Rabu, 24 September 2008

A Screening Test for Ovarian Cancer?

Here’s an ethical dilemma for you:

A promising new screening test for ovarian cancer called OvaSure can correctly identify ovarian cancer 95% of the time. Ovarian cancer is one of the most deadly cancers if it is not detected early. The company marketing the test says that the test has a false-positive rate of only 0.6%. (A false-positive is when the test indicates that cancer is present when in fact it is not.) Should the test be made widely available as a screening test for ovarian cancer?

Let’s see; 95% success rate at detecting ovarian cancer versus a 0.6% false-positive rate – it doesn’t sound like much of a dilemma, does it? You might try it out on your students to see what they think. But it is an interesting dilemma, and the reason is that ovarian cancer is rare; its prevalence is only about 0.06%. So among 100,000 women screened with the new test, 60 (0.06% of 100,000) would have ovarian cancer and 57 of them (95%) would be diagnosed correctly. Many of these 57 might be saved. However, 600 women (0.6%) would be told they had ovarian cancer when they did not. In the absence of any good corroborating tests to confirm the diagnosis, many of those 600 women might choose to have a surgery they actually didn’t need. And almost certainly they'd suffer emotionally from the belief that they had a deadly cancer.

Sometimes you have to run the numbers carefully to see the full effect of what is being said. It pays to be a skeptic.

ADDENDUM: Ovasure was pulled from the market in October, 2008, after the FDA sent the company a letter saying that the kits required FDA approval before they could be marketed and sold.

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