Friday, July 24, 2009

Too much cancer screening can be worse than none at all

A recent article in the New York Times reports that the push for increased cancer screening may only have limited benefits. It talks especially about massive screening campaigns for rare cancers like thyroid cancer (kills only 1600/year in the US) and extending testing for breast cancer or prostate cancer to populations (younger people) where it occurs rarely or using tests that are poor screening tests.

The most insightful statement from the article:
Dr. Ned Calonge, the chairman of the United States Preventive Services Task Force said, “There are five things that can happen as a result of screening tests, and four of them are bad.”
Below is a the paraphrased and bulleted list from the article with inserted statistical names for some of these outcomes if relevant.

One good result of screening:

  • Identifying a life-threatening form of cancer that actually responds to timely intervention.

Four bad results of screening:

  • False Positive: Results that falsely indicate cancer and cause needless anxiety and unnecessary procedures that can lead to complications.
  • False Negative: Results that fail to diagnose an existing cancer, which could lull a patient into ignoring real symptoms as the cancer progresses.
  • Results that detect slow-growing or stable cancers that are not life-threatening and would not otherwise have required treatment.
  • Results that detect aggressive life-threatening cancers whose outcome is not changed by early detection.

If you know the accuracy of the test and the incident rate of the cancer in the population and the cost of treatment and the value of a human life (that last one is tricky and is a minefield) then a simple cost benefit analysis will allow the determination of appropriate screening. If human lives are worth infinity then the math is impossible. When advocates of a particular approach don't understand statistics then the math is also impossible. I think the main error is the failure to understand the cost of the four bad outcomes above. Everybody focuses on the correct positives.

I think the same approach could be used with terrorism. Replace cancer with terrorism above.

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