What is the differential diagnosis for a patient presenting with melena (dark blood in stools) and left lower quadrant pain?

Differential Diagnosis for Dark Blood in Stools and Left Lower Quadrant Pain

The most likely diagnosis is acute sigmoid diverticulitis, which accounts for the majority of left lower quadrant pain cases and can present with gastrointestinal bleeding, though you must systematically exclude perforated colon cancer, ischemic colitis, inflammatory bowel disease, and complicated diverticulitis with fistula formation. 1

Primary Differential Diagnoses

Most Common: Diverticular Disease

• Acute diverticulitis is the leading cause of left lower quadrant pain, affecting 5-25% of patients with diverticulosis, with prevalence increasing 50% between 2000-2007 1
• Dark blood (melena) suggests upper GI bleeding, but diverticular bleeding from sigmoid/descending colon can present with darker maroon stools if transit time is prolonged 1
• Approximately 70% of elderly patients have diverticulosis, making this the statistical frontrunner 1

Critical to Exclude: Perforated Colon Cancer

• Perforated adenocarcinoma can perfectly mimic diverticulitis clinically and radiographically 1, 2
• Key distinguishing feature: Pericolonic lymphadenopathy >1 cm in short axis strongly suggests malignancy over diverticulitis 1, 3
• Colon cancer is the most common cause of colonic obstruction, followed by diverticulitis 4
• Cancer arising within a diverticulum itself is rare but documented, making differentiation extremely difficult 2

Inflammatory/Infectious Colitis

• Ischemic colitis presents with left-sided abdominal pain and bloody stools, particularly in elderly patients with vascular risk factors 5, 6
• Inflammatory bowel disease (IBD) - Crohn's disease or ulcerative colitis can cause left lower quadrant pain with bloody diarrhea 1, 6
• Infectious colitis including C. difficile, CMV (especially in immunocompromised), or parasitic infections must be considered 5, 6
• Radiation colitis if prior pelvic radiation history 6, 4

Other Structural Causes

• Epiploic appendagitis - self-limited inflammation of colonic appendages, can mimic diverticulitis 1, 7
• Bowel obstruction from adhesions, hernia, or malignancy - inability to pass gas/stool is present in 90% of large bowel obstructions 8, 4
• Sigmoid volvulus - more common cause of obstruction after cancer and diverticulitis 4

Genitourinary Causes

• Pyelonephritis or urolithiasis - though typically without melena 1
• Colovesical fistula from complicated diverticulitis can cause urinary symptoms 5, 2

Rare but Important

• Psoas abscess 9
• Situs inversus with right-sided appendicitis presenting as left lower quadrant pain 9

Critical Red Flags Requiring Immediate Action

Emergent surgical consultation is mandatory if any of the following are present: 8

• Fever with inability to pass gas or stool (suggests perforation/obstruction)
• Severe abdominal tenderness with guarding or rebound (peritonitis)
• Signs of shock (hypotension, tachycardia, altered mental status)
• Free air on imaging (pneumoperitoneum)
• Toxic megacolon appearance

Diagnostic Algorithm

Step 1: Immediate CT Abdomen/Pelvis with IV Contrast

• This is the single most important test - rated 8/9 (usually appropriate) by ACR with 98% diagnostic accuracy 8
• Identifies diverticulitis, complications (abscess, perforation, fistula), alternative diagnoses, and guides treatment 8
• Specifically look for: pericolonic lymphadenopathy >1 cm (suggests cancer), extraluminal air, abscess formation, bowel wall thickening pattern 1, 3

Step 2: Laboratory Workup

• Complete blood count (leukocytosis, anemia from bleeding) 5
• Stool studies including C. difficile toxin, ova and parasites if travel history 5
• Lactate if sepsis concern 8

Step 3: Risk Stratification Based on CT Findings

If uncomplicated diverticulitis (no abscess/perforation/fistula):

• Outpatient management with oral antibiotics for 7 days maximum 8
• Conservative management without antibiotics acceptable in immunocompetent patients 8

If complicated diverticulitis:

• Small abscess (<3-4 cm): antibiotics alone for 7 days 8
• Large abscess (>3-4 cm): percutaneous drainage plus antibiotics for 4 days 8
• Perforation/peritonitis: emergent surgery 8

If pericolonic lymphadenopathy >1 cm or atypical features:

• Colonoscopy is mandatory to exclude malignancy, not routine screening 1, 3, 5

Step 4: Follow-up Colonoscopy Indications

Colonoscopy is NOT routinely needed after CT-confirmed diverticulitis EXCEPT: 1, 5

• Pericolonic lymphadenopathy >1 cm detected
• Abscess, perforation, or fistula present
• Age-appropriate cancer screening not completed
• Weight loss, anemia, or change in bowel habits

Common Pitfalls to Avoid

• Do not assume diverticulitis without imaging - clinical diagnosis alone has 34-68% misdiagnosis rate 8
• Do not miss colon cancer - it can present identically to diverticulitis; lymphadenopathy is the key differentiator 1, 2
• Do not order routine colonoscopy after uncomplicated diverticulitis - this increases costs without benefit unless specific indications present 1, 5
• Do not delay surgery if peritonitis or free perforation present - mortality increases with delayed intervention 1, 8
• Consider gynecologic causes in premenopausal women - obtain pelvic ultrasound if not already done 3, 8