PRINCIPLE OF THE METHOD
-The RF-latex is a slide agglutination test for the qualitative and semiquantitative detection of RF in human serum. Latex particles coated
with human gammaglobulin are agglutinated when mixed with
samples containing RF.
CLINICAL SIGNIFICANCE
Rheumatoid factors are a group of antibodies directed to determinants in
the Fc portion of the immunoglobulin G molecule. Although rheumatoid
factors are found in a number of rheumatoid disorders, such as systemic
lupus erythematosus (SLE) and Sjögren’s syndrome, as well as in
nonrheumatic conditions, its central role in clinic lies its utility as an aid in
the diagnosis of rheumatoid arthritis (RA).
An study of the “American College of Rheumatology” shows that the
80,4% of RA patients were RF positive.
STORAGE AND STABILITY
All the kit components are ready to use, and will remain stable until the
expiration date printed on the label, when stored tightly closed at 2-8ºC
and contaminations are prevented during their use. Do not freeze: frozen
reagents could change the functionality of the test.
Mix reagents gently before use.
Reagents deterioration: Presence of particles and turbidity.
ADDITIONAL EQUIPMENT
- Mechanical rotator with adjustable speed at 80-100 r.p.m.
- Vortex mixer.
- Pippetes 50 µL.
SAMPLES
Fresh serum. Stable 7 days at 2-8ºC or 3 months at –20ºC.
Samples with presence of fibrin should be centrifuged before testing. Do
not use highly haemolized or lipemic samples.
PROCEDURE
Qualitative method
1. Allow the reagents and samples to reach room temperature. The
sensitivity of the test may be reduced at low temperatures.
2. Place 50 µL of the sample and one drop of each Positive and
Negative controls into separate circles on the slide test.
3. Mix the RF-latex reagent rigorously or on a vortex mixer before
using and add one drop (50 µL) next to the sample to be tested.
4. Mix the drops with a stirrer, spreading them over the entire surface
of the circle. Use different stirrers for each sample.
5. Place the slide on a mechanical rotator at 80-100 r.p.m. for 2
minutes. False positive results could appear if the test is read later
than two minutes.
Semi-quantitative method
1. Make serial two fold dilutions of the sample in 9 g/L saline solution.
2. Proceed for each dilution as in the qualitative method.
READING AND INTERPRETATION
Examine macroscopically the presence or absence of visible
agglutination immediately after removing the slide from the rotator.
The presence of agglutination indicates a RF concentration equal or
greater than 8 IU/mL (Note 1).
The titer, in the semi-quantitative method, is defined as the highest
dilution showing a positive result.
CALCULATIONS
The approximate RF concentration in the patient sample is calculated
as follows:
8 x RF Titer = IU/mL
QUALITY CONTROL
Positive and Negative controls are recommended to monitor the
performance of test procedure, as well as a comparative pattern for a
better results interpretation.
All result different from the negative control result, will be considered
as a positive.
PERFORMANCE CHARACTERISTICS
1. Analytical sensitivity: 8 (6-16) IU/mL, under the described assay
conditions
2. Prozone effect: No prozone effect was detected up to 1500 IU/mL.
3. Diagnostic sensitivity: 100%.
4. Diagnostic specificity: 100%.
The diagnostic sensitivity and specificity have been obtained using 139
samples compared with the same method of a competitor.
INTERFERENCES
Bilirubin (20 mg/dL), hemoglobin (10 g/L), and lipids (10 g/L), do not
interfere. Other substances may interfere6.
LIMITATIONS OF PROCEDURE
- The incidence of false positive results is about 3-5 %. Individuals
suffering from infectious mononucleosis, hepatitis, syphilis as well as
elderly people may give positive results.
- Diagnosis should not be solely based on the results of latex method
but also should be complemented with a Waaler Rose test along
with the clinical examination.
NOTES
1. Results obtained with a latex method do not compare with those
obtained with Waaler Rose test. Differences in the results between
methods do not reflect differences in the ability to detect rheumatoid
factors.
