Rheumatoid factor (RF) is a blood test that measures the amount of the RF antibody in the blood. An abnormal result means the test is positive, which means higher levels of rheumatoid factor have been detected in your blood. Most patients with rheumatoid arthritis or Sjogren syndrome have positive RF tests. The higher the level, the more likely one of these conditions is present. Not everyone with higher levels of rheumatoid factor has rheumatoid arthritis or Sjogren syndrome.