Knowing too much

We've talked a few times about the counterintuitive conclusion that screening the general population for common diseases usually does more harm than good. As an extreme example, there are companies that sell whole body CT scans to rich people, who feel that they are doing everything they can to detect cancers, vascular disease, and other potential problems early, so they can live forever, but what they are really buying is likely to be a sack 'o woe.

It isn't just the unnecessary dose of radiation. In the first place, as we've discussed before, even a test with what appears to be a high degree of "specificity" -- that is, 90% specificity means that only 10% of people who don't have the problem will test positive -- will yield more false positives than true positives if the problem exists in a fairly small percentage of the population. This results from Bayes' Theorem, which everyone should understand. If you have a positive test, you end up with additional diagnostic tests, which may be expensive, painful, or dangerous; unnecessary treatment, which may be same as the above; anxiety, lost time at work or whatever else you do, anxiety among friends and family, and wasted resources of the medical system which could have been better applied elsewhere.

Furthermore, many diseases, no matter how dread, may not end up harming you even if you have them, because something else will get you first. Prostate cancer is a good example. Autopsies find that the majority of men over 70 actually have prostate cancer, but few of them die from it or even have symptoms. But, if they had been unlucky enough to have detected it, they would have had surgery, erectile dysfunction, incontinence, all that nasty stuff.

Finally, treatments for many diseases don't work particularly well anyway. An excellent example of that is clinical depression -- which isn't exactly a disease anyway, but a set of slippery, uncertain diagnostic criteria. There is a movement underway in the United States, spearheaded by drug companies, to screen the general population, starting with school kids, for mental disorders. In the UK, the NHS wants to screen everybody for depression. Writing in the British Medical Journal, Simon Gilbody, Trevor Sheldon, and Simon Wessely blow this idea to dust. (Why are half the men in England named Simon?)

a) Most of the people who test positive on depression screening instruments don't have depression, they just happen to be unhappy at that particular moment. They will end up getting unnecessary treatment, stigma, and whatever other badness may ensue.
b) Most people who really do have important clinical depression are going to be diagnosed eventually anyway.
c) Most of the people who really do have depresion aren't treated very succesfully.

Furthermore, Trevor and the Simons say this believing that the common treatment for depression, antidepressant drugs, is actually beneficial. There is some question about that.

So if your doctor suddenly starts asking you a lot of weird questions about how you felt in the past week, such as "I had trouble keeping my mind on what I was doing," or "I felt lonely," or "I could not get 'going'," you might just say, "And how about you doc? Did you feel happy? Did you feel sad? Are you human too?" (All of these questions, except the one about being human, are from the Center for Epidemiological Studies Depression Scale, a common screening instrument.)