Compare scans to original baseline or most recent scan
Submitted by bbdg on Thu, 11/15/2007 - 08:10.
Dear Dr. Shepherd,
The first time I attended the [ISCD] certification course earlier this year, it was mentioned that a new DXA should be compared with the FIRST test on record, or the baseline. Last August, in Santa Fe, I and others inquired and were told to compare to the previous, or last, exam. The latter a[p]proach makes more clinical sense and is what many referring clinicians request. Yet, 2 other readers, who became certified in the past few years continue to compare to the very first available in the record, even if it is many years old. Please help clarify the ISCD position so we can be consistent.
Many thanks in advance for your help.
EZ MD
Dear Dr. Z, This question
Dear Dr. Z,
This question is only really relevant to matters of long-term precision since accuracy isn’t affected by which scan you compare to, just the precision.
Adults
In adults, the bone size and shape isn’t changing much. Ideally, you would like all of your scans comparable to your initial baseline since that would give you the best long term precision. If you compare to the most recent, you bring in systematic ROI placement biases, making things harder for yourself. I have reviewed the teaching materials for ISCD and I am not sure there is a concrete policy on this. However, Hologic states in their FAQs (http://www.hologic.com/cc/faqdelphi.htm), “If I have one or more follow-up scans, which scan should I compare to? Always compare all follow-up scans to the initial baseline scan.” This in my experience is also the common practice.
Kids (the special case)
Children’s bones are growing and thus the cutlines are expanding constantly. Thus, it doesn’t make sense in this case to compare a 14 year old to scans taken when he was 7.
John