By SAURABH JHA, MD
Consider this equation.
Early Diagnosis = Early Diagnosis + Overdiagnosis (1.1)
This sort of unequal algebra will fail high school mathematics. A new NHS initiative is arithmetic defying as well. Patients who think they have symptoms of cancer will be allowed to book medical imaging directly, without seeing their GP. This is to catch cancer early. The logic is impenetrable: early diagnosis of cancer saves lives.
Here is the problem. Cancer does not unequivocally announce its arrival. Early cancer presents with non-specific symptoms, such as an uncomfortable niggle in the back.
Tom
Let’s take Tom, who has advanced pancreatic cancer. He recalls that three years earlier he noticed a dull pain in his back during a misguided drinking binge. He would be correct in thinking that had he attended the emergency department and had a CAT scan of his abdomen, the cancer would have been smaller and would not have spread to other organs. He is right in contending that had the cancer been removed then, he would have a longer survival than presently.
The rationale has implications if extrapolated to everyone. To understand the consequences of extrapolation let’s visit a logical fallacy.
1) All Mr Smiths are over six feet tall.
2) He is above six feet tall so he must be Mr Smith.
This is affirming the consequent. Not all men above six feet in height are Mr Smiths. In fact, most are not.
3) Early pancreatic cancer presents with back pain.
4) All patients with back pain have early pancreatic cancer.
Similarly, (3) doesn’t imply (4).
Cancer often presents with non-specific symptoms, such as a vague discomfort, early on. But the majority of people with a vague discomfort do not have cancer. That is, the chance that someone with pancreatic cancer has dull back pain should not be confused with the chances that someone with dull back pain has pancreatic cancer. The chances of the latter are much lower than the former.
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