We know that bone mass varies greatly between individuals and can change through our lifespan. When we are young the density of bone is lower than the density of our bone as young adults. Bone density also starts to decline when we get to middle age.

The two-component model also assumes that everyone has the same bone density, which we now know is not correct. Bone density not only varies by age Jones et al. All devices that use the two-component model i. assume bone density does not change and therefore have this error built into their determination of body composition.

Additionally, the fat mass and fat-free mass are not directly measured by two-component methods, but are calculated from equations or algorithms. The accuracy of the measurement of body composition is dependant on the population in which these equations and algorithms were developed.

Differences from the population being tested and the population that the equations or algorithms are based upon may lead to additional errors in the accuracy of the estimation of body composition. A more complex model of body composition is a three-component model.

DXA directly measures bone density and allows you to separate body composition into three components. More importantly, DXA determines lean skeletal, fat and bone masses in various regions of the body versus assuming body composition is the same throughout the entire body as is done in the two-component method.

Therefore, by determining regional body compositions as well as determining lean skeletal, bone and fat masses rather than making assumptions about bone and muscle, the percent fat is a little higher than two-component methods of determining body composition. Accuracy and precision of body composition methods Finally, a couple of other factors to consider regarding the different methods of determining body composition are the accuracy and precision reliability of the measurement.

The image below demonstrates the differences between accuracy and precision. Generally speaking, DXA is more accurate and precise compared to two-component methods. In particular, when dealing with athletes with non-traditional body types i.

Accuracy can be determined by examining the standard error of the measurement. For body fat the standard error of the measurement for DXA is 2. We have observed in our studies Raymond et al. Thus, it is significant that DXA has greater accuracy than two-component methods of measuring body composition.

Precision is determined by the coefficient of variation between measurements. A measurement is more precise if the limits on the coefficient of variation are low and narrow see figure above. Precision, both when tracking an individual and comparing multiple individuals, is critical if you plan to compare athletes.

The decisions made with these athletes from a training or nutrition perspective may differ based on the results of the two-component method, but the more accurate and precise DXA methodology indicates the fat mass is actually the same. How do I communicate these differences to athletes, coaches and other performance staff that are used to seeing lower numbers?

When we started working with athletes we knew this would be one of the biggest challenges. The issues include a negative connotation with percent body fat and explaining to coaches and athletes what these numbers mean and why they are different.

One analogy used to explain this is the differences between hand-timed forties to electronic-timed forties. The difference is in the method, and the values are all relative.

The goal should be to have accurate and reliable information, which can then be used to both track an individual athlete over time as well as compare athletes to one another. Finally, when reviewing the results, it is good to use ranges rather than averages.

These ranges should be based on a set percentage for that position. Final thoughts When measuring body composition be sure to understand exactly how the numbers are generated. These scans can help indicate how effective osteoporosis treatments are because they can show whether the condition is staying the same, worsening, or improving.

The Bone Health and Osteoporosis Foundation recommends that females aged over 65 and males aged over 70 have at least one DEXA scan. The ideal frequency may vary, depending on the results.

People receiving treatment for osteoporosis may require a scan every 1 or 2 years. The foundation also recommends a scan for anyone who has gone through menopause before age 65 and has any risk factor for low bone density, such as steroid medication use or low body weight.

Overall, females tend to develop loss of bone density sooner than males. A person does not need to prepare — they can eat and drink as usual on the day of the procedure. However, anyone taking a calcium supplement may need to stop taking it around 24 hours before the scan.

An X-ray technician performs the scan on an outpatient basis. A person may need to change into a hospital gown and remove any metal objects that they are wearing, such as jewelry and eyeglasses.

At the start of the scan, the person lies on their back on the exam table. The technician places the imaging device above them and the X-ray generator below them. It is crucial to remain still throughout the scan. When the purpose is to measure bone density, the technician tends to scan the hips and spine.

These are common locations of fractures in people with osteoporosis. In some cases, the technician scans the wrist, finger, and lower arm.

At some locations, there may also be a vertebral fracture assessment during a DEXA scan. This checks the risk of a fracture in the spine. When the purpose is to measure body composition, the machine scans the entire body to check skinfold thickness at specific sites.

Using an equation, it is possible to use these measurements to calculate body fat percentage. Afterward, the person changes back into their clothes. The technician may ask them to complete a questionnaire about their medical history, as this can help a doctor determine their fracture risk.

Then, the person can leave the facility. The scan is painless and relatively quick, usually taking 10—30 minutes , depending on the equipment and the areas being scanned. Some experts report that it can take just 6 minutes or 10—20 minutes.

The cost depends on many factors, such as where a person lives, where they have the scan, and whether they have health insurance that covers it. Medicare Part B covers a DEXA scan once every 24 months, or more often if necessary, for people with certain risk factors.

Private insurance plans may also cover DEXA scans under certain circumstances as part of their preventive benefits. The DEXA scan is generally safe for most people.

However, because it uses X-ray energy, there is exposure to radiation. As such, it is not advisable for pregnant people. Anyone who is or may be pregnant should discuss other options with their doctor.

The results of a DEXA scan for bone density involve a T-score. According to the World Health Organization WHO :. A DEXA scan may also report results using a Z-score , which shows the amount of bone that a person has, compared with others of the same size, age, and sex.

It can help determine whether something uncommon is leading to bone loss. According to the International Society for Clinical Densitometry , a Z-score of over Doctors consider a score below When the test also measures body composition, the results include total fat mass and total body fat percentage measurements.

Experts typically consider a DEXA scan to be the most accurate test, or gold standard , for measuring bone density. Research from notes that a DEXA scan is the preferred method for measuring bone and body composition.

This is largely due to the accuracy and precision of the scan. Still, a DEXA scan may not be as useful for those with irregular curvature of the spine or people who have had spinal surgery, as these factors may interfere with the accuracy of the test. A DEXA scan helps doctors determine whether a person has low bone density and a higher risk of fractures.

It can also show body fat percentage. The results of the scan can help a doctor tell whether treatment for osteoporosis is necessary and help monitor the effectiveness of this treatment.

Although the scan exposes the person to a small amount of radiation, the benefits tend to outweigh the risk. A number of tools help doctors diagnose and monitor ankylosing spondylitis, including X-rays and MRIs.

Here, learn why monitoring matters and what it…. In this article, learn about osteopenia, in which bones have low density. Osteopenia is a common symptom of aging that not generally show symptoms….