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Annual Mammograms Increase Risk of False Positive

Posted Oct 19 2011 3:01am
No one's perfect, least of all physicians, try as much as we do.  It was only a few generations ago that we actually advertised that smoking tobacco was good for you!  However, the expectations aren't any lower despite the understanding of our limitations, especially financial ones given our current economic constraints.  After all, do you want to be the one who suffers needlessly because one of us makes an error, especially if there was some pecuniary rationale behind it?  Of course not!  It's that NIMBY attitude - not in my backyard .

Well, when it comes to screening tests, there are several different outcomes.  We can get it completely right in that you don't have a particular disease or condition (true negative = d).  Or we can get it right in that you have a particular disease or condition (hopefully not too much of this latter true positive = a).  In both situations, we're right.  But what if we're wrong?  It turns out there're two wrong outcomes.  We might think you have a disease or condition when you actually don't (false positive = b) or we may think that you don't have said disease or condition when you do (false negative = c).

It turns out that missing (or delaying) a diagnosis of cancer or heart attack is one of the leading reasons for malpractice cases so you can see why we might "overdiagnose" and put you through some extra tests just to be sure.  After all, it's better than the alternative (or false negative which is to say we goof and delay your diagnosis), right?

So it shouldn't be a surprise that a prospective cohort study was published yesterday in Annals of Internal Medicine that concluded that undergoing annual mammograms (as opposed to bi-annual ones) increased one's risk of false positive (test positive but disease negative) errors.  In fact, after comparing 169,456 women who underwent initial screening mammogram between 40 & 59yo compared to 4,492 women diagnosed with invasive breast cancer, the authors concluded that there was a 16.3% risk of false positive diagnosis at the initial mammogram and 9.6% risk during subsequent mammograms.  As a result, 2.5% were incorrectly referred for biopsy after their initial mammogram and 1% after subsequent mammograms.  
One might conclude that this is a small price to pay to get the right diagnosis but think in terms of the emotional cost, procedural risk, and infectious complications, to name but a few of the consequences of a false positive.  It turns out that screening every other year reduces the risk of false-positive outcomes.  Of course, the downside is a statistically insignificant increase in the risk of diagnosing breast cancer at a later stage.  But do you want that to be you?  Remember NIMBY?

So what do we do w/this information?  Just as w/previous recommendations for screening mammograms, age at initiation is an issue to be decided between the patient and her physician.  Likewise, it's best for the patient to decide upon the frequency of said screening after discussing the risk of false positive diagnosis vs late stage diagnosis.

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