20% of people with RA test serum negative for it. i too have it but test negative. also, RA DOES effect the knees, trust me, i have accidental damage in one knee and RA damage in the other. my hands, fingers, wrists, elbows and shoulders also get stiff and painful, but it isnt usually a constant thing, just comes and goes (especailly in wet and cold weather).
good luck
missy