What are the possible causes and treatments for dark sticky stool and left lower quadrant pain?

Dark Sticky Stool and Left Lower Quadrant Pain

Order CT abdomen and pelvis with IV contrast immediately—this presentation suggests gastrointestinal bleeding with possible diverticulitis, colitis, or malignancy, and CT has 98% diagnostic accuracy while identifying life-threatening complications. 1

Critical Initial Assessment

Dark sticky stool (melena) indicates upper or proximal gastrointestinal bleeding, but when combined with left lower quadrant pain, you must consider:

• Bleeding diverticulitis - inflammation with vessel erosion 2
• Colonic malignancy - particularly sigmoid colon cancer presenting with obstruction and bleeding 1
• Ischemic colitis - especially in elderly patients with atherosclerotic disease 2
• Inflammatory bowel disease - Crohn's disease or ulcerative colitis with active bleeding 3

Do not rely on clinical examination alone—misdiagnosis rates are 34-68% for left lower quadrant pathology. 1, 2

Immediate Diagnostic Workup

Laboratory Tests Required

• Complete blood count - assess for anemia from blood loss and leukocytosis suggesting infection 2
• C-reactive protein - CRP >170 mg/L predicts severe diverticulitis with 87.5% sensitivity 2
• Lactate level - elevated lactate indicates bowel ischemia or sepsis 2

Imaging Strategy

CT abdomen and pelvis with IV contrast is rated 8/9 (usually appropriate) by the American College of Radiology and should be ordered immediately. 1, 2

CT with IV contrast provides:

• Detection of bowel wall thickening, pericolonic fat stranding, and complications (abscess, perforation, fistula) 2
• Identification of active bleeding or vascular pathology 1
• Differentiation between diverticulitis and malignancy 1
• Assessment of alternative diagnoses in 49% of cases 4

Red Flags Requiring Emergency Surgery Consultation

Contact surgery immediately if any of these are present:

• Free air on CT indicating perforation 2
• Signs of peritonitis - diffuse tenderness, guarding, rebound, absent bowel sounds 2
• Hemodynamic instability - hypotension, tachycardia, signs of shock 2
• Complete bowel obstruction - inability to pass gas or stool with abdominal distension 2
• Fecal peritonitis 2

Management Based on CT Findings

If Uncomplicated Diverticulitis (No Abscess/Perforation)

• Immunocompetent patients: Conservative management WITHOUT antibiotics—clear liquid diet advancement and oral analgesics 2
• Immunocompromised or elderly patients: Antibiotics for maximum 7 days (Ertapenem 1g q24h or Eravacycline 1mg/kg q12h) 2

If Complicated Diverticulitis with Small Abscess (<4 cm)

• Antibiotics alone for 7 days without drainage 2

If Complicated Diverticulitis with Large Abscess (≥4 cm)

• Percutaneous drainage PLUS antibiotics for 4 days 2
• Stop antibiotics at 4 days if source control is adequate in immunocompetent patients 2

If Septic Shock Present

• Escalate to broad-spectrum carbapenem immediately: Meropenem 1g q6h by extended infusion, Doripenem 500mg q8h by extended infusion, or Imipenem/cilastatin 500mg q6h by extended infusion 2

Critical Pitfalls to Avoid

Do not perform colonoscopy acutely—colonic distention increases perforation risk in acute diverticulitis. 1

Watch for CT findings suggesting cancer rather than diverticulitis:

• Pericolonic lymphadenopathy >1 cm 2
• Luminal mass 2
• Asymmetric wall thickening without pericolonic inflammation 1

MRI may be more accurate than CT for differentiating cancer from diverticulitis (sensitivity 100% vs 67%, specificity 100% vs 93%), but is not appropriate for acute evaluation. 1

Follow-Up Colonoscopy Indications

Colonoscopy after acute episode is indicated ONLY for:

• Age-appropriate colon cancer screening not yet performed 2
• Abnormal pericolic lymph nodes on CT 2
• Luminal colon mass on CT 2
• Uncertain diagnosis (inflammatory bowel disease vs ischemic colitis) 1, 2
• Presence of abscess, perforation, or fistula 2

Routine colonoscopy after uncomplicated diverticulitis is NOT warranted. 1, 2

Special Population Considerations

For premenopausal women: If gynecologic pathology is suspected, order pelvic/transvaginal ultrasound first before CT. 1, 2 However, given the presence of melena (gastrointestinal bleeding), CT abdomen and pelvis remains the priority imaging.