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calculating percentage of change
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nananc posted:
I just received the results of my DXA and am confused. This is the first time they did not report the percentage of change. I did ask about the precision and was told 3%.

Previous lumbar spine was 1.065, present reading .994. How can one figure out per cent of change?

Thanks!
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NATIONAL OSTEOPOROSIS FOUNDATION
Michael Lewiecki, MD, FACP, FACE responded:
From what you tell me, it appears that the lumbar spine bone mineral density (BMD) decreased from 1.065 g/cm2 to 0.994 g/cm2. This is a loss of 0.071 g/cm2 or 7%. The 7% value is calculated by subtracting 0.994 g/cm2 from 1.065 g/cm2 and then dividing the difference (0.071 g/cm2) by the original value of 1.065 g/cm2. In order to know whether this is a statistically significant change the DXA facility must do what is called "precision assessment" and calculate the "least significant change" (LSC). Typically the LSC is expressed as an absolute value, such as 0.030 g/cm2, rather than a percentage. If the change in BMD is at least as much as the LSC, then it is considered to be a statistically significant change. This assumes that the measurements were done on the same machine, with identical positioning each time, and analyzed the same way. "Precision" is not the same as the LSC, as the LSC is usually calculated by multiplying the precision error by a factor of 2.77. The is probably more information than you wanted, but to assure that you are getting the best quality tests, ask about the credentials of the DXA staff. Certification by the International Society for Clinical Densitometry (ISCD) for the technologist doing the DXA and the clinician interpreting it is one way to assure that they have achieved a basic level of skills in bone densitometry.
 
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nananc replied to Michael Lewiecki, MD, FACP, FACE's response:
Thank you for the information.

I am wondering how the density can go from .869 in 2006 to 1.065 in 2008 to .994 now. They all were done on the same machine by a highly regarded radiology office. I have been on Actonel or Fosamax all four years. I also have been exercising in a safe way and taking calcium with D.


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