You really see this tension in cancer screening. Doctors who want to prevent cancer apply these tests as if they were treatments, as if getting a mammogram were somehow like prescribing an antibiotic. Our experience tells us these tests catch cancer in some patients — the woman in the exam room could be one of them. Complications can be handled. We’ve all handled them before.
But this is where our reliance on our instincts and experience may betray us. Screening involves a test conducted on a healthy person, not a treatment given to a sick person. It’s comparatively easy for a doctor to see whether a treatment is working; that data point shapes our instincts and informs our experience going forward.
Screening is far less instructive for physicians. We can never tell how often a test makes an individual better or improves her prospects of survival. Neither is it possible to measure the effect of screening in your own practice. You must screen hundreds of patients to prevent a handful of cancer deaths. With routine mammography, you’d have to screen more than 1,000 women in their 40s to prevent just one breast cancer death…
It is time for us to own up to our shortcomings in cancer screening, and we must start by acknowledging a hard fact: Doctors sometimes don’t know best. We are terrific at inventing new tests that can be performed on people. But we have been less good at figuring out which people should have them.