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’m skeptical of submitting asymptomatic people to medical tests. Almost any
time I look into the evidentiary power of medical tests, I’m struck by how much
work is performed by the base rate. The tests only work because we perform them
on people who appear sick, i.e. have a higher probability of actually being sick
in the first place. If we would perform them on seemingly-healthy people we
would get nonsensical results.1 Note that this is not a criticism of the
tests. They are optimised for the thing they need to do.
Thus it was with interest I read Scott Alexander’s breakdown of the benefits and
costs of a routine full-body mri as a way of screening for cancer. However, I
felt like the conclusions weren’t put into an understandable context. Here’s my
attempt at doing so.
First, a quick recap of the main points of the article. I’ll ignore the
exorbitant financial costs of us healthcare, and focus on the benefit and cost
in terms of health. This is measured in quality-adjusted life years, or
qalys. Of the hypothetical thousand people who get scanned, the estimation is
that
- 680 people will be fine, and this costs them only the doctor’s visit, which is
around 3 hours, or 0.0003 qalys. - 296 people will undergo additional waiting and testing unnecessarily, at a
total cost of 0.02 qalys coming from side effects, anxiety, and patient
time. - 10 people will have unnecessary biopsies on top of the additional waiting and
testing above, bringing their total cost to 0.06 qalys. - 6 people will detect a real problem, but in a way that doesn’t help them.
We’ll count this as no benefit, but also no cost.2 They would probably have
gone through this circus eventually anyway. - 4 people will benefit from early detection already at the mri stage, and
gain an average of 4 qalys from this, and at a cost of 0.007 qalys after
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
testing. They still gain the 4 qalys but their cost is ten more hours of
patient time and some additional anxiety. It still ends up being a net benefit
of 3.99 qalys.
Tallying up the costs and benefits into an expected value, we get a net benefit
of 0.025 qalys per person, after accounting for medical time.3 This is the
figure Scott Alexander reports as “25” in sum across all 1000 people. I just
didn’t find its very clear so I had to replicate it to make sense of it. This
doesn’t tell me a whole lot, because my intuition for qalys is weak. How
strongly should I prefer an intervention with a net benefit of 0.025 qalys
over other things I might do with my time? No idea!
However! When marketing the effect of global health interventions, a count of 27
qalys is typically considered “a life saved”. A life also happens to be a
million micromorts, and I have a much better intuition for micromorts! When we
run that maths in reverse, we get a very cheeky exchange rate between qalys
and micromorts:
One qaly is 37,000 micromorts.4 The equivalence between 27 qalys and
a life saved is based on global demographics. For people in developed countries,
where the life expectation is longer, a qaly probably corresponds to a lower
number of micromorts – probably around 30,000 or so.
Thus, an intervention that has an expected benefit of 0.025 qalys – like a
routine full-body mri – corresponds to an intervention that has a benefit of
926 micromorts. The alternative titles indicate activities that carry roughly a
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
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
number of base jumps and will work hard to get out of doing them, but that’s
because I wouldn’t enjoy the activity, not because I’m worried about the risk.
If I was put in a situation where it seemed like I had to perform two base
jumps to reunite with my family, I would spend some effort on finding
alternative ways of getting there, but perhaps not all that much before I decide
to just eat the base jumps and getting it over with.
