A 48-year-old teacher presents with a 3-month history of pain and stiffness in her hands, particularly affecting the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints bilaterally. The stiffness is worst in the mornings, lasting for about 90 minutes, and improves with activity. On examination, there is symmetrical swelling and tenderness of the affected joints. What blood test result would most strongly support a diagnosis of Rheumatoid Arthritis?