This patient most likely has rheumatoid arthritis (RA), and testing for both anti–cyclic citrullinated peptide (CCP) antibodies and rheumatoid factor will be most helpful in confirming the diagnosis. RA is an autoimmune disorder that typically presents as a symmetric inflammatory polyarthritis affecting the proximal interphalangeal and metacarpophalangeal joints of the fingers, the wrists, and the analogous joints of the feet. Prolonged morning stiffness is common. Anti-CCP antibody testing has the greatest specificity (95%) for the diagnosis of RA. Although no single laboratory test will diagnose RA, the combination of a compatible clinical presentation and a positive rheumatoid factor and positive anti-CCP antibodies is more specific for the diagnosis than any other combination of tests. Approximately 75% of patients with RA are rheumatoid factor positive, but specificity is only around 80%. Rheumatoid factor positivity frequently occurs in other autoimmune disorders and chronic infections, most notably chronic active hepatitis C virus infection.

Testing for antinuclear antibodies (ANA) is usually performed in patients with suspected systemic lupus erythematosus (SLE). A new onset of polyarticular inflammatory arthritis as seen in this patient can be indicative of SLE; however, she has no other signs or symptoms suggestive of SLE such as alopecia, aphthous ulcers, malar rash, pericardial and pleural serositis, or cytopenias. Furthermore, an ANA test may be positive in 40% of patients with RA and would not distinguish between RA and SLE with as much specificity as the combination of anti-CCP antibodies and rheumatoid factor.

Although an elevated C-reactive protein may provide laboratory evidence of inflammation that can complement the physical examination findings of inflammatory synovitis, this inflammatory marker lacks diagnostic specificity and does not distinguish RA from other forms of inflammatory arthritis.