The Cyclic Citrullinated Peptide IgG Ab. test is used to help diagnose rheumatoid arthritis (RA) and differentiate it from other types of arthritis. This test detects the presence of antibodies directed against cyclic citrullinated peptides, which are produced in the joints of people with RA. The test is considered highly specific for RA, meaning that a positive result is very likely to indicate the presence of the disease. It can also be useful in identifying early-stage RA, before symptoms become more severe. Early diagnosis and treatment of RA can help prevent joint damage and improve long-term outcomes.