Medical and Biological Sciences are rarely absolute, that privilege resides in some laws of the Physical and Mathematical world. Life has but one guarantee, that one day it will end.
For every test, intervention or therapy, there are not just two outcomes, but four. A test for example, may be positive or negative, but in each case the result may be right or wrong.
Thus performance of a test is a complex balance of the values (utility) of the possible results. So if it is very important or serious condition, we may need to accept that the test gives a measurable incorrect (false) positive result, so we don't miss it. Alternatively where a positive test result may lead to costly, embarrassing or invasive consequences, we may need to desensitise it to avoid the false positive (alarm) rate.
These are complicated and confusing concepts, and are well documented in books such as "Reckoning with risk", by Gerd Gigerenzer, Penguin books, 2002.
Unfortunately these concepts have to be accepted, whether they are understood or not by those having Medical therapy. Much is accepted under what is called implied consent, ie. that by asking for a medical opinion you realise that details will be asked of you (the history), that examinations appropriate to your condition may be made (the physical), and investigation requested that may lead to recommendations for intervention.
That the summation of these will be appropriate and satisfactory is part of professionalism, and consequently part of the service that I offer in my areas of practice.