Medical Decisions–Risk Saavy

screeningLearnMoreThis post looks at the medical/health component of decision making as addressed in Gerd Gigerenzer’s new book, Risk Saavy, How to Make Good Decisions. First, Gigerenzer has contributed greatly to improving health decision making. This blog includes three consecutive posts on the Statistics of Health Decision Making based on Gigerenzer’s work.

He points out both the weaknesses of screening tests and our understanding of the results. We have to overcome our tendency to see linear relationships when they are nonlinear. Doctors are no different. The classic problem is an imperfect screening test for a relatively rare disease. You cannot think in fractions or percentages. You must think in absolute frequencies. Breast cancer screening is one example. Generally, it can catch about 90% of breast cancers and only about 9% test positive who do not have breast cancer. So if you have a positive test, that means chances are you have breast cancer. No! You cannot let your intuition get involved especially when the disease is more rare than the test’s mistakes. If we assume that 10 out of 1000 women have breast cancer, then 90% or 9 will be detected, but about 90 of the 1000 women will test positive who do not have disease. Thus only 9 of the 99 who test positive actually have breast cancer. I know this, but give me a new disease or a slightly different scenario and let a month pass, I will still be tempted to shortcut the absolute frequencies and get it wrong.

Gigerenzer points out the weaknesses in medical screening tests, but does not point out that as more people participate over time, they tend to be refined and treatment improves. To some extent, he wants you to avoid being a “guinea pig”, but more importantly he wants you to at least be told that you are a guinea pig and make your own decision.

Gigerenzer examines the major problems in breast cancer and prostate cancer screening. Few lives are saved while many are put through stress, actual operations and procedures, and even a few die based on screening finding false positives or finding cancer that would never have progressed. Gigerenzer cites Cochrane Reports that say that such screening cannot be recommended and frankly says to not undergo those screenings. I must side with Michael Marmot, a distinguished public health advocate who after reviewing the literature on breast cancer screening found that it saves lives. However, he noted that you should be told before your mammography that you have about a 1 in 100 chance of being diagnosed and subsequently treated for a cancer that would never have impacted your life otherwise, while between 3 and 5 lives for each 1000 screened would be saved. (See British Journal of Cancer (2013), 108, p 2205-2240.)

Gigerenzer no longer derides colonoscopies as he did in Gut Feelings, Intelligence of the Unconscious. The screening test is getting more effective, cheaper, and safer, and if detected early, colon cancer can often be treated effectively. Screening tests do tend to be oversold and I suppose this is a form of a “nudge” and it can be an unethical nudge.

Gigerenzer, himself, does considerable nudging in the book. On page 179, he states:  “Pap smear screening for cervical cancer, in contrast, appears to save lives, this has not yet been tested in a randomized trial.”  No randomized trial could ethically be done for the pap smear since as Gigerenzer notes, it appears to save lives. In contrast, randomized control trials have been done for mammography, but that has not stopped the arguments about which one is best or was based on the appropriate protocols. One issue with screening tests is the difficulty of being able to withhold them once they seem to be helpful. Nearly twenty years ago, I recall a professor talking about how the pap smear was not very good, but once it became normal, it was difficult to improve. Setting up a randomized clinical trial pitting a new or improved cervical screening method must enroll people in a method that is unproven. This requires considerable altruism for people who are not sick.

One of the headings in the book states “Fight cancer with prevention, not screening.”  I must ask, why not both?  Gigerenzer talks about prevention especially with respect to cancer and talks for instance about how 20 to 30% of cancers can be attributed to smoking and 10 to 20% to other lifestyle and dietary factors. Although these are reasonable numbers, there definitely has not been a randomized clinical trial to prove it. Gigerenzer attributes between 30 and 50% of cancer to behavior, lifestyle and dietary risks. The World Health Organization says 30%. If some people act in ways that they believe will prevent cancer that are actually harmful, then supposed prevention will have some of the same impacts as supposed screening. I think Gigerenzer’s “nudge” on this might not be appropriate. Risk communication is difficult and usually requires feedback between those trying to communicate and those trying to understand the risk.

Gigerenzer suggests that CT scans cause about 2% of cancers and that other diagnostic tests or checklists (decision trees) coupled with diagnostic tests would be better and safer. I agree. Gigerenzer has a little story about not wanting his six year old daughter to get dental x-rays at her first appointment and getting in an argument with the dentist. The point of this seems to be that a lot of doctors are stupid jerks. I have my own stupid jerk CT scan story.  The point of my story is that when you are in a hospital, you can only rock the boat so hard, and that your decision making skills although helpful are inadequate.

My sister was admitted to the hospital after messing up the doses of pain meds and oral chemo drugs. She had terrible diarrhea and was incoherent or nearly so. After several very rough days in the hospital but having stabilized, the hospitalist came in and told my sister that she wanted to do a CT scan of her abdomen to check for an obstruction. The hospitalist gave several reasons that it probably was not an obstruction, but she wanted to be certain. She then told my sister that she could do whatever she wanted, but it was the hospitalist’s recommendation to do the scan. My sister said that she would do it if her oncologist agreed that it should be done. The hospitalist was apparently surprised by this reaction, she continued with her “You can do what you want, but I think we should do the scan” and continued to get the same reaction.  On the fourth try, she even invoked her years of experience as a hospitalist. My sister as the smart aleck arising from delirium, said: “That is the fourth time you have said that. I have given you my instructions.” The hospitalist emerged from the room looking like a poker player who’s bluff had been met by “You’ve got nothing.”

It was a classic example of winning the battle and losing the war. The scan was eventually done forcing Roxanne to drink a half gallon of nasty diarrhea inducing medicine, diarrhea being the original reason she was in the hospital, and, of course, the scan was negative. The oncologist was not as brave as Roxanne or the scan would not have been done. Gigerenzer might properly consider this as more evidence of the defensive medicine that he decries, but I see it as something else. You may try to avoid healthy screening, etc. but when you get really sick you need a doctor who is a friend or nearly so, and to have learned the best you can what to avoid in the medical system. Your best efforts may not be fully successful, but you ignore and completely avoid the medical system at your own risk.









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