Rare and difficult diseases, as well as any that are yet to be identified or for which tests still do not exist, can take decades to be identified.
Its why I find the assertion that evidence shows that rediagnosis for psychosomatic disorders only occurs at a 4% rate is laughable. This refers to research done at a hospital where they don't do further tests on these patients, and only has a follow-up period of a year. What nonsense. Its deeply implicit that testing is adequate, and doctors are competent to diagnose these problems, whereas neither is an accurate assumption. I have to ask the question as to what the rediagnosis rate would be if adequate testing were done and doctors were following up on the patient for the 20+ years it might take.
Not to mention the fact that often - the patient never returns to see the doctor who made the misdiagnosis in the first place. Each time some dismissive, patronising, sexist doctor has misdiagnosed my (what turned out to be autoimmune disease) symptoms as "functional/psychosomatic/the result of being an emotional woman" - I have refused to go back to that doctor for any further consultations so they don't ever find out that they were wrong in the first place and don't necessarily have any feedback on their diagnostic incompetence.
That's why I always find the whole "40-50% of clinic appointments are for MUS" that gets bandied about by GPs and specialists on twitter and other social media so laughable - the statistics aren't based on any reality - they are just designed to make doctors feel good about their rubbish diagnostic skills. Not to mention I believe that the statistic is derived from one of Sir Wes's jokes of a "study".