Written by MicroDok

AIM: To detect the presence of rheumatoid factor autoantibody in the serum of patient’s as an aid in the diagnosis of rheumatoid arthritis.

PRINCIPLE:  The principle is based on the immunologic reaction between the rheumatoid factor autoantibody in patient’s serum with the corresponding IgG coated onto latex particles, resulting in visible agglutination.

MATERIAL/APPARATUS: Blood specimen, centrifuge machine, timer, test card, Pasteur’s pipette, rheumatoid factor latex reagent (a component of latex particles, commercially prepared IgG, & 0.1% sodium azide serving as a preservative).


  1. Bring the rheumatoid factor test reagent to room temperature (23oC – 29oC).
  2. Centrifuge the blood specimen at 400rpm for 5 minutes in the centrifuge machine in order to sediment the red cell component of the blood, and obtain the serum required for this investigation.
  3. Place a drop of the serum on a test card using the Pasteur’s pipette.
  4. Shake the vial containing the rheumatoid factor test reagent, and place a drop of it on to the serum on the test card.
  5. Mix the mixture together using a mixing stick or the paddle end of the Pasteur’s pipette.
  6. Rock/rotate the test card for about 3 minutes.
  7. Observe the test card macroscopically for any visible agglutination within the time interval.


Presence of agglutination indicates that the patient’s serum contains rheumatoid factor autoantibodies, and is thus reported as positive. Absence of agglutination is a negative result, and it shows that the patient’s serum does not contain rheumatoid factor autoantibodies.

Rheumatoid arthritis is a chronic systemic inflammatory autoimmune disease generally characterized by swelling and pain in the joints of our body. Its onset is characterized by the detection of rheumatoid factors in the serum of patient’s whole blood, who suffer from rheumatoid arthritis.

Autoantibody is an antibody produced by the body, and directed against normal body tissue. It results when autoimmunity sets in. Autoimmunity is the process by which the body mounts an immune response against a normal body component. This results to autoimmune diseases (e.g. rheumatoid arthritis) which are diseases that emanate when the body’s immune system fails to recognize and differentiate self from non – self (pathogens). Rheumatoid arthritis is organ specific, and it is most common in women and afflicts them between 40 & 60 years of age.

Rheumatoid factors are autoantibodies produced in the body of rheumatoid arthritis patients. It is an IgM class of antibody. It is reactive at the crystallizable fragment (Fc) region of IgG, and this forms the basis for this test. The rheumatoid factor autoantibody reacts with IgG to form IgM – IgG complex which deposits in the joints together with other mediators of the immune system e.g. plasma cell, neutrophils e.t.c. These complexes activate complement reaction leading to a type hypersensitivity reaction. This finally results to the chronic inflammation of the joints.

Serum (Sera – plural) is the yellowish/golden fluid that remains after blood coagulates, and the red cells have formed clot. It is plasma minus the clotting factor fibrinogen. Serum contains mainly antibodies produced during the pathologic state of a patient, and that is why serum is often required to perform most serological tests carried out in the clinical microbiology laboratory.

Plasma is the red blood cell free, fluid portion of blood which contains all the clotting factors of blood. It consist of H2O containing a large number of dissolved substances, including proteins, salts (Na, K, Chlorides, & bicarbonates), food materials (glucose, fats, amino acids), hormones, vitamins, & excretory materials. Plasma is obtained from a whole blood by placing/collecting the blood specimen in an anticoagulant bottle i.e. bottles that contains substances that prevent blood from clotting. Examples of anticoagulants are: ethylene diamine tetra – acetic acid and heparin. The above serological tests explained in this chapter can be performed using plasma, with the exception of rheumatoid factor test which can only be performed with serum because plasma contains fibrinogen (a blood clotting factor) which may cause non – specific agglutination of the latex particles contained in the rheumatoid factor test reagent. This gives a false positive result.

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