Medical test systems, like any system, have a certain level of accuracy and precision. Do doctors know about the accuracy of their tests? Do they inform their patients?
For example, I have a friend who is diabetic. She went in to get her "A1C" tested and it was measured to be 6.4 %. Since the American Diabetes Association recommends that number to be 6.5 or less, she felt pretty good.
So I did some checking on WIkipedia and found the following quote:
"Interpreting results is somewhat difficult because (1) laboratory results can differ depending on the analytical technique and (2) biological variation between individuals can be up to 1 percentage point (i.e., two individuals with the same average blood sugar can have A1C values that differ by up to 1 percentage point)."
So, the error is at least ±0.5%. The Wikipedia article also states that the results can be off if there was a recent change in diet.
This particular doctor did not discuss uncertainty levels, or maybe he didn't know. Maybe he just assumed that 6.4 really is 6.4. So I guess the lesson to take away from this is that you should always ask your doctor about the test itself. If he doesn't know, then do your own research.