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Rheumatoid factor measured by fluoroimmunoassay: a responsive measure of rheumatoid arthritis disease activity that is associated with joint damage

E Knijff-Dutmer¹, W Drossaers-Bakker¹, A Verhoeven², G van der Sluijs Veer³, M Boers⁴, S van der Linden² and M van de Laar¹

¹ Rheumatology Twente, Medisch Spectrum Twente, Enschede, The Netherlands
² Rheumatology Department, University Hospital Maastricht, The Netherlands
³ Laboratory for Clinical Chemistry, Medisch Spectrum Twente, Enschede, The Netherlands
⁴ Department of Clinical Epidemiology and Biostatistics, Vrije Universiteit Medical Centre, Amsterdam, The Netherlands

Correspondence to:
Correspondence to:
Dr E A J Knijff-Dutmer, Rheumatology Twente, Department for Rheumatology, Medisch Spectrum Twente, PO Box 50.000, 7500 KA Enschede, The Netherlands;
eajknijff{at}hotmail.com

Objectives: To determine whether rheumatoid factors (RFs), measured as continuous variables by time resolved fluoroimmunoassay, reflect disease activity in rheumatoid arthritis (RA). Further, to study the association of RFs and other disease activity parameters with radiological joint damage, especially in individual patients.

Methods: In active, early RA, IgM and IgA RFs, as well as erythrocyte sedimentation rate (ESR), C reactive protein (CRP), tender joint score, and swollen joint score were assessed regularly. At the study start and at 56 and 80 weeks, radiographs of hands and feet were assessed by the Sharp score (van der Heijde modification). Associations between RFs and disease activity parameters were studied. In addition, associations between radiographic damage and disease activity parameters (baseline and time integrated) were analysed by non-parametric tests and multiple regression analysis. The relation between time integrated disease activity parameters and radiological damage in individual patients was analysed and visualised.

Results: 155 patients were included. RF levels were strongly associated with the disease activity parameters (especially ESR and CRP) and with each other. All disease activity parameters, at baseline as well as time integrated parameters, were associated with (the progression of) radiographic damage. Moreover, in individual patients, a linear relationship between time integrated disease activity parameters and progression of radiological damage was seen.

Conclusion: RFs, measured as continuous variables, can be considered as disease activity parameters in patients with RA. The level of RF at baseline and the exposure to RF over time is associated with radiological damage. In individual patients, there is a constant relation between disease activity and radiological damage.

Keywords: rheumatoid factor; disease activity; outcome; fluoroimmunoassay

Abbreviations: AUC, area under the curve; CRP, C reactive protein; ELISA, enzyme linked immunosorbent assay; ESR, erythrocyte sedimentation rate; RA, rheumatoid arthritis; RF, rheumatoid factor; SHS, Sharp score, modification van der Heijde; SJC, swollen joint count; TJC, tender joint count

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