A 42-year-old woman presents with a 3-month history of symmetrical polyarthritis affecting the small joints of both hands and wrists, morning stiffness lasting 2 hours, and constitutional symptoms. Physical examination reveals swollen, tender metacarpophalangeal and proximal interphalangeal joints. ESR is 68 mm/h and CRP is 12 mg/dL. Which investigation is most specific for confirming the diagnosis of rheumatoid arthritis?