When do tests hurt more than help?

Manil Suri and Daniel Morgan are an unusual team: Manil is a mathematics professor and author (of both fiction and nonfiction), while Daniel is a physician and professor of epidemiology, public health, and infectious diseases. But — in what they say is a typical “Smalltimore” moment — both a neighbor and a student had told them they should work together because of a shared interest in false positives on diagnostics tests. The result was a recent First Opinion essay, “Diagnostic tests for rare conditions present a mathematical conundrum,” in which they write about how the more rare a disease ease, the more likely a test will return a false positive.

On this episode of the “First Opinion Podcast,” I spoke with Manil and Daniel about how false positives can cause major problems, how both physicians and patients misunderstand statistics, and how their work plays out in their own lives.

Manil imagined a test for a rare condition: “If you tested 20,000 people, only one person would actually have that condition. But remember, if you have a 1% false positive rate, then out of 20,200 people, that is 1% will test positive. And so out of that 200 people, only one person will actually have the disease,” he said. For the other 199, the result may be devastating for no good reason.

Daniel has created a website, TestingWisely.com, where you can learn about how to think through diagnostic testing in a more practical way.

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