Posted On: March 18, 2012 by Patrick A. Malone

Doctors Often Misunderstand the Science Behind Screening Tests

Here’s another arrow in the quiver of patients well-armed against deficiencies in (well-meaning but often wrong) preventive medical care.

A survey published in the Annals of Internal Medicine concluded that primary care doctors can be as confused as the rest of us when they ponder information about screening tests.

“Most primary care physicians mistakenly interpreted improved survival and increased detection with screening as evidence that screening saves lives,” the authors wrote. “Few correctly recognized that only reduced mortality in a randomized trial constitutes evidence of the benefit of screening.”

An accompanying editorial in the same publication claimed that what physicians don’t know can harm their patients, and that screening for things such as breast and prostate cancer are widely overused.

Not that we love saying, “We told you so,” but we told you so.

As reported on MedPage Today, in a hypothetical scenario, about 3 in 4 physicians incorrectly said that increased five-year survival and early detection of cancer proves that a screening test saves lives. About 8 in 10 correctly said that a reduction in mortality in a randomized trial proves the efficacy of a screening test.

According to the study authors, "Misunderstanding of statistics ... matters, because it may influence how physicians discuss screening with their patients or how they teach trainees."

The researchers explained how measuring survival rates can be subject to bias. In a group of individuals who will die at 70, the five-year survival rate for those diagnosed with cancer because of symptoms they had at 67 will be 0 percent. But the five-year survival rate for those diagnosed through screening at 60 will be 100 percent.

"Yet, despite this dramatic improvement in survival ... nothing has changed about how many people die or when," they said.

This is such a common error in medicine that it has a name: Lead time bias. Take two imaginary people, who are both destined by fate to die on the same day from the same type of cancer. One learns of his disease three years earlier than the other, thanks to a screening test. But the test only added three years of worry to his life. It did not extend his life span a single day. That is why medical researchers always look to see if a screening test results in lower mortality (death) rates from the disease being screened for. Increase in survival means nothing without that, since it can all be lead time bias.

Screening for cancer that ultimately does not progress (as is often the case with, say, prostate cancer) also can boost survival rates but not affect mortality.

But in a randomized trial, which is the basis for solid science, mortality rates aren’t affected by these biases. So that’s the only measure that can prove that a screening test saves lives.

If even doctors, who are trained in the science if not always the art of medicine, have trouble weighting the evidence when deciding whether or not to order a cancer screening, what’s a patient to do?

Ask questions:


  • Why are you recommending this test?

  • What are its risks?

  • What other ways are there to find out more about my problem?

  • What do you think will or could happen if I don’t have this test?


If your doctor isn’t interested in entertaining these questions, consider seeking another care provider. Also, our Better Health Care Newsletter from December can help you navigate these discussions, especially the article “Refusing Health Care Is OK, as Long as You Have the Facts.”

First published on Technorati.

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