This is an important and unintuitive reality about all kinds of tests. You have to take into account the base rate - the likelihood that you have the disease given what you know prior to the test.
If you're otherwise healthy and would have a 1/1,000,000 chance of having the disease before the test, and then you test positive with a test that is 99% accurate, you are ~100x more likely to have the disease than before - but that's still only 1/10,000 - not at all 99% likely, even though the test was "99% accurate"
That said, I think with this knowledge the test still confers helpful information. I might decide to spend $1000 on an additional diagnostic, even knowing that I'm still very likely to be negative. Depends on how wealthy I am, and how serious the disease is, and what the treatments for it are.
> I might decide to spend $1000 on an additional diagnostic, even knowing that I'm still very likely to be negative.
The problem is that diagnostics aren't necessarily risk-free. For example, there's a non-zero risk of death while getting a colonoscopy, to the point that false positives from unnecessary testing can increase all-cause mortality for patients.
Case I read in the JAMA which I get because I signed up during covid.
Late 20's Hispanic lady shows up in the ER with what they think is probably food poisoning. But they do a CT. Which shows changes in her liver which probably is a fatty liver. But they do a biopsy just in case. Biopsy results in a bleed which requires a transfusion and 4 days in the hospital. Biopsy result, fatty liver.
There is also a nontrivial chance of every CT scan to cause fatal cancer. It's a ton of radiation[1], and ERs love pushing it indiscriminately on vulnerable people.
Surprisingly few ER docs anywhere in the world have even a rudimentary understanding of the risks of CT scanning patients. There's a lot of information around about this, but my own first hand (anecdotal) experience is that I've had ER docs try to convince me that it's basically the same as an X-Ray and act like I'm a crazy person when I explain that it's orders of magnitude higher and cumulative over a lifetime. On one hand, it's not their job to care about your long term health - they need to rule out an emergency and get you out the door as quickly as possible - but it's very concerning.
It's a bit like how general practitioners aren't taught about nutrition at all, so give out really poor advice for heart disease patients (the leading cause of mortality in Western economies).
Edit: For comparison, a chest X-ray is around 0.1mSv, a chest CT at 6.1mSv, so a factor of 61 between (https://www.radiologyinfo.org/en/info/safety-xray ). Compared to natural exposure (usually 1 to 3mSv/a) however, a chest CT isn't that bad at 2 to 3 years natural dose, 2 polar flights or 1 year of living at higher altitude or Ramsar (https://aerb.gov.in/images/PDF/image/34086353.pdf ). Acute one-time dose damage has been shown above 100mSv, below that there is no damage shown, only statistical extrapolations.
So I'd say that the risk of using a CT right away should be lower than the risk of overlooking a bleed or a clot in an emergency, where time is of the essence and the dance of "let's do an X-ray first..." might kill more patients than the cancers caused by those CTs.
> On one hand, it's not their job to care about your long term health - they need to rule out an emergency and get you out the door as quickly as possible - but it's very concerning.
Yeah, but what's the alternative when you have a stroke? They need to understand it's type and the mistake here is likely to be fatal since they require opposite treatments.
There's also a real psychic cost to even a tentative positive result, which is why I'm much more likely to get an actual colonoscopy than a Cologuard test.
I had the same thought. I got the colonoscopy and I don’t regret it at all. And for anyone thinking about it or stressing about it, the prep was not that bad. If you’re over 45 just do it.
I had put it off for several years because everyone kept telling me how awful the prep was. My doctor kept pestering me to agree to do it so I eventually relented.
Turns out, I needed to go to the bathroom frequently during the day which was an annoyance but I never had anything close to an "accident" nor did I feel any strong urge to evacuate at night. So the whole experience turned out to be a huge nothing-burger and I had a few polyps that got sniped that weren't cancerous - so now I have peace of mind that I didn't have before.
If you're otherwise healthy and would have a 1/1,000,000 chance of having the disease before the test, and then you test positive with a test that is 99% accurate, you are ~100x more likely to have the disease than before - but that's still only 1/10,000 - not at all 99% likely, even though the test was "99% accurate"
That said, I think with this knowledge the test still confers helpful information. I might decide to spend $1000 on an additional diagnostic, even knowing that I'm still very likely to be negative. Depends on how wealthy I am, and how serious the disease is, and what the treatments for it are.