rheumatoid factor test and principle

overview

RF is an  autoantibody, an IgM (immunoglobulin M) protein that is produced by the body‘s   immune system. Autoantibodies usually attack the patient‘s own tissues, mistakenly identifying   them as ―foreign.‖ While the role of RF is not well understood, it is not thought to directly cause   joint damage but to promote the body's inflammation reaction, which in turn contributes to   autoimmune tissue destruction.

 Not all people with rheumatoid arthritis have detectable rheumatoid factor. Those who do notare said to be "seronegative".

 Why It Is Done

 The rheumatoid factor (RF) test is primarily used to help diagnose  rheumatoid arthritis (RA) andto distinguish it from other forms of arthritis and other conditi ons that cause similar symptoms ofjoint pain, inflammation, and stiffness as gout.

Sample

Only serum sample

Note:

• The specimens may be stored at 2-8ºC for 48 hours. For longer periods of time
  freeze serum at or below –20ºC
• Do not use haemolysed, lipaemic , contaminated serum specimens or  high lipimic
  samples which may cause a falsely high rheumatoid factor (RF) result.

Procedure

Principle

The earliest tests, and those still most widely used clinically rely on the agglutinating properties   of the IgM  class of RF. IgG, usually human or rabbit, is bound to a particulate carrier, and the   presence of RF is then detected by agglutination or flocculation of the respective indicator   system. Carrier particles frequently used include latex, and erythrocytes.

Semi-quantitative analysis

to determine the antibody content of a serum involves  serial  dilutions of the serum and
determination of an end point (the last dilution at which agglutination can be visualized). The
reciprocal of this dilution is known as "antibody titer".

latex agglutanation test

Qualitative assay

• Positive: The reagent will agglutinate (clump) in the presence of RF.
• Negative: No agglutination generally indicates absence of RF

Semi-quantitative assay 

The titer is expressed as the highest dilution showing agglutination

Normal levels

Normal levels of RF in adults are < 8 mg/l

Normally about 5% to 10% of people over age 65 have an elevated RF level.

Interpretation 

• In patients with symptoms and clinical signs of  rheumatoid arthritis, the presence of
  significant concentrations of RF indicates that it is likely that they have RA.
• High concentrations of RF are often found in people with active cases of rheumatoid arthritis,
  and it correlates with the severity of signs or symptoms  (Prognosis worse with high titer at
  onset). 
• RF Incidence increases with duration of disease
• • Three months: 33% 
  • Six months: 45% 
  • One year: 75% 
  • Eighteen months: 90% 

Note:  A negative RF test does not rule out RA. About 20% of patients with RA will be
persistently negative for RF and/or may have very low levels of RF

Note:  The RF test is sensitive but nonspecific. It is most closely associated with
rheumatoid arthritis (RA) but may also be present in a variety of other conditions such as:

• Autoimmune disease
• • Sjogren's Syndrome (75 to 95%)
  • Systemic Lupus Erythematosus (15 to 35%)
  • Scleroderma (20 to 30%)
  
  
• Infections
  
  • Osteomyelitis
  • Tuberculosis
  • Syphilis
  • parasitic diseases (e.g., malaria, schistosomiasis, filariasis)
  • Hepatitis (Acute and Chronic) and hepatic cirrhosis (10% to 40%)
  • Leukemia, and multiple myeloma