ISSN 2398-2942      

Rheumatoid factor


Michael Day

Helen Milner

Synonym(s): RF


  • Rheumatoid factor (RF) is an autoantibody (usually of the IgM class, less often IgG or IgA) specific for 'self' immunoglobulin (IgG Blood biochemistry: gamma globulin ).
  • The target immunoglobulin is considered to be bound in turn to an antigen, and this interaction of antigen and antibody creates a structural (conformational) change in the IgG that permits binding of the RF.
  • The identity of the target antigen/s is presently unknown.


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  • Two tests are recommended for detection of serum or synovial fluid rheumatoid factor in veterinary species.
  • The most commonly used test is the Rose Waller Assay:
    • In this test, a washed suspension of sheep or canine erythrocytes are preincubated with an antiserum specific for sheep or dog erythrocytes.
    • The antiserum must be used at a sub-agglutinating dose, so that the red cells are coated with antibody but do not agglutinate.
    • This preparation of antibody-coated red blood cells (RBC) is then incubated with serum or synovial fluid from the patient.
    • The test samples are serially diluted, usually in a microtiter system and a standard volume of antibody-coated RBC suspension is added to each dilution.
    • The antibody-RBC complex mimics the antibody-antigen complex to which RF binds in vivo.
    • If RF is present in the serum or synovial fluid sample it will bind to the anti-RBC antibody and cause cross-linking of these molecules, and thus agglutination of the red cells.
    • This agglutination is scored visually, and the last well in the microtiter plate in which agglutination has occurred is recorded in order to calculate the titer of the reaction.
    • A duplicate titration of the sample is incubated with a suspension of RBC that has not been pre-incubated with anti-RBC antibody as a negative control.
  • The second method used for detection of RF is ELISA Enzyme linked immunosorbent assay (ELISA) :
    • In this assay, a purified immunoglobulin is coated to the plastic wells of the ELISA microplate.
    • Serum or synovial fluid from the patient is titrated into replicate wells, and if RF is present it will bind to the immunoglobulin.
    • Presumptively, the process of immunoglobulin adhering to the plastic surface of the ELISA plate is sufficient to alter the conformation of the molecule to permit RF binding.
    • Any bound RF is subsequently detected using an antiserum, eg specific for canine IgM, that is conjugated to an enzyme (typically alkaline phosphatase Blood biochemistry: alkaline phosphatase (ALP) ).
    • The appropriate substrate is added and the ensuing color change is recorded spectrophotometrically.


  • In addition to the internal control for RF binding (non-coated RBC), the RF assay will include a negative and positive control serum of known RF content.
  • The positive control serum will produce an agglutination reaction with antibody-coated RBC, but not uncoated RBC.
  • In the ELISA method, the positive control serum may be used to generate a standard curve, the slope of which can be compared to those obtained with test sera for calculation of the result.


  • The RF assay will usually only be available from specialist Clinical Immunology diagnostic laboratories.



  • The ELISA is a more sensitive test than the Rose Waller Assay, however the ELISA is generally used as a research tool rather than a routine diagnostic test.


  • RF is a non-specific autoantibody.
  • It may be present in clinically normal animals, or in animals with non-autoimmune disease.
  • In most cases these will be relatively low-titered RFs.

Result Data

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Further Reading


Refereed papers

  • Recent references from VetMedResource and PubMed.
  • Bell S C, Carter S D, May C & Bennett D (1993) IgA and IgM rheumatoid factors in canine rheumatoid arthritis. JSAP 34, 259-264.
  • Chabanne L, Fournel C, Faure J R et al (1993) IgM and IgA rheumatoid factors in canine polyarthritis. Vet Immunol Immunopathol 39, 365-379.
  • Nielsen O L (1992) Detection of IgM rheumatoid factor in canine serum using a standardized enzyme-linked immunosorbent assay. Vet Immunol Immunopathol 34,139-47.
  • Halliwell R E et al (1989) Incidence and characterisation of canine rheumatoid factorVet Immunol Immunopathol 21, 161-75.
  • Bennett D & Kirkham D (1987) The laboratory identification of serum rheumatoid factor in the dogJ Comp Pathol 97, 541-550.

Other sources of information

  • Day M J (1999) Clinical Immunology of the Dog and Cat. Manson Publishing, UK.


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