Multiplying by zero

Last week a friend sent me an a news article saying someone had just discovered that a medication he has been taking for many years “increases the risk of heart attacks”. I related a story to him of how I myself got a slight shock a few years ago, when I read that having had two CT scans of my head as a child tripled my risk of brain cancer.

Beyond relating health risks that neither my friend nor I can do anything about, these two situations have something important in common: the risk of each of the events they increase the risk of, in me and my friend, is negligible. Nothing multiplied by three is still nothing.

As a woman in her late twenties, my risk of brain cancer next year is around 1 in 100,000. Having had those two CT scans, it is now approximately three in 100,000 (or one in 33,000 if you prefer).

Likewise, my friend’s risk of a heart attack or another acute cardiac event as a man in his twenties is around 15 per 100,000 or one in 6,700. The 16% increase that has been estimated from his medication puts him at one in 5,700.

Numbers like these don’t make headlines, and it’s much easier for a paper to report (as they did) that something “increases the risk of heart attack” without providing any figures at all than it is to explain the actual magnitude of the risk with relation to specific groups, such as men under 30.

Identifying these risks is important – if a doctor was considering prescribing my friend’s medication to an obese smoker in his 60s with a strong family history of heart disease, a 16% increase in risk might be unacceptable. But news stories like this cause concern even among young healthy people whose baseline risk of most serious health events is low – we see this every few years with the pill, for instance.

Practically, I would suggest the following: Those of you who work in medicine should be up front with your patients about these issues. If young women who were prescribed the pill were told “the pill has been shown to increase the risk of blood clots, however, because you are a non-smoker under thirty the risk to you is extremely low” they would be less liable to panic when the next round of news stories about a one in a million event goes around.

Those of you who are patients, when you see stories like this, need to consider what you know about your own risk of the event in the news story. Do you have a family history of heart disease or a particular type of cancer? Are you of an age where your risk of that event is actually meaningful, or do you know that this event is very rare in people your age? Knowing the numbers may be useful, but you’ll probably have an intuitive sense – do you know anybody who’s had a heart attack under 40, for instance? I don’t. They’re rare.

If you think the risk is meaningful or you just want to be sure, talk to your doctor and they should be able to walk you through it. But remember that the newspapers aren’t worried about your health – they want clicks. And when you see a story like this, and freak out, and share it with all your friends, who freak out, and share it with all their friends, the papers gets what they want. All you get is a bad night’s sleep.

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