Clinically, bloody diarrhoea is seen in nearly all patients with ulcerative colitis, but only half of those with Crohn's. An abdominal mass is common in Crohn's but rare in UC; perianal disease is common in Crohn's, but again rare in UC. There is never rectal sparing in UC.
Histologically, UC is a mucosal disease with distorted glands and no granulomata; Crohn's is transmural with normal glands and granulomata.
Radiologically, UC is a symmetrical, continuous disease causing shallow ulcers but very rarely strictures and never any fistulae. Crohn's is discontinuous with deep ulcers with common strictures and fistulae.
This said, a firm diagnosis is not made in 15-20% of patients.
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