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’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]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 that680 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]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