What is the appropriate diagnosis and treatment for an adult patient with no significant past medical history presenting with left lower quadrant pain and nausea?

Left Lower Quadrant Pain and Nausea: Diagnostic and Treatment Approach

Order CT abdomen and pelvis with IV contrast immediately as the first-line diagnostic test for this patient, as it has 98% diagnostic accuracy and is rated 8/9 (usually appropriate) by the American College of Radiology. 1, 2

Initial Diagnostic Strategy

The most likely diagnosis is acute diverticulitis, which is the most common cause of left lower quadrant pain in adults. 1 However, clinical diagnosis alone is unreliable with misdiagnosis rates between 34-68%, making imaging essential. 1, 2

Key Clinical Features to Assess

• Classic triad: Left lower quadrant pain, fever, and leukocytosis—but this is present in only 25% of diverticulitis cases 1, 2
• High-accuracy criteria: When left lower quadrant tenderness, CRP >50 mg/L, and absence of vomiting are all present, diagnostic accuracy reaches 97%, though this occurs in only 24% of patients 2
• Red flag symptoms requiring immediate emergency evaluation: 2
  • Fever with inability to pass gas or stool
  • Severe abdominal tenderness with guarding or rebound
  • Vomiting (especially if persistent)
  • Bloody stools
  • Signs of shock (hypotension, tachycardia, altered mental status)
  • Progressively worsening pain over several days

Why CT with IV Contrast is Essential

CT abdomen and pelvis with IV contrast provides: 1, 2

• Detection of complications (abscess, perforation, fistula, obstruction) with near 100% sensitivity
• Identification of alternative diagnoses in up to 49% of cases 1
• Guidance for treatment decisions (medical vs. interventional vs. surgical)
• Risk stratification that reduces unnecessary hospital admissions by >50% 2

IV contrast specifically improves: 1

• Characterization of bowel wall pathology
• Detection of pericolic abnormalities
• Identification of vascular pathology
• Visualization of intra-abdominal fluid collections

Alternative Imaging Considerations

For premenopausal women with suspected gynecologic pathology (ectopic pregnancy, pelvic inflammatory disease, ovarian torsion), order pelvic/transvaginal ultrasound as the initial test instead. 1, 2 Gynecologic and non-gynecologic pathology can present identically in this population.

If IV contrast is contraindicated, CT without contrast is acceptable (rated 6/9) but less accurate for detecting abscesses. 2 Unenhanced CT has similar accuracy to contrast-enhanced CT for general abdominal pain evaluation in some studies, but this is suboptimal for diverticulitis complications. 1

Ultrasound with graded compression has variable sensitivity (77-98%) and is highly operator-dependent and limited by body habitus. 1 It may reduce unnecessary CT examinations but is not standard in the United States for non-gynecologic left lower quadrant pain. 1

Treatment Algorithm Based on CT Findings

Uncomplicated Diverticulitis (No abscess, perforation, or fistula)

For immunocompetent patients: 2

• Conservative management WITHOUT antibiotics is the recommended approach
• Clear liquid diet with gradual advancement as tolerated
• Oral analgesics for pain control
• Outpatient management is appropriate if patient can tolerate oral intake

For immunocompromised or elderly patients: 2

• Antibiotics for maximum 7 days
• Recommended regimens: Ertapenem 1 g q24h OR Eravacycline 1 mg/kg q12h 2

Complicated Diverticulitis with Small Abscess (<3-4 cm)

• Antibiotic therapy alone for 7 days without drainage 2
• Same antibiotic regimens as above 2

Complicated Diverticulitis with Large Abscess (≥3-4 cm)

• Percutaneous drainage PLUS antibiotics for 4 days in immunocompetent, non-critically ill patients 2
• Stop antibiotics at 4 days if source control is adequate 2
• Continue up to 7 days in immunocompromised or critically ill patients, guided by clinical response and inflammatory markers 2

Perforation with Peritonitis or Septic Shock

Immediate interventions required: 2

• Emergency surgical consultation for operative management
• Broad-spectrum carbapenem therapy: Meropenem 1 g q6h by extended infusion, Doripenem 500 mg q8h by extended infusion, OR Imipenem/cilastatin 500 mg q6h by extended infusion 2
• NPO status, IV fluid resuscitation, nasogastric decompression 3

Critical Pitfalls to Avoid

Do not order plain radiography as the initial test—it has extremely limited diagnostic value and will not change management. 1 It can only detect large volume extraluminal air or bowel obstruction, both of which CT detects with far greater sensitivity. 1

Do not order contrast enema as the initial test—it increases risk of colonic perforation in acute diverticulitis and does not show extraluminal abnormalities like abscesses. 1

Do not order MRI for initial evaluation—it is less sensitive for extraluminal air and urinary calculi, more time-consuming, and more subject to motion artifacts in symptomatic patients. 1

Watch for CT findings suggesting cancer rather than diverticulitis: 2

• Pericolonic lymphadenopathy >1 cm with or without pericolonic edema
• Luminal colon mass
• These findings warrant colonoscopy even after acute episode resolves

Do not routinely order colonoscopy after CT-confirmed uncomplicated diverticulitis, except for: 2

• Age-appropriate colon cancer screening not yet performed
• Abnormal pericolic lymph nodes on CT
• Luminal colon mass on CT
• Uncertain diagnosis
• Presence of abscess, perforation, or fistula

Differential Diagnosis Beyond Diverticulitis

CT will also identify these alternative causes of left lower quadrant pain: 1, 2, 4

• Colitis (infectious, inflammatory bowel disease, ischemic)
• Epiploic appendagitis
• Bowel obstruction
• Incarcerated hernia
• Urolithiasis or pyelonephritis
• Ovarian/fallopian tube pathology (in women)
• Perforated colon cancer
• Abdominal wall pathology (rectus sheath hematoma)

In rare cases, appendicitis with intestinal malrotation can present with left-sided pain, which CT will readily identify. 5