How an ounce of Rx prevention can cause a pound of hurt.

In a revealing 2002 paper in the Journal of the National Cancer Institute, William Black and colleagues from Dartmouth-Hitchcock Medical Center explain how those randomized studies—which form the backbone of some screening guidelines—actually emphasized the wrong outcome. Routine chest X-rays, for example, are supposed to reduce the death rate from lung cancer, and that's what the studies typically measure and report. Studies also routinely show that mammograms reduce breast-cancer deaths. But that's not really what people care about. What they want is an overall lower death rate.  What good, after all, is a test that may lower the risk of lung-cancer death but increase the overall risk of death from side effects, such as pointless operations (as in neuroblastoma)?

Unfortunately, according to the Dartmouth analysis, none of the studies demonstrated any measurable overall reduction in mortality from cancer screening. Most worrisome, in half the studies, the overall mortality rates tended to be worse in screened groups than in unscreened groups—erasing any benefit of screening.

Without better studies on which to base national screening policies, efforts to prevent disease may do more harm than good. It's hard to hold off on strategies as seductive as CT scanning to detect early lung cancers and study them further. But if we don't—to paraphrase—we must be prepared to accept the consequences of going to war with the data we have, instead of the data we really need.