1. In whites, the prevalence of either ulcerative colitis or Crohn's disease is between 50 and 100 in 100,000.
2. Peripheral arthritis occurs in 10% to 25% of patients and is slightly more common in Crohn's disease. Men and women are equally affected, and the peak age of onset is between 25 and 44 years. There is no association with HLA-B27.
3. Axial involvement is more frequent in Crohn's disease (5% to 22%) than in ulcerative colitis (2% to 6%). There is no male predominance, as there is in ankylosing spondylitis, and only 50% to 70% of patients are positive for HLA-B27. In fact, the presence of ankylosing spondylitis in patients negative for HLA-B27 should predict inflammatory bowel disease or psoriatic arthritis.
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