If KA is to be a useful support to me, I need to be able to trust everyone's statements about their diagnosis and treatments. Lon - I appreciate very much when you tell us that they are changing your diagnosis. Autoimmune diseases are very complex and I think it helps us to remember that. I love your honesty about what the rheumy is telling you.
DISH is a very serious disease, just as AS is and I don't believe it is an old person's disease. The other person I know who has it is not old either. Doing some more resarch yourself and making a list of your symptoms and, as Kat suggested, a time-line may be helpful. You could also ask for a second opinion.
I tend to think it's good that rheumys don't jump to conclusions and are willing to think outside the box and consider other possibilies. The only time I don't think that's ok is when they are trying to deny someone their right to disability payments (as in Jimmy's case).
I also think it's important to recognize that back pain and tendonitis may not alway mean autoimmune. There are many, many causes of these things.