A positive anticitrullinated protein antibody indicates what likely diagnosis in a patient with joint pain in her proximal interphalangeal joints?

A positive anticitrullinated protein antibody (ACPA) test is highly suggestive of rheumatoid arthritis (RA), especially in a patient presenting with joint pain in the proximal interphalangeal joints. The presence of ACPAs is known to be specific to RA and can often be detected even before the clinical symptoms appear. These antibodies are believed to play a role in the pathogenesis of the disease and aid in both diagnosis and prognostication.

Joint pain in the proximal interphalangeal joints is characteristic of RA, as the disease typically affects symmetric small joints, particularly in the hands. The early diagnosis and treatment of RA are crucial because they can significantly affect the long-term outcome and prevent joint destruction.

In contrast, conditions like gout primarily involve monosodium urate crystal deposition and do not correlate with the presence of ACPAs. Polymyalgia rheumatica involves symptoms like shoulder and hip stiffness and is not associated with ACPAs. Systemic lupus erythematosus, while it can present with joint symptoms, is associated with a different set of antibodies and does not typically present with positive ACPAs. Therefore, the presence of a positive anticitrullinated protein antibody alongside joint pain in these specific joints strongly points towards rheumatoid arthritis as the likely