ESR may be normal with poly and dermatomyositis , however a normal ESR almost excludes poly myalgia Rheumatica.
The most specific sign for dermatomyositis is papules over extensor surface of the fingers ( + muscle pain ,weak,high CPK).
PMR has a common association with giant cell arteritis .The most specific sign of GCA is jaw claudication .Temporal artery biopsy may be negative .