Wednesday, March 6, 2019

Wisdom Wednesday: DEXA

The dual-energy X-ray absorptiometry (DEXA) scan uses two low-energy X-ray beams that separates the images into two components, including soft tissue and bone. When determining whether a person has low bone density or whether the condition is getting worse, a DEXA scan tends to be more accurate than a typical X-ray because it can detect even small changes in bone loss. Doctors also consider it more reliable than other methods of calculating body fat percentage, including underwater weighing. Scan results may indicate the severity of bone loss and help doctors determine a person's risk of developing a fracture. When assessing body composition, the results may also help determine a person's level of visceral fat, which the body stores around certain internal organs.

The most common purpose of a DEXA scan is to assess whether a person's bones are weak and or at risk of fracture. It also helps a doctor diagnose osteoporosis. Osteoporosis causes the bones to lose density or become thin. When the bones get thin, they also become fragile, which makes them more susceptible to breaks. Determining whether a person has osteoporosis as early as possible is important to prevent the condition from becoming worse. It also reduces the risk of fractures. A DEXA scan is also a useful diagnostic tool to assess whether or not osteoporosis is getting worse. After the first DEXA scan, a doctor will usually order another scan in a couple of years to determine whether the bone density has changed.

DEXA scans can also evaluate the effectiveness of any prescribed osteoporosis treatments. The scan may indicate worsening or improving bone density, or bone density that remains the same. According to the National Osteoporosis Foundation, females over the age of 65 and males over age 70 should have a DEXA scan. The ideal frequency of scans may vary depending on the results. People receiving treatment for osteoporosis may require a scan every 1 or 2 years. Females who are postmenopausal but younger than 65 who also have risk factors for low bone density, such as steroid medication use or low body weight, should also have the scan. The recommendation for females to undergo a DEXA scan at an earlier age than males is because they tend to develop loss of bone density sooner.

The results of a DEXA scan for bone density use a system called a T-score. The T-score involves comparing a person's scan with the bone density of a healthy young adult of the same sex. The World Health Organization (WHO) provide the following definitions of bone density levels: A T-score of -1.0 or higher is normal bone density. A T-score of -1.1 to -2.4 indicates osteopenia, or low bone density. A T-score of -2.5 or lower means a diagnosis of osteoporosis. A DEXA scan may also report results using a Z-score, which reports the amount of bone a person has in comparison with other people of the same size, age, and sex. It can help determine whether something uncommon is leading to bone loss.

My Take:
Bone scans have become increasingly popular for aging women. However, the DEXA scan has significant limitations both as a diagnostic tool and for evaluating the effectiveness of treatment.

The scan measures the density of bone, not the health or strength of bone. Most of the medications prescribed for osteoporosis increase bone density, so the DEXA test will improve with medication. However, the drug works by preventing osteoclasts from dismantling old, brittle, dense bone and therefore also prevent the osteoblasts from creating new, strong, less dense bone.

The end result is an increase in old, dense, brittle bone that looks good on the scan but actually increases the risk of hip fracture. That’s why a black box warning is printed on the prescription.

Bottom Line:
The DEXA test has some value, but also some limitations. If your T-score or Z-score is indicates osteopenia or osteoporosis, consider diet, supplementation and weight bearing exercise as your first line of treatment.

Source: February 26, 2019 NIH

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