What is the differential diagnosis and initial management for a patient presenting with abdominal pain?

Differential Diagnosis for Abdominal Pain

For acute nonlocalized abdominal pain, CT of the abdomen and pelvis with IV contrast is the optimal initial imaging choice, particularly when fever is present or serious pathology is suspected. 1, 2

Initial Clinical Assessment

Vital Signs and Physical Examination

• Tachycardia is the most sensitive early warning sign of surgical complications and should trigger urgent investigation even before other symptoms develop 2
• The combination of fever, tachycardia, and tachypnea predicts serious complications including anastomotic leak, perforation, or sepsis 2
• Assess for peritoneal signs (rebound tenderness, rigid abdomen) which indicate intestinal ischemia or perforation 2
• Severe pain out of proportion to physical examination findings should raise immediate suspicion for acute mesenteric ischemia 2
• Asymmetric gaseous distention with emptiness of the left iliac fossa is pathognomonic for sigmoid volvulus 2

Critical History Elements

• Document pain location, as this guides both differential diagnosis and imaging selection 2
• Ask specifically about last bowel movement and passage of gas (85% sensitivity, 78% specificity for adhesive small bowel obstruction in patients with prior abdominal surgery) 2
• Colicky pain indicates bowel obstruction as the bowel attempts to overcome occlusion 2
• The triad of abdominal pain, constipation, and vomiting suggests sigmoid volvulus 2
• Any prior laparotomy makes adhesive obstruction the leading diagnosis (55-75% of small bowel obstructions) 2

Mandatory Laboratory Testing

Initial Laboratory Panel

• Beta-hCG testing is mandatory in all women of reproductive age before any imaging 2, 3
• Complete blood count to evaluate for leukocytosis suggesting infection or inflammation 2, 3
• Comprehensive metabolic panel including liver function tests 3
• Serum lipase (more specific than amylase for pancreatitis) 3
• Urinalysis to evaluate for urinary tract infection or nephrolithiasis 3

Advanced Laboratory Markers

• High C-reactive protein has superior sensitivity and specificity compared to white blood cell count for ruling in surgical disease 2
• Elevated lactate suggests ischemia or sepsis, though normal levels do not exclude internal herniation or early ischemia 2
• Blood cultures should be obtained if sepsis is suspected 3
• D-dimer and lactate levels if mesenteric ischemia is suspected 2

Differential Diagnosis by Frequency

Most Common Causes (Emergency Department Presentations)

• Non-specific abdominal pain: 24-44.3% of cases 4
• Acute appendicitis: 15.9-28.1% of surgical cases 1, 2, 4
• Acute cholecystitis: 2.9-9.7% 4
• Small bowel obstruction: 15% of acute abdominal pain admissions 2, 4
• Pancreatitis 1
• Renal colic 1
• Diverticulitis 1, 4
• Perforated peptic ulcer 1

Age-Specific Considerations

• Elderly patients have higher likelihood of malignancy, diverticulitis, and vascular causes 2
• In elderly patients with fever and abdominal pain, many laboratory tests may be normal despite serious infection 1
• Acute appendicitis represents two-thirds of surgical interventions in children with acute abdomen 4

Special Population Red Flags

• Post-bariatric surgery patients often present with atypical symptoms, and classic peritoneal signs are frequently absent 2
• Internal herniation should be considered even with normal lactate in post-bariatric patients 2
• Neutropenic patients present a diagnostic challenge as typical signs of abdominal sepsis may be masked, with high mortality rates 1

Imaging Algorithm by Clinical Presentation

Nonlocalized Abdominal Pain with Fever

• CT abdomen and pelvis with IV contrast is the preferred imaging option due to broad differential and need for fast imaging 1, 2
• Imaging is especially helpful in elderly patients where laboratory tests are nonspecific 1
• Radiography has limited role with low sensitivity for sources of abdominal pain and fever 1

Pain Location-Specific Imaging

• Right lower quadrant pain: Abdominal ultrasound is the most appropriate initial imaging 2
• Pelvic pain: CT abdomen and pelvis with contrast 2
• Right upper quadrant pain: Ultrasound for suspected cholecystitis 2, 3
• Left lower quadrant pain: CT abdomen and pelvis with contrast for suspected diverticulitis 2
• Suspected kidney stones: Non-contrast CT abdomen and pelvis 2

Condition-Specific Imaging

• Bowel obstruction: CT abdomen and pelvis with contrast 2
• Mesenteric ischemia: CT angiography of the abdomen 2
• Acute appendicitis: CT abdomen and pelvis with contrast (consider ultrasound first to minimize radiation in young patients) 2

Critical Pitfalls to Avoid

• The absence of peritonitis on examination does not exclude bowel ischemia—patients with sigmoid volvulus often lack peritoneal signs despite established ischemia due to chronic distension masking the examination 2
• Classic peritoneal signs are often absent in post-bariatric surgery patients 2
• Normal lactate does not exclude internal herniation or early ischemia 2
• Normal laboratory tests in elderly patients do not exclude serious infection 1
• Failing to obtain pregnancy testing before imaging can delay diagnosis of ectopic pregnancy 3
• Conventional radiography has limited diagnostic value and should not be routinely ordered 3

Management Principles

When to Consider Urgent Surgical Consultation

• Signs of peritonitis (rigid abdomen, rebound tenderness) 2
• Hemodynamic instability suggesting bleeding or sepsis 2
• Severe pain out of proportion to physical findings (mesenteric ischemia) 2
• Abdominal distension with vomiting (bowel obstruction) 2
• Fever with abdominal pain suggesting abscess or perforation 2

Diagnostic Laparoscopy Indications

• Patients without specific diagnosis after appropriate imaging 4
• ICU patients with acute abdomen or sepsis of unknown origin 4
• Suspicion of acute cholecystitis, diffuse gut hypoperfusion, or mesenteric ischemia 4
• Refractory lactic acidosis, especially after cardiac surgery 4