A couple weeks ago I had a conversation with a patient that surprised her. Her test result had been incorrect. She is a 57-year old woman who had been having chest pain for the last several months, though the symptoms weren’t necessarily with exertion. I had ordered a stress echocardiogram (see “Services” section of this website for a description of what this test is all about) in order to see if the symptoms she was having could be due to coronary artery disease. The test result came back indicating that a portion of her heart was not getting a good blood supply, indirect evidence that she had a significant blockage in an important coronary artery.
I therefore referred her to one of my partners for a coronary angiogram (aka a cardiac catheterization, also described in the “Services” section of this website) the following week. But, after performing the procedure, he called to tell me that she had no significant blockage. So, in her follow-up visit with me, my patient’s obvious question was “How come the stress test indicated a blockage and the angiogram didn’t find one?”
I explained to her that tests can give false positive results, meaning that the test is “positive” (abnormal), but the patient doesn’t have the condition. Every test we order has a certain frequency of false positive results (as well as false negative results—the test is normal, but the patient DOES have the disease). For a stress echocardiogram, the false positive rate is about 10-20%—so, for every 10 tests that come back abnormal, 1 or 2 of those patients don’t actually have a significant coronary blockage.
There are several reasons for this. First, stress echocardiograms are looking at the heart’s function, not at the coronary arteries (which can’t be visualized by echocardiography). It is looking for whether each section of the heart improves (as it is supposed to) with exercise or worsens (an abnormal result). Sometimes there can be microvascular disease (very small vessels that don’t work right) that creates poor blood flow even if the major conduits are open. Second, the images may not be optimal, so that a part of the heart doesn’t appear to move well due to not being able to see that segment adequately. This can occur due to obesity, lung disease, or just that some people don’t have good transmission of ultrasound waves. Finally, there are times we get a false positive test result and don’t have an explanation.
We would like to think that the tests we do are 100% accurate and give unequivocal results. Unfortunately, technology has its limitations and we find that the appropriate test sometimes gives incorrect results. At such times, we have to accept that we live with imperfection and remind ourselves that medicine is an art as much as a science.
Greg Koshkarian, MD, FACC