Becky is 28 years old and presents with a 4 month history of pain and swelling in her wrists and fingers of both hands. On examination there is mild bilateral synovitis of the metacarpophalangeal joints. You suspect she may have Rheumatoid Arthritis and refer her urgently to the rheumatology team. Whilst awaiting her referral you carry out some initial blood tests including a check for rheumatoid factor and explain to her the limitations of this test.
What proportion of patients with Rheumatoid Arthritis will be positive for rheumatoid factor?