RA -Factor


                                    Kit used: QUIMICA CLINICA APLICADA S.A.


1.     Objectives:

In most patients suffering from e chronic polyarthritis, the presence of macro globulins (Rheumatoid Factors), which are able to agglutinate inert particles sensitized with human gamma-globulin are detected. The test of the latex agglutination allows distinguishing between this disease and the other one known as articular rheumatism or rheumatic fever, in which Rheumatoid Factors are not present.



2.     Principle:


The RF Direct Latex reagent is a suspension of polystyrene particles sensitized with human gamma-globulin. When the reagent is faced against the Rheumatoid Factors of serum, an antigen-antibody reaction takes place being easily visualized because of the latex agglutination.


3.     Materials:


Kit (Ref. 99 45 21) for 50 tests. Contents:

 A. 1 x 2.0 ml Latex reagent Ref. 99 10 75

B. 1 x 0.5 ml Positive control Ref. 99 61 81

 C. 1 x 0.5 ml Negative control Ref. 99 59 24

D. Slide and disposable stirrers.


Latex reagent:      Suspension of polystyrene latex particles coated with human gamma-Globulin

Positive control:   Pool of human sera known to have a RF titer higher than 20 IU/ml

Negative control: Pool of human sera known to have a RF titer lower than 20 IU/m


4.     Precautions:  

Human sera used in controls have been found negative in the reaction with HBsAg and HIV I/II. However they should be handled with care. All reagents are preserved with 0.09% sodium azide.

Waste products must be handled as per local regulations.

Storage and stability of Test reagents:

The components of the kit, when stored at 2 - 8ºC, will remain stable until the expiration date stated

 on the label.  Do not freeze. Discard those controls in which, in spite of containing sodium azide, a microbial growth is apparent.



Fresh serum or stored at 2 - 8ºC for no longer than 48 h. It is necessary to freeze the sample when the assay is to be carried out after that period of time.  Discard contaminated or hemolyzed sera.


5.     Procedure:   

Qualitative assay:

1. Bring reagents and serum samples to room temperature.
2. Place 40 μl of undiluted serum onto a slide’s black area.

3. Mix well the Latex reagent and add one drop over the serum drop.

4. Mix with the aid of a stirrer both drops and tilt the slide.

5. Observe the presence or absence of agglutination within a period not longer than 3 min.


Semi quantitative procedure :

Prepare serial two-fold dilutions in physiological saline (Na Cl 0.9%) and then proceeded as described for undiluted serum with each diluted sample.

The approximate Rheumatoid Factor level in a serum sample can be calculated by the following formula:

 RF (IU/ml)= Highest dilution with positive reaction x Reagent sensitivity (20 IU/ml)


Normal values : Not clearly established. However, the existence of significantly high titers (above 30

                             IU/ml) in 70% of cases of rheumatoid arthritis have been found.




6.     Result & Interpretation


Result : Appearance of agglutination within 3 minutes : Positive for macro globulins

                                                                                                       (Rheumatoid Factors),


Interpretation of results : Latex agglutination will mean that the serum Rheumatoid Factor level is

                                            higher than 20 IU/ml


7.     Clinical Significance:

Performance Characteristics Sensitivity: The reagent is designed to agglutinate in the presence of levels of RF higher than 20 IU/ml.


Standardized against WHO international standard for RF Specificity: The reagent agglutinates only in the presence of Rheumatoid factors


Interferences: There is no interference by ASO until 1000 IU/ml or by CRP until 448 mg/L

There is no prozone phenomenon for titers ≤ 1129 IU/ml

Lipemic and highly hemolyzed sera as well as plasma interfere with the assay.


Limitations of the procedure:  The reagent is for screening of Rheumatoid factors in serum samples. In some cases (20 - 30%) samples from patients suffering from Rheumatoid Arthritis are negative Occasional agglutinations produced after 4 min have no diagnostic significance.


8.     References:

Ø  Waaler, E. (1940), Acad. Path. Microbiol. Scan. 17, 172–187.

Ø  Singer, J.M, Plotz, C.M., (1965), Am. J. Mad., 21, 888-892

Ø  Edelman, G.M., Kunkel, H.G., Franklin, E.C., (1958), J. Exp. Med., 108 , 105-120.

Ø  Singer, J.M. (1961), Am. J. Med. , 31, 766-779.

Ø  Jones, W.L., Wiggins, G.L. (1973), Am. J. Clin. Pathol., 60, 703-706

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