Last month, an article in the New York Times proudly proclaimed a new 100% accurate test for the prediction of future Alzheimer’s disease. (This is the part where you should scroll down and read the retraction.).
Confused about the issue? Don’t feel bad – many doctors also fail at this type of reasoning.
Let’s say for the sake of argument that I tell you that I have a brand-new medical test. It’s called the “sleep gives you cancer” test, and it is one question long: Do you sleep? If you answered “yes”, then you will get cancer! As everyone who has ever had cancer has slept, this test is (according to the logic of the New York Times) 100% accurate. But, as a smart reader you will now tell me that my test isn’t so great because there are plenty of people in the world who sleep all the time and have never had cancer.
A test must do 2 things in order to be accurate: it must predict which people will get the disease (this is called sensitivity in the medical literature, and hit rate in psychology), and it also must predict which people won’t (the specificity in medical-speak, correct rejections in psych-speak). The test in question had a 100% sensitivity (everyone in their sample who later got Alzheimer’s tested positive), but 36% of people in the sample who didn’t get Alzheimer’s also tested positive.
So, how good is this test really? Fortunately, some useful math exists to help us figure this out. Let’s say we have 1000 55-year olds. We know that 10% of them will develop Alzheimer’s by age 60. We give all 1000 people this test, and wait 5 years. Looking at our sample, we’ll find that all 100 patients with Alzheimer’s tested positive for the test, as well as 324 (36%) of the non-Alzheimer group. Therefore, if one participant tested positive for the test there is only a 100/424 chance that s/he will have AD.
We also need to examine how useful an Alzheimer’s prediction test would be because, as of this writing, there isn’t a whole lot that can be done for AD. As pointed out here, the test described in the New York Times is based on a painful and invasive spinal tap, which makes the cost-benefit ratio quite large. However, there exist several predictive tests for AD based in neuroimaging that are less invasive. However, given the high degree of uncertainty in the tests, coupled with the lack of meaningful therapeutic options spells years of needless anxiety for patients and families, in the opinion of this writer.