Rheumatoid factor testing in Spanish primary care: A population-based cohort study including 4.8 million subjects and almost half a million measurements

Abstract

Objective: Rheumatoid factor (RF) testing is used in primary care in the diagnosis of rheumatoid arthritis (RA); however a positive RF may occur without RA. Incorrect use of RF testing may lead to increased costs and delayed diagnoses. The aim was to assess the performance of RF as a test for RA and to estimate the costs associated with its use in a primary care setting. Material and methods: A retrospective cohort study using the Information System for the Development of Research in Primary Care database (contains primary care records and laboratory results of >80% of the Catalonian population, Spain). Participants were patients ≥18 years with ≥1 RF test performed between 01/01/2006 and 31/12/2011, without a pre-existing diagnosis of RA. Outcome measures were an incident diagnosis of RA within 1 year of testing, and the cost of testing per case of RA. Results: 495,434/4,796,498 (10.3%) patients were tested at least once. 107,362 (21.7%) of those tested were sero-positive of which 2,768 (2.6%) were diagnosed with RA within 1 year as were 1,141/388,072 (0.3%) seronegative participants. The sensitivity of RF was 70.8% (95% CI 69.4 to 72.2), specificity 78.7% (78.6 to 78.8), and positive and negative predictive values 2.6% (2.5 to 2.7) and 99.7% (99.6 to 99.7) respectively. Approximately €3,963,472 was spent, with a cost of €1,432 per true positive case. Conclusions: Although 10% of patients were tested for RF, most did not have RA. Limiting testing to patients with a higher pre-test probability would significantly reduce the cost of testing. Ethical disclosures Protection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this study. Confidentiality of data. The authors declare that they have followed the protocols of their work center on the publication of patient data. Right to privacy and informed consent. The authors declare that no patient data appear in this article. *Responsabilidades eticas TITLE (in English): "Rheumatoid factor testing in Spanish primary care: a populationbased cohort study including 4.8 million subjects and almost half a million measurements." Title in Spanish: "Uso del factor reumatoide en atención primaria en España: estudio de cohortes poblacional incluyendo 4.8 millones de participantes y casi medio millón de mediciones."