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)