What is the most appropriate initial step in evaluating a 37‑year‑old man with left lower‑quadrant abdominal pain?

Initial Evaluation of Left Lower Quadrant Pain in a 37-Year-Old Male

Order CT abdomen and pelvis with IV contrast immediately—this is the single most accurate diagnostic test with 98% accuracy and is rated 8/9 (usually appropriate) by the American College of Radiology. 1

Why CT with IV Contrast is the Correct First Step

CT with IV contrast should be your default imaging choice because it simultaneously:

• Confirms or excludes diverticulitis (the most common cause of LLQ pain in adults) with near-perfect accuracy 1, 2
• Detects life-threatening complications including abscess, perforation, fistula, and bowel obstruction that determine whether the patient requires admission, drainage, or surgery 1
• Identifies alternative diagnoses in approximately 49% of patients, preventing missed diagnoses of conditions like pyelonephritis, nephrolithiasis, inflammatory bowel disease, perforated colon cancer, or epiploic appendagitis 1, 2
• Reduces unnecessary hospital admissions by more than 50% through accurate risk stratification 1

Clinical examination alone misclassifies acute diverticulitis in 34-68% of cases, making imaging essential rather than optional. 1

Essential Laboratory Tests Before Imaging

Order these labs immediately alongside the CT:

• Complete blood count with differential – Leukocytosis with left shift ≥75% neutrophils suggests bacterial infection, though the classic triad (LLQ pain + fever + leukocytosis) is present in only 25% of diverticulitis cases 3
• C-reactive protein (CRP) – CRP >170 mg/L predicts severe diverticulitis with 87.5% sensitivity and 91.1% specificity; when CRP >50 mg/L is combined with LLQ tenderness and absence of vomiting, diagnostic accuracy reaches 97% 3
• Urinalysis – Rules out urinary tract infection or nephrolithiasis as alternative causes 3
• Lactate level – If the patient appears systemically ill, elevated lactate signals sepsis and metabolic derangement requiring aggressive resuscitation 3

Critical Red Flags Requiring Emergency Department Evaluation

Send the patient to the ED immediately if any of these are present:

• Peritoneal signs (guarding, rebound tenderness, rigidity, or absent bowel sounds) indicating possible perforation 1
• Fever combined with LLQ pain and elevated WBC—the classic diverticulitis triad 1
• Inability to pass gas or stool with abdominal distension (present in 90% of large bowel obstructions) 1
• Vomiting, bloody stools, or signs of shock 1
• Progressively worsening pain over several days 1

Treatment Algorithm Based on CT Findings

Uncomplicated Diverticulitis (No Abscess/Perforation/Fistula)

• Immunocompetent patients: Conservative management WITHOUT antibiotics—antibiotics do not improve outcomes and promote resistance 1
• Immunocompromised or elderly patients: Antibiotics for maximum 7 days (Ertapenem 1g q24h or Eravacycline 1mg/kg q12h) 1
• Outpatient management requires ability to tolerate oral intake, no peritoneal signs, and reliable follow-up 1

Complicated Diverticulitis with Small Abscess (<4 cm)

• Antibiotics alone for 7 days without drainage 1

Complicated Diverticulitis with Large Abscess (≥4 cm)

• Percutaneous CT-guided drainage PLUS antibiotics for 4 days 1
• Stop antibiotics at 4 days if source control is adequate in immunocompetent, non-critically ill patients 1

Perforation with Diffuse Peritonitis

• Emergency surgical consultation for primary resection with or without diverting stoma (stable patients) or Hartmann's procedure (critically ill patients) 1

Septic Shock

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

Special Considerations for This 37-Year-Old Male

While diverticulitis is the most common cause of LLQ pain in adults and its prevalence increased 50% between 2000-2007, it typically affects older patients. 2 In a 37-year-old, maintain a broader differential including:

• Inflammatory bowel disease (Crohn's disease or ulcerative colitis) 1
• Epiploic appendagitis 1
• Nephrolithiasis or pyelonephritis 1
• Rarely, left-sided appendicitis with situs inversus totalis (though extremely uncommon) 4, 5

Critical Pitfalls to Avoid

• Do not rely on clinical examination alone—misdiagnosis rates are 34-68% without imaging 1
• Do not prescribe empiric antibiotics in immunocompetent patients with uncomplicated diverticulitis—this increases resistance without benefit 1
• Do not skip imaging in mild cases—complications like abscess or perforation can be clinically silent initially 1
• Normal inflammatory markers do not exclude serious disease—only 25% of diverticulitis patients present with the classic triad 3
• Watch for CT findings suggesting cancer rather than diverticulitis: pericolonic lymphadenopathy >1 cm with or without pericolonic edema requires colonoscopy 1

When Repeat Imaging is Needed

Order repeat CT abdomen/pelvis with IV contrast if:

• Symptoms persist beyond 2-3 days despite appropriate conservative management 1
• Fever and leukocytosis persist beyond 48-72 hours despite treatment 1
• Clinical deterioration occurs at any point 1