Overdiagnosis occurs when a medical professional diagnoses a patient with a disorder that will neither have deleterious symptoms nor alter the course of the patient's life in any meaningful way. When the individual making the diagnosis recognizes this and, accordingly, advises the patient not to seek treatment, no harm is done. When, on the other hand, the medical professional advises a course of treatment, the patient who is, for all practical purposes, in good health may enter a course of treatment that includes harmful side effects. Overdiagnosis occurs most commonly in advance screening for cancer. Early cancer screenings often detect apparently-cancerous tissue and recommend treatment despite the fact that, in many cases, there is little or no potential for long-term damage.
One essential element of overdiagnosis is that the condition diagnosed is real and present. It is not necessarily a false diagnosis — it is simply unnecessary because addressing or even knowing about the issue can bring no possible benefits. Some definitions even consider diagnosis of an untreatable condition to be overdiagnosis because the diagnosis, though correct, is entirely irrelevant. A diagnosis may also be considered irrelevant if the patient has no interest in seeking treatment because of the excessive pain and inconvenience it would cause, the limited chance of success, or other personal or cultural reasons.
Unfortunately, it is often impossible to tell whether or not overdiagnosis has occurred until long after the diagnosis and subsequent treatment, or lack thereof. For many issues, such as some early signs of cancer, it is impossible to tell early on if treatment is strictly necessary. Some individuals who avoid treatment may never exhibit symptoms or experience any harmful effects. Others, on the other hand, could develop serious and harmful conditions if they refrain from seeking treatment. The only sure way to assess whether or not overdiagnosis has occurred is to avoid treatment for a diagnosed condition and to notice no harmful effects from that condition for the rest of one's life.
The single most significant consequence of overdiagnosis is unnecessary treatment. In minor cases, this may mean taking inexpensive antibiotics or following some other equally unobtrusive treatment regimen for a short period of time. In more severe cases of overdiagnosis, individuals may follow costly, intrusive, and painful treatment regimens involving chemotherapy, radiation, surgery, or a variety of other unpleasant practices. All of the unpleasantness of treatment is, in such cases, an unnecessary response to the diagnosis.