Which is seen more commonly in rheumatoid arthritis than in ankylosing spondylitis?

An autoantibody against IgG (usually IgM) describes the rheumatoid factor (RF), which is more likely to be seen in rheumatoid arthritis (RA) than in ankylosing spondylitis (AS). Ankylosing spondylitis (AS) is a sero­negative (RF negative) spondyloarthropathy that is either seen alone or in association with other disorders such as Reiter syndrome, psoriatic spondylitis, intesti­nal infections, and inflammatory bowel disease. AS most commonly involves young men between 15 and 30 years of age who are HLA-B27 positive (95% of cases). Approximately 20% of patients who are HLA­B27 positive will develop AS or one of the other variants upon exposure to the environmental factors listed above. Characteristic features include an insidi­ous onset of sacroiliitis, persistence for more than 3 months, and morning stiffness in the lower back that is relieved with exercise, as well as a diminished anterior flexion of the spine, which can be identified with the Schoeber test. The vertebral column is eventually involved with subsequent fusion of the vertebral col­umn and production of the "bamboo spine." Compli­cations include (1) restricted chest movement with development of a restrictive type of lung disease; (2) iridocyclitis, with a potential for visual loss; and (3) aortic valve incompetence with regurgitation in a small percentage of cases. Indomethacin is the treatment of choice.