Rheumatoid factor (RA Latex)
(Immunology)
Investigation : Rheumatoid factor (RA Latex)
Spec container : Serum Gel 
Volume required : 5ml
Reference range : Negative
Turnaround : 14 days

Methodology: Latex agglutination. IgM-RF. Follow-on enzyme immunoassay test is reflexed for quantitation if the screen is positive.

Indications: Due to the low specificity of the test for rheumatoid arthritis, it is best reserved for specialist use. Therefore, its routine use as a screening test is not advised. If an inflammatory arthritis is suspected, patients should be referred for specialist opinion regardless of test results (NG100, July 2018). If there is joint discomfort without inflammation, a rheumatoid factor rarely contributes usefully to investigation.

Reference range: Negative: <3.5 IU/mL | Equivocal: 3.5-5 IU/mL | Positive: >5.0 IU/mL

Clinical interpretation: Rheumatoid factor can be seen in multiple clinical situations (see table below); also sometimes seen when there is no obvious clinical correlate. Therefore, must be interpreted with caution

Sample requirements/patient preparation: No specific requirements

Retesting interval: Repeat testing is not normally recommended.

References: Nice guidelines, NG100, July 2018; www.choosingwisely.co.uk (accessed 17/7/19)

Prevalence of RF (by latex agglutination) in different diseases
Disease
Approximate prevalence
RA80% (high titre)
 
Sjogrenís syndrome 70% (high titre)
Mixed cryoglobulinaemia 70% (high titre)
Systemic lupus erythematosus 30%
Mixed connective tissue disease 25%
Polymyositis 20%
Systemic sclerosis 20%
Juvenile chronic arthritis 15%
 
Subacute bacterial endocarditis 40%
Infectious hepatitis 25%
EBV, CMV infections 20%
Leprosy 25%
Tuberculosis 15%
Trypanosomiasis 15%
Syphilis 10%
Sarcoidosis 10%
 
Waldenstomís macroglobulinaemia 30%
Liver cirrhosis 25%
Pulmonary interstitial disease 5%
 
Healthy controls <5%
Elderly (>70yrs) 15%
Reference: Manual of Biological Markers of Disease; W. J. van Venrooij & R. N. Maini


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