that of a young, healthy adult while a Z-score, on the other hand, compares your bone density to the bone densities of people that are your same age, weight, gender, and ethnicity. In general, the T-score is the one that you will follow. T-scores are generally reported in the negative numbers; a T-score above -1 are considered normal, a T-score between -1 and -2.5 is consistent with osteopenia, and a T-score below -2.5 indicates osteoporosis. The lower your T-score, on a negative number scale, the lower your bone density. In other words, you will want to keep or reach a T-score that is higher than -1. When comparing your T-scores over time, it is best to use the same area of bone each time you have a DEXA scan and, ideally, the same DEXA scanning device.
One downside is that DEXA is a two-dimensional look at a bone and yields a value in g/cm2 rather than a volumetric density (g/cm3) This results in a systematic overestimation of bone density in larger bones, and if DEXA's are calibrated for women, they miss some low density readings in male patients.
A DEXA scan is a painless, ultra-low risk test that can detect a potentially dangerous disease before it causes a catastrophic bone fracture. Ask your physician if a DEXA scan is right for you.
Common Questions
Q. Do bone density tests measure how strong my bones are?
A. Not directly, but they provide a proxy for strength. More importantly they are easy to do. A better measure of bone strength would require close physical examination of the bone outside the body or through invasive sampling.
Q. How good is bone density testing?
A. Good enough that it is widely accepted by the medical community.
Q. Does it hurt?
A. No! You lie on an exam table and an x-ray machine scans you. No injections are made to the body for conventional DEXA scans and there is no pain.
Q. How much radiation does my body get?
A. Not enough to worry about. Less than a chest X-ray, for instance.
Q. How often should I get a bone density test?
A. Consult your doctor about your specific situation. Generally people who take certain medicines or have certain conditions get another density every one or two years. People with risk factors for osteoporosis are also tested every one or two years, but people past age 65 without other risk factors had been getting them every 3 to 5 years. In 2011 the U.S. Preventive Services Task Force issued a recommendation for all women past age 65 to get regular screening. The press release is here. The Task Force did not say how often "regular" meant.
Bone Quality
Recognizing there is not a direct correlation between the results of DEXA scans and the tendency to bone fractures, the medical field has started talking about "bone quality". An NIH conference defined this as "the sum total of characteristics of the bone that influence the bone's resistance to fracture." Bone quality is supposed to capture everything, but in an essay in PLoS Medicine several Finish scientists pointed out that a metric that can't be measured isn't much of a metric and concluded the term was "an empty term."
Cost Concerns
A paper published in the Archives of Internal Medicine in Oct 2011 listed bone density scans as among the five most commonly overused clinical activities in the US. There is concern about cost control and unnecessary tests.
Related Tests
The 25-hydroxy vitamin D test is given to bone patients to determin how much vitamin D is in the body. Blood counts (white cells, red cells, platelets) are also taken when trying to diagnose bone problems. Parathyroid hormone levels are examined to screen for hyperparathyroidism. Testosterone levels may be measured in men with low bone densities. These are all blood tests. Doctors may also order a test to measure calcium levels in the urine. Ultrasound, commonly used for imaging, can be used to estimate bone density in the heel.
CT Scans to find Bone Density
CT scans can be employed to estimate bone density and are of particular use for examination of the spinal cord. It is not often used because of the cost and the large amount of radiation applied to the body can have negative effects.
Last updated: Jan 21, 2012
