Septic arthritis in patients with pre-existing inflammatory arthritis

The case: A 56-year-old woman presents to her family physician with a 7-day history of tenderness, swelling and redness of 2 metacarpophalangeal joints and 1 proximal interphalangeal joint in her right hand. She also reports pain and redness of her right knee that makes it difficult to walk. On examination, her right knee appears swollen and red. The patient has a 10-year history of rheumatoid arthritis with intermittent polyarticular flares, primarily involving her wrists, fingers ( metacarpophalangeal and proximal interphalangeal joints), feet ( metatarsophalangeal joints) and occasionally her knees. However, the condition has been well managed with methotrexate and hydroxychloroquine therapy. She had no flares in the year before presentation and is able to perform household chores and to work as a dry-cleaning attendant. What other history, physical examination and laboratory investigations would you seek to distinguish a simple flare of inflammatory polyarthritis from a superimposed infection?