overdiagnosis

Overdiagnosis is the detection of a disease that would never have caused symptoms or harm had it gone unnoticed. The US National Cancer Institute defines it, for cancer, as finding cases with a screening test such as a mammogram or PSA test that will never cause any symptoms: cancers that may stop growing or go away on their own.

It is distinct from a false positive. A false positive is a test result that suggests disease where there is none. An overdiagnosed cancer is real and confirmed under the microscope, but slow-growing or dormant, and the person would have lived out their life without ever knowing it was there. The institute's own PSA fact sheet draws the line: overdiagnosis is detecting tumours that would not have caused problems during someone's lifetime, and overtreatment is treating them.

That is where the harm sits. The institute names anxiety and treatments that are not needed, with the side effects those treatments bring, as the main costs. Overdiagnosis is the central trade-off in any screening programme: the more people screened, the more lives can be saved and the more harmless disease is found alongside them, which is why screening trials count both.