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I give an annual lecture to first year medical students on what makes a good screening test. One of the hardest points to get across is that early detection does not necessarily lead to improved outcomes. Why isn’t it the case, they ask, that finding cancer early is not always better than finding it later? It’s so counterintuitive.
To answer I cite the randomised controlled trials (RCTs) done in the 1970s that tested chest x ray pictures and sputum cytology as screening tests for lung cancer. Despite the fact that the screening tests found many asymptomatic lung cancers, none of the trials decreased mortality rates among the smokers who were screened, in comparison with controls. We would never have known this without RCTs, given that early detection nicely increased “survival times” (but really just the length of time that patients knew they had the disease).