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Immunology

Rheumatoid Factor

Rheumatoid Factor is the original serological marker for Rheumatoid Arthritis (RA), a chronic, systemic autoimmune disease that primarily attacks the synovium (the lining of the joints), leading to painful inflammation, joint destruction, and disability. The discovery of RF was a landmark event that proved RA was an autoimmune disease, a condition of immunological “friendly fire.”

The Target: What Exactly IS Rheumatoid Factor?

This is the most important concept to master. Rheumatoid Factor is an autoantibody—an antibody that mistakenly targets one of the body’s own proteins. But what makes RF unique is that its target is another antibody

  • Definition: Rheumatoid Factor is an antibody (most commonly of the IgM class) that is directed against the Fc portion of the patient’s own IgG molecules
  • Analogy: Imagine your IgG antibodies are soldiers. The Fc portion is the “handle” or base of the Y-shaped antibody molecule. RF is like a second type of soldier (an IgM) that comes along and, instead of fighting the enemy, starts grabbing the handles of its own IgG soldiers, tying them up and rendering them useless, or worse, turning them into a problem
  • Isotypes: While the classic RF is an IgM autoantibody (and the one most commonly measured), RF can also exist as IgG or IgA isotypes. These are less frequently tested for but can also be clinically significant

Pathophysiology: How RF Causes Damage

The presence of RF is not just a marker; it actively contributes to the disease process in the joints. This is a classic example of a Type III Hypersensitivity reaction

  1. Immune Complex Formation In the joint space, the IgM Rheumatoid Factor binds to the patient’s normal IgG, forming large, cage-like immune complexes
  2. Complement Activation These immune complexes are potent activators of the classical complement pathway. The resulting cascade generates inflammatory byproducts (like C3a and C5a)
  3. Inflammation C3a and C5a are powerful chemoattractants that call in a swarm of neutrophils and other inflammatory cells into the joint
  4. Tissue Damage These frustrated neutrophils release their destructive lysosomal enzymes and reactive oxygen species, causing significant damage to the cartilage and bone of the joint, leading to the characteristic pain, swelling, and erosion seen in RA

Laboratory Detection Methods: From Clumps to Light Scatter

We have several ways to detect RF in the lab, ranging from classic manual methods to modern automated immunoassays

1. Latex Agglutination (Classic Screening Method)

  • The Principle: This is a rapid, qualitative or semi-quantitative slide test. The reagent consists of microscopic latex beads that have been coated with purified human IgG (this is the antigen)
  • The Procedure: A drop of the patient’s serum is mixed with a drop of the latex reagent on a slide
  • The Result
    • Positive: If the patient’s serum contains IgM Rheumatoid Factor, the large, pentameric IgM molecules will bind to the IgG on multiple latex beads, cross-linking them and causing them to clump together in a process called agglutination. This is visible to the naked eye
    • Negative: If no RF is present, the mixture remains smooth and milky
  • Quantitation: By performing serial dilutions of the patient’s serum, we can determine a titer, which is the reciprocal of the last dilution to show visible agglutination

2. Nephelometry and Turbidimetry (Modern Quantitative Methods)

  • The Principle: These are automated methods performed on chemistry or immunology analyzers. Instead of using latex beads, the patient’s serum is mixed with a reagent of soluble, purified human IgG
  • The Procedure: When RF in the serum binds to the IgG antigen, it forms insoluble immune complexes that make the solution cloudy. A beam of light is passed through the cuvette
    • Nephelometry: measures the amount of light that is scattered at an angle by the immune complexes
    • Turbidimetry: measures the amount of light that is blocked (or the decrease in light that passes directly through) by the immune complexes
  • The Result: The amount of light scatter or blockage is directly proportional to the concentration of RF in the sample. The result is reported as a quantitative value in International Units per milliliter (IU/mL), with a specific cut-off for positivity (e.g., >14 IU/mL). This is far more objective and reproducible than a manual agglutination test

3. Enzyme-Linked Immunosorbent Assay (ELISA)

  • The Principle: The wells of a microtiter plate are coated with IgG. Patient serum is added, and any RF present binds to the coated IgG. A secondary, enzyme-labeled antibody is then used to detect the bound RF, producing a color change that is read by a spectrophotometer
  • The Advantage: ELISA is highly sensitive and is the best method for specifically detecting the different isotypes of RF (e.g., IgA-RF or IgG-RF) if needed

Interpretation and Clinical Utility: The Crucial Caveats

This is the most important section for an MLS. A positive RF test does not equal a diagnosis of Rheumatoid Arthritis

  • Sensitivity: The test is not perfectly sensitive. About 20-30% of patients with clinically confirmed RA are RF-negative. This is known as “seronegative rheumatoid arthritis.” Therefore, a negative RF test cannot rule out RA

  • Specificity: The test is notoriously not specific. A positive RF result can be found in a variety of other conditions, including:

    • Other Autoimmune Diseases: Especially Sjögren’s Syndrome (where it is found in >75% of patients), SLE, and Scleroderma
    • Chronic Infections: Hepatitis C, tuberculosis, and endocarditis can all stimulate RF production
    • Malignancies.
    • Healthy Elderly Population: A low-titer positive RF can be found in up to 15% of healthy older individuals

The Modern Partner: Anti-CCP, The More Specific Test

Because of the poor specificity of Rheumatoid Factor, a second, far more specific autoantibody test is now considered essential for the diagnosis of RA

  • Anti-Cyclic Citrullinated Peptide (Anti-CCP) Antibody: This autoantibody is highly specific for Rheumatoid Arthritis (specificity >95%). A positive Anti-CCP test is a very strong indicator of RA

Today, rheumatologists almost always order both RF and Anti-CCP tests together. A patient who is positive for both has a very high probability of having RA, and it often predicts a more aggressive course of disease