When “Positive” Turns Out To Be Negative

This past week I had a conversation with a patient that surprised him.  His test result had been incorrect.  He is a 76-year old gentleman who had been having shortness of breath for the last several months, though the symptoms weren’t necessarily with exertion.  I had ordered a nuclear stress test (see “Services” section of this website for a description of what this test is all about) in order to see if the symptoms he was having could be due to coronary artery disease.  The test result came back indicating that a large portion of his heart was not getting a good blood supply, indirect evidence that he had a significant blockage in an important coronary artery.

I therefore referred him 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, my partner called to tell me that he had no significant blockage.  So, in his follow-up visit with me last week, my patient’s obvious question was “How come the stress test indicated a blockage and the angiogram didn’t find one?”

I had to explain to him 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 a problem).  For a nuclear stress test, the false positive rate is about 10%—so, for every 10 tests that come back abnormal, 1 of those patients don’t actually have a significant coronary blockage.

There are several reasons for this.  First, a nuclear stress test is a perfusion study, not a look at the coronary arteries.  It is looking for where the blood is (or isn’t) flowing (perfusing).  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 show a deficit that is not caused by poor blood flow, but by a “shadow” cast over the heart by another body structure.  This can be seen with the diaphragm and with breast tissue.  Finally, there are times we get a false positive test result and don’t have an explanation—perhaps there is a technical problem with the nuclear isotope or the imaging process.

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


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Gregory Koshkarian, MD, FACC