In the US, if you actually care about fixing a problem, it really helps to do tests yourself privately and quickly if the doctor or specialist is unwilling. Let's say there are 30-50 blood tests that are relevant for a set of symptoms. I would do up to $500 worth of tests at a time, then repeat the loop, continuing with the next batch until an answer is found or until all applicable tests are exhausted. Each normal result helps refine the next batch.