Screening for disease is a real pain. I was reminded of this by the publication of a study in BMJ the very day of the Science-Based Medicine Conference a week and a half ago. Unfortunately, between The Amaz!ng Meeting and other activities, I was too busy to give this study the attention it deserved last Monday. Given the media coverage of the study, which in essence tried to paint mammography screening for breast cancer as being either useless or doing more harm than good, I thought it was imperative for me still to write about it. Better late than never, and I was further prodded by an article that was published late last week in the New York Times about screening for cancer.
If there’s one aspect of medicine that causes more confusion among the public and even among physicians, I’d be hard-pressed to come up with one more contentious than screening for disease, be it cancer, heart disease, or whatever. The reason is that any screening test is by definition looking for disease in an asymptomatic population, which is very different from looking for a cause of a patient’s symptoms. In the latter case, the patient is already being troubled by something that is bothering him. There may or may not be a cause in the form of a disease or syndrome that is responsible for the symptoms, but the very existence of the symptoms clues the physician in that there may be something going on that requires treatment. The doctor can then narrow down range of possibilities for what may be the cause of the patient’s symptoms by taking a careful history and physical examination (which will by themselves most often lead to the diagnosis). Diagnostic tests, be they blood tests, X-rays, or other tests, then tend to be more confirmatory of the suspected diagnosis than the main evidence supporting a diagnosis.