In patients with SLE and coexisting knee osteoarthritis, how useful is musculoskeletal ultrasound particularly power Doppler in distinguishing an SLE inflammatory arthritis flare from an OA flare when serologies and inflammatory markers may not be helpful?

Unfortunately you are correct. Ultrasound alone is not able to distinguish between an effusion or synovitis due to the patient's OA or SLE. You would have to assess clinically for other evidence of an SLE exacerbation, and/or aspirate and analyze the fluid (which may be easier using ultrasound guida...