A full body MRI earns you a year of smoking

Alternative titles:

… earns you a high-risk pregnancy

… earns you an ascent of Matterhorn

… earns you 10,000 km on a motorcycle

… earns you two base jumps

… earns you a day on the frontline in Ukraine

These are all about equivalent to the risk of one year of smoking.

I&rsquo;m skeptical of submitting asymptomatic people to medical tests. Almost any<br>time I look into the evidentiary power of medical tests, I&rsquo;m struck by how much<br>work is performed by the base rate. The tests only work because we perform them<br>on people who appear sick, i.e. have a higher probability of actually being sick<br>in the first place. If we would perform them on seemingly-healthy people we<br>would get nonsensical results.11 Note that this is not a criticism of the<br>tests. They are optimised for the thing they need to do.

Thus it was with interest I read Scott Alexander&rsquo;s breakdown of the benefits and<br>costs of a routine full-body mri as a way of screening for cancer. However, I<br>felt like the conclusions weren&rsquo;t put into an understandable context. Here&rsquo;s my<br>attempt at doing so.

First, a quick recap of the main points of the article. I&rsquo;ll ignore the<br>exorbitant financial costs of us healthcare, and focus on the benefit and cost<br>in terms of health. This is measured in quality-adjusted life years, or<br>qalys. Of the hypothetical thousand people who get scanned, the estimation is<br>that

680 people will be fine, and this costs them only the doctor&rsquo;s visit, which is<br>around 3 hours, or 0.0003 qalys.

296 people will undergo additional waiting and testing unnecessarily, at a<br>total cost of 0.02 qalys coming from side effects, anxiety, and patient<br>time.

10 people will have unnecessary biopsies on top of the additional waiting and<br>testing above, bringing their total cost to 0.06 qalys.

6 people will detect a real problem, but in a way that doesn&rsquo;t help them.<br>We&rsquo;ll count this as no benefit, but also no cost.22 They would probably have<br>gone through this circus eventually anyway.

4 people will benefit from early detection already at the mri stage, and<br>gain an average of 4 qalys from this, and at a cost of 0.007 qalys after<br>patient time and side effects, this is still a gain of 3.99 qalys.

4 people will benefit from early detection only after additional rounds of<br>testing. They still gain the 4 qalys but their cost is ten more hours of<br>patient time and some additional anxiety. It still ends up being a net benefit<br>of 3.99 qalys.

Tallying up the costs and benefits into an expected value, we get a net benefit<br>of 0.025 qalys per person, after accounting for medical time.33 This is the<br>figure Scott Alexander reports as &ldquo;25&rdquo; in sum across all 1000 people. I just<br>didn&rsquo;t find its very clear so I had to replicate it to make sense of it. This<br>doesn&rsquo;t tell me a whole lot, because my intuition for qalys is weak. How<br>strongly should I prefer an intervention with a net benefit of 0.025 qalys<br>over other things I might do with my time? No idea!

However! When marketing the effect of global health interventions, a count of 27<br>qalys is typically considered &ldquo;a life saved&rdquo;. A life also happens to be a<br>million micromorts, and I have a much better intuition for micromorts! When we<br>run that maths in reverse, we get a very cheeky exchange rate between qalys<br>and micromorts:

One qaly is 37,000 micromorts. 44 The equivalence between 27 qalys and<br>a life saved is based on global demographics. For people in developed countries,<br>where the life expectation is longer, a qaly probably corresponds to a lower<br>number of micromorts – probably around 30,000 or so.

Thus, an intervention that has an expected benefit of 0.025 qalys – like a<br>routine full-body mri – corresponds to an intervention that has a benefit of<br>926 micromorts. The alternative titles indicate activities that carry roughly a<br>risk of 1000 micromorts:

a year of smoking,

a high-risk pregnancy,

an ascent of Matterhorn,

riding 10,000 km on a motorcycle,

two base jumps, and

one day on the frontline in Ukraine.

So the same effort you would expend to get out of those activities on account of<br>their risk, the same effort you should be willing to expend to get a full-body mri.

Note the on account of their risk phrasing. I have no interest in doing any<br>number of base jumps and will work hard to get out of doing them, but that&rsquo;s<br>because I wouldn&rsquo;t enjoy the activity, not because I&rsquo;m worried about the risk.<br>If I was put in a situation where it seemed like I had to perform two base<br>jumps to reunite with my family, I would spend some effort on finding<br>alternative ways of getting there, but perhaps not all that much before I decide<br>to just eat the base jumps and getting it over with.