A 45-year-old woman presents with a 6-month history of joint pain and stiffness. She reports morning stiffness lasting more than an hour, improving throughout the day. On examination, there is symmetric swelling and tenderness of the metacarpophalangeal and proximal interphalangeal joints bilaterally, with decreased grip strength. Laboratory studies show elevated ESR and CRP. Which of the following laboratory findings is most specific for her condition?