Saturday, February 26, 2011

My thoughts about "How false positives can kill"

Kevin MD posted about a case report that was in the Archives of Internal Medicine. I added my thoughts (quoted below) to his reply thread. No one else has chimed in, but what do you think?

Case report from Archives of Internal Medicine:
A 52-year-old woman presented to a community hospital with atypical chest pain. Her low-density lipoprotein cholesterol and high-sensitivity C-reactive protein levels were not elevated. She underwent cardiac computed tomography angiography, which showed both calcified and noncalcified coronary plaques in several locations. Her physicians subsequently performed coronary angiography, which was complicated by dissection of the left main coronary artery, requiring emergency coronary artery bypass graft surgery. Her subsequent clinical course was complicated, but eventually she required orthotropic heart transplantation for refractory heart failure. This case illustrates the hazards of the inappropriate use of cardiac computed tomography angiography in low-risk patients and emphasizes the need for restraint in applying this new technology to the evaluation of patients with atypical chest pain.
 Kevin opines:

I’ve written in the past that more medicine and tests do not necessarily reflect better care.
There is no test that is 100% specific or sensitive.  That means tests may be positive, when, in fact, there is no disease (“false positive”), or tests may be negative in the presence of disease (“false negative”).
It’s the latter that often gets the most media attention, often trumpeted as missed diagnoses, but false positives can be just as dangerous.
Consider this frightening case report from the Archives of Internal Medicine:
With the proliferation of CT scans that can detect coronary artery disease, there have been some who advocate more aggressive use of the tests. In the emergency room, for instance, a scan that can accurately detect heart disease would be a huge diagnostic leap forward.
But we’re not there yet.
As this case shows, there are real consequences to false positives. Often times, they lead to more invasive tests, like biopsies, or in this case, a cardiac catheterization that went horrifically awry.
So, if you or your doctor believe in getting a scan “just to be safe,” consider this case where such a mentality led to a heart transplant.
False positives can potentially kill.

My response:
Using invasive and noninvasive testing to accurately diagnose (without under or over diagnosing illness) is very challenging. However, the authors in Archives have not established that this patient’s test was inappropriately ordered
Based on Diamond and Kaul’s table for establishing the pre-test likelihood of CAD, a 52 year old woman with atypical angina has an intermediate likelihood of CAD (the authors refer to her as “low-risk”, but it is not clear what risk they were referring to). Intermediate risk patients with symptoms suggestive of CAD are considered appropriate candidates for noninvasive testing based on guidelines for stress echo, SPECT, and CT angio. Therefore based on the information provided, the test was ordered appropriately.
Complications are an unfortunate risk of invasive tests, however, consider some alternative situations.
-A 52 year old woman with atypical chest pain is reassured and undergoes no testing. She has an MI 2 days later
-A 52 year old woman with atypical chest pain undergoes treadmill ECG testing and falls, breaking her hip and suffers massive DVT/PE post-op from hip repair
-A 52 year old woman with atypical chest pain undergoes SPECT which reveals an inferior defect. Ischemia cannot be distinguished from diaphragmatic artifact and she undergoes uneventful coronary catheterization.
Anecdotes are perhaps the weakest form of data and decisions about the clinical utility of a test cannot be made based on one patient’s outcome.

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