What lab finding is used to confirm the diagnosis of rheumatoid arthritis (RA)?

The correct answer is the presence of anti-cyclic citrullinated peptide antibodies (anti-CCP), which is a significant laboratory finding used to confirm the diagnosis of rheumatoid arthritis (RA). Anti-CCP antibodies are highly specific for RA and their presence in the blood can indicate a higher likelihood of the disease, even in the early stages. These antibodies target proteins modified by citrullination, which is a process that occurs in chronic inflammatory conditions. Their specificity for RA makes them crucial for diagnostic confirmation, especially when considered alongside clinical symptoms.

Other laboratory tests like rheumatoid factor (RF) and anti-nuclear antibodies (ANA) may be present in patients with RA but are not as specific. RF can be found in various autoimmune diseases and in some healthy individuals, while ANA positivity is often associated with systemic lupus erythematosus and other autoimmune conditions rather than being specific for RA. A complete blood count (CBC) is a standard test that can indicate inflammation and other issues but does not confirm a diagnosis of rheumatoid arthritis on its own. Thus, the presence of anti-CCP antibodies stands out as the definitive confirmatory test for RA diagnosis.