Why patients should worry about a normal cardiac stress test

Jennifer is a middle-aged woman who recently began feeling shorter of breath than usual while carrying groceries up the stairs and when she’s chasing her grandson in the park. Deep down, she’s worried that her symptoms could be due to something sinister – like heart disease. She considers her risk factors: She’s overweight and also has high blood pressure, diabetes and a family history – and decides to see her doctor. He’s worried too, and orders a cardiac stress test. A week later, he calls to tell her that the results were normal. She is relieved, but should she be?

If Jennifer’s symptoms are actually due to coronary heart disease, the stress test will accurately diagnose her only about half the time. Yes, half the time. This seems like a startling statistic, but it has proven true in studies from Mayo Clinic and University of California, Irvine, and in a large review that I and my colleagues at NYU School of Medicine and Duke University published two years ago. So why didn’t Jennifer’s doctor tell her this? And why are the millions of patients evaluated each year in the U.S. for suspected CHD also in the dark?

One needs to understand three things:

▪ The prevalence of heart disease has been declining in the United States for at least 30 years, thanks to public health reductions in smoking, high blood pressure and high cholesterol. For adults in the U.S., this changing epidemiology has meant that patients’ symptoms are less likely to be due to heart disease than compared with a few decades ago. For doctors, it means they are frequently lulled into believing that the stress test is functioning well in diagnosing CHD because so many people have a negative test result, and these people do well afterward.

▪ The traditional way that doctors determined how well a cardiac stress test worked was to compare its results with the results of an invasive coronary angiography. This is a procedure in which a patient has a catheter placed in the groin or arm, which is then threaded to the heart to visualize the coronary arteries. If a person’s cardiac stress test was positive and the coronary angiogram showed coronary disease, that meant the test worked. While this sounds fine in principle, a major problem with evaluating a test this way is that it ignores how well the test worked – or didn’t – in patients who had the test but were never referred for a coronary angiogram. The biased population of people referred for a coronary angiogram over-represents individuals who were so high risk that their doctor felt that an invasive procedure was necessary, and it makes the test appear to be more sensitive than it actually is.

▪ Many doctors are just not very good at math and statistics when it comes to interpreting test results. Some of my former colleagues at Harvard Medical School recently published a paper in which they asked attending physicians, residents and medical students to interpret laboratory test results using information about the prevalence of the disease and how well the test worked. The same exact study was done in 1978 using a similar mix of doctors and students. About 3 in 4 respondents answered the question incorrectly – unchanged from the proportion of respondents who answered the question incorrectly 37 years ago. Doctors weren’t very good at interpreting diagnostic test information back then, and they’re not very good now.

So if you were the patient, how would you feel about your normal cardiac stress test? If you’re worried that you might be one of those people with CHD who the stress test misses, there are other tests available, including some with less radiation. The choice should be yours, but it’s one that patients haven’t had because they have rarely been told how well – or not – cardiac stress testing truly works. A change is long overdue.

This story was originally published November 6, 2015 at 6:04 PM with the headline "Why patients should worry about a normal cardiac stress test."