Rheumatoid arthritis is a clinical syndrome, and the diagnosis requires the presence of pain, swelling, and tenderness in the joints. In the absence of these features, identification of rheumatoid factor in the serum is of little use. Because rheumatoid factor is an immune complex, it is a marker of immune activation and, therefore, may be present in the circulation of persons with a variety of inflammatory conditions that stimulate the immune system. Also, 5% of the healthy population have significant titers of rheumatoid factor in their serum.