I think the reason it isn't mentioned except for a mention of a negative equivocal and positive range somewhere, I forget where, is that it doesn't really matter for the most part. It is only one piece of many. Its interest and diagnostic value depend on what else is going on. There might be a lot of interest in an academic or research setting in the fine details, which sort of anti RNP and what test method.
I diagnose infoverloaditis
You already have a firm diagnosis haven't you?