A 40-year-old woman presents with pain and swelling in her hands, shoulders,

and feet for the last 2 months. She also has morning stiffness that lasts for about an hour and malaise with night sweats. She denies any hair loss, oral ulcers, or dry eyes. She also denies dry mouth, rash, fever, or discoloration of hands when exposed to cold. She has no history of psoriasis or inflammatory bowel disease.

On physical examination, she has swelling with tenderness in the metacarpophalangeal, proximal phalangeal, and metatarsophalangeal joints. Laboratory tests reveal a hemoglobin of 10.0 g/dL, MCV of 82 fL, and a platelet count of 450,000/μL. Which of the following tests is most specific for establishing the diagnosis?