But if you have a test that is wrong 20% of the time, and you take this on all your patients every year, you will end up with a huge amount of mental suffering or possibly physical side-effects (unnecessary biopsies, or even surgery). You might end up giving people long runs of unnecessary meds. OTOH, you might end up missing some diagnoses if you don't do some tests. Balancing all of this is difficult.
Of course this is not news to the medical profession, this is a big input to the standard flowcharts. And you can always find anecdotal edge-cases that think their particular case was crystal clear and should have immediately been diagnosed. But don't assume that medical diagnosis and treatment is as trivial as doing a blood test and some googling.
I will note that in my case the test is routine and reliable and has been since before I was born.
I often wonder why such an expensive device as an MRI machine isn't utilized 24/7. I'd gladly go in for a scan at 3am if it meant paying even as little as 20% less.
It's just information about your body. You shouldn't need a doctor's approval for that.
I feel like I see this story all the time: "a simple test found the issue that they denied I had".
What we should be examining are the incentives for the doctor to resist running tests. Are they too invasive? Some can be, but blood tests certainly aren't. Are they too expensive? Ah, there's the ticket. The doctors are either losing money on tests that find nothing, or they are getting berated by the managers for wasting money on tests that find nothing, or the insurance company is saying no when they ask to run tests.
We have the technology to have proactive medicine. We could be catching cancers in early stages all the time with some standard sets of annual tests. But the economic incentives are all out of whack, and we're waiting until the last minute to do these tests.