My Doctor Told Me I Had To Get A Test For Rheumatoid Factor. What Is That?
The test for rheumatoid factor (RF) is ordered when you have symptoms of RA, such as stiffness in your joints for a long time in the morning, swelling, nodules under your skin, and evidence on x-rays of swollen joints and loss of cartilage and bone. Rheumatoid factor is an antibody that is measurable in the blood. Although antibodies are normal proteins in our blood and are important parts of our immune system, the RF antibody is not usually present in the normal individual (although it can be found in 1% to 2% of healthy people). When healthy people have RF, it is usually present in only a small amount. The incidence of RF in healthy people increases with age, such that as many as 20% of people older than age 65 have an elevated RF level.
Approximately 80% to 90% of patients with RA have high amounts of RF in their blood. This antibody can usually be detected after a person starts noticing joint pain and stiffness. The amount of RF in the bloodstream increases slowly after symptoms first occur. That is, the incidence of RF increases with the duration of disease in RA. Commonly, only 33% of people who have had RA for three months have detectable amounts of RF; one year after the onset of the disease, the incidence of elevated RF levels jumps to 75%.
High levels (titers) of RF are associated with more severe RA and a higher mortality risk in patients with RA than individuals without this disease. In addition, RF is known to be abundant in the synovium and cartilage of patients with RA. High levels of RF have also been associated with an increased tendency to develop the non-joint complications of RA, such as rheumatoid nodules and rheumatoid lung disease. There is a much lower prevalence of RF in patients with juvenile RA.
A blood test is used to detect the presence of RF. It requires a small blood sample, which is usually drawn from your arm in the doctor's office or a clinical laboratory.