by Marcelle Pick, OB/GYN NP
DXA technology was introduced in 1988 and has become the most popular tool for measuring bone density. The National Osteoporosis Foundation does not recommend screening women under age 65 unless they have more than two major risk factors.
DXA measures the bone mineral content (BMC) of the spine, hip, wrist, femur or any other part of the skeleton so desired. Using computer software, it then divides that number by the surface area of the bone being measured to create bone mineral density (BMD)
A picture is printed out of where a patient’s BMD falls compared to norm: the accepted standard peak bone mass (PMB) set by a selected reference group of young adults. There are no international standards of PMB because density varies so dramatically according to race and region.
Women are given a diagnosis of osteopenia if their scans deviate -1-1.5 points from the norm (although I’ve had patients who were put on Fosamax at 1.3). Anywhere below 2 standard deviations is diagnosed as osteoporosis.
When you are measured against the younger reference group, it is called a T-score. When you are measured against the average BMD for your gender and age, it is called a Z-score. Since T-scores are the gold standard for traditional diagnosis, natural bone loss that results in even one standard deviation requires treatment. Since everyone loses bone as they age, eventually every woman will deviate. No wonder there’s an epidemic!
Errors in DXA measurement can be 8-10% depending on the machine, almost one standard deviation. Also, reference groups change from institution to institution. The best DXA results come from comparison to a reference group of the same race, age, region, and gender as yourself.
Several studies in Canada, Turkey, and the third National Health and Nutrition Examination Survey here in the U.S. compared BMD samples from a diverse but local population with those supplied by DXA’s manufacturers. The results showed that the percentage of women who would be called osteoporitic by the manufacturer’s standards dropped 20% under the customized standards.
The worrisome thing about these discrepancies is that women are changing their behavior dramatically when they think they’re abnormal. They feel frightened and immensely fragile and stop lifting and exercising when that’s just what they need to be doing! They start a course of preventative—and speculative—drugs that may continue for the rest of their lives.