Rheumatoid Factor (RF) is a protein known as an autoantibody, which is mistakenly produced by the immune system. Instead of targeting foreign invaders like viruses or bacteria, this protein attacks the body’s own healthy tissues. The specific target of Rheumatoid Factor is usually the Fc portion of another antibody type called Immunoglobulin G (IgG). The RF test is a common blood analysis designed to measure the concentration of this particular autoantibody circulating in the bloodstream.
The Purpose of the Rheumatoid Factor Test
A healthcare provider typically orders the Rheumatoid Factor test when a patient exhibits symptoms suggesting an autoimmune disorder, such as persistent joint pain, stiffness, and swelling. Its primary function is to serve as an aid in diagnosing Rheumatoid Arthritis (RA), a chronic inflammatory disorder. RF is detected in the majority of individuals with confirmed RA, making it a valuable marker for the condition.
The RF test is not considered a definitive diagnostic tool on its own because its presence is not exclusive to RA. The test also helps in evaluating other systemic autoimmune conditions, including Sjögren’s syndrome and Systemic Lupus Erythematosus (SLE). Chronic infections, such as Hepatitis C or bacterial endocarditis, can also prompt the immune system to produce RF, which complicates the interpretation.
Doctors commonly order additional laboratory tests to measure overall inflammation, such as the Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP). Another specific blood test, the anti-cyclic citrullinated peptide (anti-CCP) antibody test, is often used alongside RF because it is a more specific marker for RA.
How the Test is Performed
Performing the Rheumatoid Factor test involves a standard blood draw, known as venipuncture. A medical professional collects a small sample of blood, usually from a vein in the arm or the back of the hand. This collection process is quick and generally requires no special preparation from the patient beforehand, though patients should confirm any specific instructions with their doctor.
Once the blood is collected, the sample is sent to a laboratory for analysis. The lab measures the concentration of RF in the serum, which is the clear liquid portion of the blood, to determine the quantitative result.
Interpreting the Results and Clinical Context
The results of the Rheumatoid Factor test are typically reported in International Units per milliliter (IU/mL). While the specific numerical cut-off varies between laboratories, a result below the threshold is generally considered negative, suggesting a normal amount of RF is present. A result above the threshold is considered a positive or abnormal finding.
A positive result suggests an elevated level of autoimmune activity in the body. This finding does not automatically confirm a diagnosis of Rheumatoid Arthritis (RA) due to False Positives, where other non-RA conditions elevate RF levels. For example, chronic conditions like liver disease, specific viral infections such as mononucleosis or Hepatitis C, and the normal process of aging can cause an increase in RF.
The clinical context is important because up to 20% of healthy individuals, especially older adults, may have a low positive RF result without having any form of arthritis. The higher the concentration of RF, the more likely an underlying autoimmune condition is present, but this must be cross-referenced with other clinical data.
Conversely, a negative RF result does not entirely rule out RA, which is known as a False Negative. Between 20% and 40% of people who meet the criteria for RA are considered “seronegative” because their blood tests do not show elevated RF levels. In these cases, the diagnosis relies much more heavily on the presence of joint pain, swelling, and results from other inflammatory markers like CRP.