A 65-year-old woman presents with intermittent pain in small joints of both hands for 2 years, without swelling, accompanied by morning stiffness lasting about 20 minutes. No other joints are involved. Urinalysis (UA) and RBC count are normal, erythrocyte sedimentation rate (ESR) is normal, and rheumatoid factor (RF) is negative. Which of the following ancillary tests would be most helpful for diagnosis?