Most commonly, rheumatoid factor is used as a blood test for the diagnosis of rheumatoid arthritis. Rheumatoid factor is present in about 80% of adults (but a much lower proportion of children) with rheumatoid arthritis.heumatoid factor (RF) facts. 1  Rheumatoid factor is an antibody that is detectable in the blood of approximately 80% of adults with rheumatoid arthritis.