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There's an amazing mathematical paradox, maybe even better than the birthday paradox, related to screening for rare diseases, that dramatically changes the formula. Imagine there's some terrible disease affects 1 in 1 million people. And there's a test that's 99% accurate. You go get screened for the disease, and it comes up positive. Oh no! What are the chances that it was a false positive? Intuition would tell you 1%. In reality? It's 99.99% likely to be a false positive.

Why? Imagine 1 million people get tested. Well we know exactly 1 person (on average) in that group is going to have the disease. But our 99% accurate test will ring a positive 1%, or 10,000 times. So the odds that you really have the disease are the odds that you're that 1 in 10,000 which is 99.99% against! Well just run the test again. Oh no! It turns up positive again! What are the odds it's two false positives? 99%! Same math. Now we know that 1 person has the disease, but our test will show 1%, 100 people, in the 10,000 as being positive. So your odds of having it are 1 in 100, or 99% against.

I'm not especially interested in being tested for rare conditions.

Oh no! But what if we run the test a third time?

You have exhausted your region's capacity for testing. Game over womp womp

Well if I only have to re-test 1% of cases maybe I can even use a more accurate test the second go-round.

Aye that’s a great one. Even testing and treating not so rare conditions can lead to statements that seem different when you know the details.

Imagine you test early and often for a condition in country A much more often than country B which waits until some more late stage easier to detect symptoms occur. Now compare survivor rates. Much higher in country A! We should clearly also be testing in country B, right?

Depends. If the condition is something that doesn’t often actually kill and typically remains at a non lethal but detectable state then all you might have done is treat a lot of extra non fatal conditions that usually only is detected in country B once it evolves to a more advanced and dangerous state. You may have put many people in country A through an unnecessary, expensive, and frightening treatment regime.

The point is that these things are complex and need thorough analysis.

> Still: This is like saying home insurance is a bad deal because on average the insurance companies make money. Screening is an insurance policy (not a free one, to be sure) against a catastrophic outcome.

I'd say this depends on the nature of the diffuse negative effects you mention - if it's car accidents on the way to the doctor's office that's one thing, but if it's people dying during surgery they actually didn't need that's another

The article quotes some one saying this isnt a call to abandon screenings in totality.

The risks are not 'car crash from doctors visit' they are 'got cancer from imaging', 'unnecessary surgery and complications', etc - the risks are directly related to the screening.

This is an active topic in medical ethics as well which you seem ignored of (no offense intended), given that you are framing this in terms of insurance or public health from the perspective of a beaurocrat - the bottom line is that if your screening is more likely to kill or maim you than the thing being screened for then that screening shouldn't be standard practice, and when it is less clear cut than that you still have to make a determination about which screenings make sense to perform on a population level.

That is something that a caring doctor has to think about as part of their duty, there is the very real potential to do much more harm than good by being thoughtless about the interventions you perform.

I’m not arguing that all screenings are worthwhile, or that the questions you raise aren’t vital to answer for good medical practice.

I’m arguing that this specific analysis has very little to tell individuals about how they should perceive the value of any particular test. A different analysis—looking at the particular negative outcomes of the testing itself or the reaction to false positives—would be a different story entirely.

What do you think the individual patient should take away from this analysis in actionable terms?

I see! Who are you arguing against? The article gives zero indication that any of this is actionable, seems to advocate not changing anything with respect to your regular medical care.

But to answer your question; 'nothing'. This is interesting but it isn't actionable.

I thought the article was pretty sloppy in its description of the results. Here’s the lede:

Most cancer screenings don’t ultimately give someone extra time beyond their regular lifespan, according to a new review of clinical trials involving more than 2.1 million people who had six kinds of common tests for cancer.

This is trivially misreadable as “an individual with cancer who catches it early via screening doesn’t see appreciable lifespan benefits”, which is decidedly not what the analysis purports to show.

The use of “someone” makes me question whether the author of the CNN article understood the analysis in the first place. The phrase “extra time” is also particularly strange, since it’s something of a loaded term in the world of serious disease. I don’t get a cancer screening because I want “extra time”. I look to get on the early-access list for an unapproved chemo drug because I want “extra time”. The article goes on to frame the results in a way that easily could be misunderstood to be the benefits for sick patients to early detection rather than the average benefits across the entire population, sick and healthy.

Ok, you've convinced me. This article could have given more information and context so that folk without that benefit would walk away with a more nuanced understanding of screening and its seemingly paradoxical costs. A short article that glosses over the details was not appropriate for this subject. Seeing plenty of evidence for that in this comment section.

If you add "on average" that's exactly what the original paper says. The averaging is over "someone who doesn't get cancer but died of screening complications", "someone who got false positive and decreased their lifespan with unnecessary treatments", and of course "someone who got true positive and increased his lifespan due to treatment". Are you sure you will be in the last group?

I agree with your meta-analysis-analysis completely. Just chiming in to say that I chuckled when you reminded me of this classic: https://twitter.com/dril/status/464802196060917762?lang=en

what are some cancer screening that one should actively pursue?

Speak with your doctor about this, especially because I am not one, but from what I understand, the ones that you have a family history for, are pre-diposed by environment, the common ones, and the ones caught by usual body checks / tests during check ups. Again, not giving advice but things you can start a conversation with your doctor with.

The other comment basically got it. Ask doctor particularly with family history. All cancers are better treated if caught earlier but some like colorectal cancer have much less severe outcomes if caught early on (eg removing a large segment of colon vs a small polyp)