Both knee pain since 1.5 yr. MRI OK. Now left hand elbow and finger joints burning. RA factor8.6IU/ml. ESR/RPR/ASO/CRP/uric acid normal. Possible reasons?

Unclear polyarthriti. The RF (Rheumatoid factor) assay -which is, I assume the "RA factor"- varies widely from lab to lab, so I am not sure if 8.6 IU/mL is in the reference range or not (the lab should give both the result and the ref range). It's best to talk to a rheumatologist about your symptoms. It sounds complex. There are forms of arthritis which don't show up on blood tests (seronegative); you may have 1 of tho.