Illness is identified by taking a history, examining symptoms and signs, and often by taking some tests.
Many of us seem unwell as much testing is happening – roughly 50 million diagnostic tests, 500 million biochemistry and 130 million haematology tests are performed annually in the NHS. In the U.S., it’s another order of scale, with 14 billion laboratory tests ordered annually. Testing is also on the increase: in primary care, it increased by 8.5% per year between 2000 and 2015 across all ages. The proportion having more than one test has also increased significantly. However, there are wide variations in testing, which is unlikely to be explained by clinical need.
The CDC reports that 70% of medical decisions depend on laboratory test results, but what happens when these decisions do not benefit patients or would never have caused any symptoms or problems?
Overdiagnosis transforms people into patients unnecessarily by identifying problems that were never going to cause harm or by medicalising ordinary life experiences through expanded definitions of diseases.
As an example, the most extensive study to date of the Prostate-Specific Antigen (PSA) blood test to screen for prostate cancer found it only had a negligible impact on reducing deaths. Still, it led to overdiagnosis and missed the early detection of some aggressive cancers.
The trial included 415,357 men between the ages of 50 and 69. They were randomly assigned to a single invitation for PSA screening or a control group without PSA screening. The participants were followed up for a median of 15 years.
So, while a single invitation for PSA screening reduced prostate cancer deaths by a tiny amount, roughly one in 1000 men tested, it had no impact on all-cause deaths, which is the outcome you are concerned about.
PSA screening did increase the detection of low-grade and localised disease but not intermediate, high-grade or distally advanced tumours. As a consequence, about one in six cancers were over-diagnosed. These men went on to have invasive treatments they didn’t need, while the test failed to spot aggressive cancers requiring intervention – creating stress and worry for no reason.
You might expect all this extra testing to translate into better outcomes. Yet, medical practices with the highest PSA rates do not see reduced prostate cancer mortality. However, they do see increases in the number of downstream diagnostic and surgical procedures with potentially harmful consequences.
A slight reduction in prostate cancer deaths weighed against the lack of all-cause mortality and overdiagnosis that comes with all the worry and stress means the benefits of testing often do not outweigh the potential harms.