Hi Brannybob
My mum's consultant explained that they have to rule out everything else which could cause the same symptoms first (eg Thyroid deficiency, B12 deficiency, CJD etc). Scans etc will sometimes show up shrinkage of the brain, but not always, especialy in younger people. At that point, if all the other possibilities have been ruled out, they end up with an AD diagnosis. Apparently an autopsy can confirm the existence of the sticky clusters which interfere with the workings of the brain but, until then, they do work a bit in the dark (his words not mine!).
I am so sorry you have had to face this, much love and lots of hugs x