Overdiagnosis is not the same as misdiagnosis (for example, many people are concerned that ADHD is often misdiagnosed, but call it overdiagnosis.*)
Overdiagnosis and overtreatment are intertwined. Diagnostic tests are considered "useful" if treatment decisions are affected by the results. Although it is extremely difficult to assess when overdiagnosis has occurred in an individual, it is relatively easy to assess when over diagnosis has occurred in a population. Rapidly rising rates of testing and disease diagnosis in the setting of stable death rates are suggestive of overdiagnosis.
There is some debate about how to best describe the problem, but narrowly defined, overdiagnosis can occur in at least three ways:
when screening tests become increasingly sensitive, identifying abnormalities that are minor, non-progressive, or likely to resolve on their own
when the definition of a disease changes such that conditions formerly to be thought "compatible with normal" are now classified as disease identifying as "at risk" people who will never suffer ill-effects from their condition
when tests performed unnecessarily show an abnormality for which the threshold of "normal" is unknown
Having a diagnosis makes well people think they're ill (overdiagnosis). If they are then treated for this overdiagnosis, they are overtreated.
An example of the first is in the newer, more sensitive tests for screening for thyroid cancer discussed in the NYT article. The overtreatment in this case is the unnecessary surgery (and complications thereof) because of the diagnosis.
An example involving changing definitions pertains to diabetes. When the official definition of diabetes changed from having a fasting blood sugar (FBS) of "X or greater" to an FBS of "(X-y) or greater" (a bit oversimplified, but still a good example), 1.6 million new diabetics were instantly diagnosed, some of whom are not likely to ever develop symptoms and complications and are not likely to benefit from treatment.
An example involving tests performed unnecessarily would be getting a head CT scan on a young, healthy person for a single seizure which could easily be explained by circumstance (e.g. being supported in an upright position during a feint), and finding an unrelated (and, say, benign) lesion, which someone then wants to biopsy.
...[We] are in the midst of an epidemic of diagnosis. Conventional wisdom tells us that finding problems early saves lives because we have the opportunity to fix the problems early. [That t]here is no risk in finding things early. The truth is that early diagnosis is a double-edged sword; while it has the potential to help some, it has a potential to harm us. Such overdiagnosis leads to overtreatment when these “pseudo-diseases” are conventionally managed and treated as if they were real abnormalities; because these findings have a benign prognosis, treatment can only do harm.
Prostate cancer is the poster child for overdiagnosis. Until we started widely screening for it with the prostate-specific antigen test, it was considered a disease with a uniformly bad prognosis. Only once screening (and treatment of detected tumors) was well underway did it come to light that the majority of prostate cancers detected by screening are clinically unimportant.
Overdiagnosis should not be confused with false-positive results, that is, a positive test in an individual who is subsequently recognized not to have cancer. By contrast, an overdiagnosed patient has a tumor that fulfills the pathological criteria for cancer. Studies now estimate that 1 in 2 prostate cancers, 1 in 3 breast cancers, and 1 in 5 lung cancers are most likely overdiagnosed.
The impact of false-positive test results is largely transitory, but the impact of overdiagnosis can be life-long and affects patients’ sense of well-being and their ability to get health insurance; overtreatment impacts their physical health, and even their life expectancy.
What can be done? Resistance to overdiagnosis needs to be multi-pronged. The medical community, through testing and experience, needs to raise the threshold to label a test as “abnormal” or raise the threshold to intervene. (This has been done with the Prostate Specific Antigen test for prostate cancer.)
The other is harder. Many doctors believe patients can't make informed decisions in this area. However, studies (with women who've been diagnosed with breast cancer detected on screening) have shown that patients can make good decisions when presented with the appropriate facts.
When your doctor recommends screening for a particular cancer because of age or other demographic factors (e.g. you're a smoker), ask. Ask how you're likely to benefit from the test if it comes back positive, ask if there is any controversy about the screening test, ask if there are hand-outs explaining the risks and benefits of the screening test.
N.B. This does not pertain to all tests ordered by physicians. Not all screening tests are bad, and tests need to be done when you have symptoms.
Cancer overdiagnosis may have of one of two explanations: 1) The cancer never progresses (or, in fact, regresses) or 2) the cancer progresses slowly enough that the patient dies of other causes before the cancer becomes symptomatic. Note that this second explanation incorporates the interaction of three variables: the cancer size at detection, its growth rate, and the patient’s competing risks for mortality. Thus, even a rapidly growing cancer may still represent overdiagnosis if detected when it is very small or in a patient with limited life expectancy.
The effect of information about overdetection of breast cancer on women’s decision-making about mammography screening:study protocol for a randomised controlled trial
Mammography for Women Aged 40 and Older: A Decision Aid for Breast Cancer Screening in Canada
Overdiagnosis in Cancer
Overdiagnosed: Making People Sick in the Pursuit of Health
Using Evidence to Combat Overdiagnosis and Overtreatment
The prostate specific antigen era in the United States is over for prostate cancer: what happened in the last 20 years?