What is the differential diagnosis for a patient presenting with left lower quadrant abdominal pain, nausea, vomiting, and bloody stools?

Differential Diagnosis for Left Lower Quadrant Pain with Nausea, Vomiting, and Bloody Stools

Acute diverticulitis is the most likely diagnosis in this presentation, but you must immediately obtain CT abdomen/pelvis with IV contrast to exclude perforated colon cancer, which can present identically and requires urgent surgical intervention. 1, 2

Primary Differential Diagnoses

Most Likely: Acute Diverticulitis

• Diverticulitis is the leading cause of left lower quadrant pain, affecting 5-25% of patients with diverticulosis, with approximately 70% of elderly patients having underlying diverticulosis. 1
• The combination of left lower quadrant pain, nausea/vomiting, and bloody stools fits the classic presentation, though the complete triad of left lower quadrant pain, fever, and leukocytosis occurs in only 25% of cases. 3, 2
• Dark or maroon bloody stools can occur with diverticular bleeding from the sigmoid/descending colon when transit time is prolonged. 1
• Clinical diagnosis alone has a 34-68% misdiagnosis rate, making imaging mandatory. 1, 3

Critical Alternative: Perforated Colon Cancer

• Perforated adenocarcinoma mimics diverticulitis perfectly on both clinical and radiographic grounds. 1, 2
• The key distinguishing CT finding is pericolonic lymphadenopathy >1 cm in short axis, which strongly suggests malignancy over diverticulitis. 1, 2
• Colorectal cancer risk is 7.9% in complicated diverticulitis versus 1.3% in uncomplicated cases. 2

Other Considerations

• Inflammatory bowel disease (Crohn's disease or ulcerative colitis) can present with left lower quadrant pain, bloody stools, and systemic symptoms, though typically has a more chronic course. 4
• Ischemic colitis presents with sudden left-sided abdominal pain and bloody diarrhea, particularly in elderly patients with vascular risk factors. 5
• Infectious colitis (including C. difficile, cytomegalovirus, or tuberculosis in endemic areas) should be considered, especially in immunocompromised patients. 4

Immediate Diagnostic Approach

Mandatory Imaging

Order CT abdomen and pelvis with IV contrast immediately—this is the single most important test with 98% diagnostic accuracy. 1, 3, 2

Specific CT findings to identify:

• Pericolonic lymphadenopathy >1 cm = suspect cancer, not diverticulitis. 1, 2
• Bowel wall thickening with pericolonic fat stranding = diverticulitis. 3
• Extraluminal air or free air = perforation requiring emergency surgery. 1, 3
• Abscess size and location = determines drainage strategy (<4 cm vs ≥4 cm). 3

Initial Laboratory Tests

• Complete blood count (elevated WBC suggests infection). 2, 6
• C-reactive protein >50 mg/L combined with left lower quadrant tenderness and absence of vomiting has 97% accuracy for diverticulitis (though only present in 24% of patients). 3
• Basic metabolic panel and urinalysis to exclude urinary tract pathology. 6

Red Flags Requiring Emergency Surgical Consultation

Obtain emergent surgical consultation if any of the following are present: 1, 3

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

Management Algorithm Based on CT Findings

Uncomplicated Diverticulitis (No Abscess, Perforation, or Fistula)

• Conservative management without antibiotics is appropriate for immunocompetent patients. 3, 2
• Antibiotics (Ertapenem 1g q24h or Eravacycline 1mg/kg q12h) for maximum 7 days only in immunocompromised or elderly patients. 3
• Outpatient management with clear liquid diet advancement and oral analgesics. 3

Complicated Diverticulitis with Small Abscess (<4 cm)

• Antibiotic therapy alone for 7 days without drainage. 3

Complicated Diverticulitis with Large Abscess (≥4 cm)

• Percutaneous drainage combined with antibiotics for 4 days. 3
• Stop antibiotics at 4 days if source control is adequate in immunocompetent, non-critically ill patients. 3

Perforation or Peritonitis

• Emergent surgery is mandatory. 3
• If septic shock is present, use Meropenem 1g q6h by extended infusion, Doripenem 500mg q8h by extended infusion, or Imipenem/cilastatin 500mg q6h by extended infusion. 3

Mandatory Follow-Up

Colonoscopy is mandatory 6-8 weeks after resolution of symptoms to exclude malignancy, particularly if: 2

• Pericolonic lymphadenopathy >1 cm was present on CT
• This was a first episode of diverticulitis
• Complicated diverticulitis occurred (abscess, perforation, or fistula)
• No high-quality colonoscopy was performed within the past year

Critical Pitfalls to Avoid

• Never assume diverticulitis without CT imaging—clinical diagnosis alone is wrong 34-68% of the time. 1, 3
• Never miss colon cancer—lymphadenopathy >1 cm is the key differentiator. 1, 2
• Never continue antibiotics beyond 7 days in uncomplicated cases—this increases resistance without improving outcomes. 3
• Never perform colonoscopy during acute diverticulitis—wait 6-8 weeks for complete resolution. 2