PRINCIPLE OF THE METHOD
The RF-latex is a slide agglutination test for the qualitative and semi-quantitative 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.
PRECAUTIONS
Components from human origin havebeen tested and found to be negative for the presence
of HBsAg, HCV, and antibody to HIV (1/2). However handle cautiously as potentially
infectious.
CALIBRATION
The RF-latex sensitivity is calibrated against the RF International Standard from NIBSC 64/002.
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
Allow the reagents and samples to reach room temperature. The sensitivity of the test
may be reduced at low temperatures.
Place 50 µL of the sample and one drop of each Positive and Negative controls into
separate circles on the slide test.
Mixthe RF-latex reagent rigorously or on a vortex mixerbefore using and add one drop
(50 µL) next to the sample to be tested.
Mix the drops with a stirrer, spreading them over the entire surface of the circle. Use
different stirrers for each sample.
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
Make serial two fold dilutions of the sample in 9 g/L saline solution.
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 concentrationin 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 comparativepattern for a better results interpretation.
All result different from the negative control result, will be considered as a positive.
REFERENCE VALUES
Up to 8 IU/mL. Each laboratory should establish its own reference range.
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 1500IU/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 witha 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 detectrheumatoid factors.
