Overdiagnosis is the detection of disease when treatment is likely to be redundant or harmful, and awareness of the condition likely to induce anxiety rather than lead to meaningful empowerment . The disease is typically at its earlier stages, inhabiting a milder spectrum of phenotype, and clinically silent.
Overdiagnosis is not the same as a false positive diagnosis, although both lead to overmedicalization. Overdiagnosis is bona fide disease, a true positive, whereas a person who tests positive for a condition and is later shown not to have that condition is a false positive.
The distinction between false positives and overdiagnosis is important to appreciate to understand why overdiagnosis is controversial. The arbiter of a diagnostic test, that is whether the positive test is a true or false positive, is verification by a truth. The truth is known as the gold standard. Disease can be defined at anatomic pathology which is considered the most indisputable truth. Often disease is defined by imaging, clinical features, and laboratory tests, which comprise criteria established through consensus by an expert panel. The criteria becomes the reference standard for the disease.
One accepts that a diagnostic test can yield a false positive, as diagnostic tests are imperfect, and the diagnosis rendered by the test is a provisional assumption. For example, a positive cardiac stress test can either be a true or false positive for obstructive coronary artery disease (CAD). The truthfulness of the positive finding on a stress test can be verified at cardiac catheter angiogram, which is considered the gold standard for CAD . However, it is less intuitive that the diagnosis of CAD at catheterization, which defines CAD, can be an overdiagnosis, a false positive of kind. What then arbitrates the gold standard for disease? What arbitrates the arbiter, the speaker of truth?
Overdiagnosis is often deduced, after the fact, rather than proven ex ante. Let us take thyroid cancer for which there is very strong evidence of overdiagnosis . The incidence of papillary cancer of the thyroid has risen several fold in South Korea because of mass screening. The screen-detected cancers have been treated by surgery, yet the mortality from thyroid cancer has not reduced. The redundancy of treatment is deduced from the fact that the metric which measures the success of treatment of cancer, mortality rates specific to that cancer, is unmoved. This points to overtreatment which, in turn, points to overdiagnosis.
Overdiagnosis of papillary carcinoma of the thyroid in South Korea is incontrovertible at a population level. Despite such high level of evidence, it is difficult to prove with certainty that any particular individual with biopsy-proven papillary cancer will be overdiagnosed. Although most thyroid cancers detected through mass screening will be an overdiagnosis, not all cancers will, as some cancers may turn out not to be as indolent as others. What fuels the debate on overdiagnosis is that a subset of patients benefit from the early diagnosis, but one does not know who they are, before the fact.
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