I'm simplifying in my statement, but generally the way it works is that basic tests give evidence for more involved tests, and those tests evidence for even mor

I'm simplifying in my statement, but generally the way it works is that basic tests give evidence for more involved tests, and those tests evidence for even more involved ones. If all the basics look good, doctors generally don't want to start shooting in the dark with much pricier and time consuming tests. So for example if you have cardiac muscle damage, but it only shows up on a cardiac MRI, it can be very hard to get one when your EKG is normal, and they even went so far as to do an echo, and that came back normal too. There are tons of examples of this in long covid. High end expensive tests may show something, but there is no path there because the book says you need to be showing xyz common with known illness abc before that test is warranted.