Rheumatoid arthritis (RA) is characterized by the presence of various circulating autoantibodies, including rheumatoid factor (RF), anti-keratin antibodies (AKA). RF is present in 70-90% of patients with RA and is included in the ACR (American College of Rheumatology) classification criteria. According to these criteria, if RF is positive in patients with arthritis of three or more joints then the patient has rheumatoid arthritis. Arthritis of less than three joints with RF negative results excludes rheumatoid arthritis. The measurement of RF is important in the diagnosis, prognosis and management of rheumatoid arthritis as high titers of RF occur in sera of patients who tend to develop extra-articular complications. ELISA can detect RF of various immunoglobulin isotypes and offer improved specificity, sensitivity and reliability over existing agglutination methods.

Anti-CCP antibody levels have proven to provide strong diagnostic and prognostic value in the diagnosis of RA. The anti-CCP ELISA exhibits superior sensitivity and specificity over other laboratory tests. Anti-CCP ELISA demonstrates 82% sensitivity for RA and can be detected in 40% of RA patients that test negative for Rheumatoid Factor (RF). Clinical studies that include non-RA patients and healthy individuals demonstrate 97% and 99% specificities, respectively. Elevated levels have also been correlated to joint involvement and radiological damage in early RA as well as a positive predictive value for the development of RA in up to 93% of undifferentiated arthritis cases.

Antibodies to Keratin (AKA) are found to be highly specific for rheumatoid arthritis as detected by indirect immunofluorescence (IFA) on rat esophagus substrate. They occur in approximately 40% of patients with RA. AKA are present in approximately 33% of RA patients who are RF negative.

AKA on rat esophagus