Relative risk is just another way of lying. ~ Aron Sousa
In order to make an informed decision about proposed medical treatments, one should consider two pieces of information:
- What is the probability of positive outcome from the treatment?
- What is the probability the treatment will cause harm?
So let me ask you, the last time you were advised to pursue a particular medical treatment, whether it be a pill or a procedure, did the doctor give you these data? I suspect if you were given any information at all, the effects of treatment were likely reported in highly misleading relative numbers rather than more useful absolute numbers.
To understand why relative numbers are misleading, consider a hypothetical population of 100,000 adults. Absent any treatment, assume for the sake of this example, 1000 of these adults are statistically likely to have a heart attack. Now, if it was discovered that after, say, five years of treatment, 750 adults had heart attacks, one could legitimately claim a 25 percent (relative) reduction in heart attacks. This sounds great, but upon closer examination, only 250 heart attacks were prevented out of a population of 100,000. In absolute terms, the success rate of the treatment is a meager .25 percent. (That’s one-quarter of one percent in case you missed that decimal point.)
Needless to say, it’s easy to understand why medical companies would rather report relative numbers than absolute numbers. After all, advertising a 25 percent reduction in heart attacks is going to sell a lot more pills than stating a decrease in heart attack rates from 1 to .75 percent of the population. However, for those of us wanting to make an informed decision about a proposed medical treatment, we have to dig a little deeper to find the more relevant absolute numbers.
But how? Many doctors get their information from the pharmaceutical companies that profit directly from the very treatments they are proposing. Where can one turn to find unbiased information?
It turns out that a group of medical professionals maintaining a site called the NNT (h/t ProPublica) do a fantastic job of distilling data in a way that is meaningful for patients and doctors alike. NNT is an abbreviation for “number needed to treat.” That is: How many patients need to be treated to get the advertised benefit?
Considering the hypothetical example noted above, since for every 400 patients who got treatment one was spared a heart attack, the NNT, in this case, would be 400. Taking it a step further, a treatment’s NNT is much more useful when viewed alongside risk data reported by its NNH — “number needed to harm.” In our hypothetical example, if the NNH is 10 for that treatment, that means for every one spared a heart attack, 40 are likely to suffer harm in the form of side effects (i.e., 400/10 = 40).
Now that we understand what NNT and NNH are, let’s use these numbers to examine an actual drug like aspirin.
This AHRQ article* evaluates the benefits and harms of aspirin for primary prevention of cardiovascular events. It found that for women between the ages of 65 to 74 who take aspirin daily for a year to prevent a first heart attack, aspirin has an estimated NNT of 1520 and NNH (for major gastrointestinal bleeding) of 701.
In other words, for every woman (65 to 74) who avoids a heart attack, twice as many will suffer serious harm in the form of GI bleeding. Any wonder it was recently announced that doctors no longer recommend daily low-dose aspirin for healthy individuals (NYTimes)?
Returning to the NNT site we learn:
- There is no statistically significant mortality benefit to taking statins if you are a person who is at low risk for cardiovascular disease. Despite this and the fact that 1 in 21 statin uses will experience muscle damage and 1 in 204 will develop diabetes mellitus, the global cholesterol-lowering industry is forecast to be worth over $20 billion in 2019 (source: reportlinker).
- For patients with stable non-acute coronary artery disease, there is no benefit to having a stent installed. Given that 1 in 50 who have a stent operation will experience serious complications as a result of the surgery, I’d say that’s a valuable piece of information.
- Postmenopausal women receiving hormone therapy for cardiovascular disease prevention receive no benefit but are now at risk for stroke (1 in 165) and blood clots (1 in 118).
And here I was worried about the legalization of marijuana.
On that note, time for me to wrap it up and tend to my day job. Before I do, how about some music for our Manufacturing Peace of Mind™ Spotify playlist? Today I’m going to leave you with Ravyn Lenae and Steve Lacy with their song “4 Leaf Clover” (unless you’re working at a dispensary, this video is probably NSFW).
* Hat tip Propublica.
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