Judith is 44 years old and presents with a 4 month history of painful joints in her hands. She describes that her wrists and knuckles look swollen from time to time and that the stiffness in her hands is worst in the morning. There are no other affected joints or skin rashes. Past medical history includes Hypothyroidism and there is no family history of Arthritis. On examination she has mild synovitis of both wrists and you suspect she may have Rheumatoid Arthritis.
Which one of the following tests is most likely to be diagnostic?