What is the differential diagnosis and initial management of bloody stool in a patient with a history of inflammatory bowel disease or diverticulitis?

Differential Diagnosis of Bloody Stool in Patients with IBD or Diverticulitis History

Immediate Assessment Priority

In patients with a history of IBD or diverticulitis presenting with bloody stool, immediately exclude infectious colitis (particularly C. difficile), IBD flare, ischemic colitis, and diverticular bleeding through targeted history, examination, and faecal calprotectin testing, with urgent gastroenterology referral for faecal calprotectin >250 μg/g. 1, 2

Key Clinical Features to Differentiate Causes

IBD Flare vs. Other Causes

• Rectal bleeding with abdominal pain, change in bowel habit, weight loss, or iron deficiency anemia strongly suggests IBD flare and warrants urgent gastroenterology referral 1, 2
• Faecal calprotectin >250 μg/g indicates active mucosal inflammation requiring urgent colonoscopy 1
• Faecal calprotectin 100-250 μg/g suggests repeat testing or routine gastroenterology referral 1
• Faecal calprotectin <100 μg/g makes IBD flare unlikely 1, 3

Infectious Colitis

• Recent antibiotic use (within 2 months) mandates immediate C. difficile PCR or toxin assay, as IBD patients have increased risk for C. difficile-associated colectomy 2
• Fever with bloody diarrhea and systemic toxicity suggests bacterial dysentery (Salmonella, Shigella, Campylobacter, enterohemorrhagic E. coli) requiring stool cultures 2
• Obtain stool multiplex antimicrobial testing before diagnosing IBD flare 2, 4

Diverticular Bleeding

• Painless, large-volume bright red rectal bleeding in a patient with known diverticulosis suggests diverticular hemorrhage 1
• Hemodynamic instability despite resuscitation requires immediate surgery 1
• Diverticular bleeding typically presents without abdominal pain, unlike diverticulitis 1

Ischemic Colitis

• Older patients with vascular risk factors presenting with bloody diarrhea and abdominal pain suggest ischemic colitis 2, 4
• Normal rectum with sharply defined segments involving "watershed territory" (sigmoid to splenic flexure) on colonoscopy indicates ischemia 2
• Rapid resolution on serial examinations supports ischemic colitis over IBD 2

Segmental Colitis Associated with Diverticulosis (SCAD)

• Localized sigmoid inflammation in patients with diverticulosis, with rectal sparing suggests SCAD rather than IBD 5, 6
• SCAD has a more benign outcome than IBD with low complication rates 6
• Most patients respond to 5-aminosalicylate therapy 5

Diagnostic Algorithm

Step 1: Initial Laboratory Assessment

Obtain in all patients: 1, 2, 3

• Full blood count (assess anemia, leukocytosis)
• CRP (elevated suggests active inflammation)
• Urea & electrolytes, renal function (assess dehydration, complications)
• Albumin (low suggests severe inflammation or malnutrition)
• Faecal calprotectin (unless NSAID use in past 6 weeks)
• Stool culture and C. difficile testing (exclude infection)

Step 2: Risk Stratification for Complicated Disease

Predictors of progression to complicated disease include: 1

• Symptoms lasting >5 days
• Vomiting
• Systemic comorbidity
• CRP >140 mg/L
• CT findings of pericolic extraluminal air, fluid collection, or longer inflamed colon segment

Step 3: Endoscopic Evaluation

• For acute severe colitis presentation: Perform unprepared flexible sigmoidoscopy (NOT colonoscopy due to perforation risk) with biopsies from at least one site 1, 2
• For stable patients with suspected IBD flare: Perform complete ileocolonoscopy with biopsies from at least five sites, taking at least two biopsies per site, even from normal-appearing mucosa 4
• For hemodynamically stable patients with bleeding: Perform sigmoidoscopy and esophagogastroduodenoscopy to localize bleeding source 1, 2

Step 4: Imaging When Indicated

• CT abdomen/pelvis with IV contrast for hemodynamically stable patients with ongoing bleeding to localize source 1
• CT angiography for patients with ongoing bleeding after resuscitation 1
• Avoid colonoscopy in acute severe colitis due to perforation risk 1, 2

Management Based on Diagnosis

IBD Flare

• Exclude infectious cause first before escalating immunosuppression 1
• Refer for further investigation if faecal calprotectin elevated 1
• Consider admission if systemically unwell or suspected acute severe colitis 1

Diverticular Bleeding

• Immediate surgery for hemorrhagic shock non-responsive to resuscitation 1
• Subtotal colectomy with ileostomy for acute severe ulcerative colitis with refractory hemorrhage 1
• Outpatient management acceptable for uncomplicated diverticulitis in immunocompetent patients without systemic inflammatory response 1

Infectious Colitis

• Supportive care with hydration 2
• Avoid antimotility agents in bloody diarrhea due to toxic megacolon risk 2
• Treat C. difficile if identified 2

Critical Pitfalls to Avoid

• Do NOT delay infectious workup before diagnosing IBD flare – always exclude C. difficile and bacterial pathogens first 1, 2, 4
• Do NOT perform colonoscopy in acute severe colitis – use flexible sigmoidoscopy only to avoid perforation 1, 2
• Do NOT assume negative stool culture excludes infection – cultures are positive in only 40-60% of infectious colitis cases 2
• Do NOT rely solely on CRP to rule out IBD activity – false negatives occur with proximal/ileal disease 2
• Do NOT use antimotility agents (loperamide) in bloody diarrhea – risk of toxic megacolon 2
• Do NOT delay surgery in hemodynamically unstable patients with bleeding – immediate surgical exploration is mandatory 1