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RF Assay Methods — SCE Rheumatology MCQ

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ModerateAutoimmune SerologyRF Assay MethodsSCE Rheumatology

A 55-year-old man with RA on Methotrexate develops a rash. His rheumatology nurse measures his RF by nephelometry at 85 IU/mL. Previously it was 45 IU/mL measured by latex agglutination. Can RF values be compared between different methods?

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Correct answer: D — No RF results from different assay methods are not directly comparable; the method should be documented and trends followed using the same assay

RF can be measured by several methods including latex agglutination nephelometry ELISA and Waaler-Rose. Each method has different sensitivities specificities and reference ranges. Results from different methods are NOT directly comparable. For clinical monitoring the same assay method should be used consistently to allow meaningful trend assessment. RF is an IgM antibody directed against the Fc portion of IgG. It is present in approximately 70-80% of RA patients but is also found in other conditions (Sjogren syndrome infections hepatitis C cryoglobulinaemia) and in up to 5% of healthy individuals (increasing with age).

Reference: ACR/EULAR 2010 RA criteria; Nishimura K et al 2007 RF meta-analysis