What is the initial approach to a patient presenting with acute abdominal pain?

Differential Diagnosis for Acute Abdominal Pain

Initial Systematic Approach

Begin with a structured evaluation using pain location as the primary guide, combined with vital signs assessment, targeted laboratory testing, and location-specific imaging to rapidly identify life-threatening conditions.

Immediate Clinical Assessment

Vital Signs and Red Flags

• Assess for hemodynamic instability: Tachycardia is the most sensitive early warning sign of surgical complications and should trigger urgent investigation, even before other symptoms develop 1
• Evaluate for shock indicators: The combination of fever, tachycardia, and tachypnea predicts serious complications including anastomotic leak, perforation, or sepsis 1
• Recognize atypical presentations: Classic peritoneal signs may be absent, particularly in obese or elderly patients, making tachycardia and vital sign abnormalities critical 1, 2
• Identify pain-physical examination mismatch: Severe pain out of proportion to physical findings strongly suggests mesenteric ischemia requiring immediate vascular imaging 2

Physical Examination Findings

• Assess for peritonitis: Rigid abdomen, rebound tenderness, and guarding indicate perforation or ischemia requiring urgent surgical evaluation 2
• Evaluate abdominal distension with vomiting: This combination strongly suggests bowel obstruction 2
• Check Murphy's sign: Pain with right upper quadrant palpation during inspiration suggests acute cholecystitis 2

Mandatory Laboratory Evaluation

Core Laboratory Panel

• Complete blood count: Leukocytosis suggests infection or inflammation, though normal values do not exclude serious pathology 2, 3
• Comprehensive metabolic panel: Including liver function tests (ALT, AST, alkaline phosphatase, bilirubin), renal function, and electrolytes 2, 3
• C-reactive protein: High CRP has superior sensitivity and specificity compared to white blood cell count for ruling in surgical disease, though normal CRP does not exclude complications 1
• Serum lactate: Elevated lactate suggests ischemia or sepsis, but normal levels do not exclude internal herniation or early ischemia 1, 2
• Serum lipase: More specific than amylase for pancreatitis diagnosis 3
• Urinalysis: To evaluate for urinary tract infection or nephrolithiasis 3
• Beta-hCG: Mandatory in all women of childbearing age before any imaging to rule out ectopic pregnancy 2, 3

Additional Testing Based on Clinical Suspicion

• Procalcitonin: Helpful for assessing inflammatory response in suspected sepsis 1, 3
• Blood cultures: If fever present with suspected sepsis 3
• D-dimer: Consider if mesenteric ischemia suspected, though lacks specificity 2, 3

Location-Based Differential Diagnosis and Imaging

Right Upper Quadrant Pain

• Primary differential: Acute cholecystitis, biliary colic, hepatitis, peptic ulcer disease 2, 4
• Initial imaging: Abdominal ultrasound is the most appropriate first-line study 2, 4
• Laboratory focus: Liver function tests and hepatobiliary markers 3

Right Lower Quadrant Pain

• Primary differential: Acute appendicitis (most common surgical cause at 15.9-28.1%), cecal diverticulitis, ovarian pathology, ectopic pregnancy 1, 4, 5
• Initial imaging: Abdominal ultrasound is recommended as first-line, with CT abdomen/pelvis with contrast if ultrasound non-diagnostic 2
• Alternative approach: CT abdomen/pelvis with contrast directly if high clinical suspicion 2

Left Lower Quadrant Pain

• Primary differential: Diverticulitis, sigmoid volvulus, ovarian pathology 2
• Initial imaging: CT abdomen/pelvis with contrast 2

Pelvic Pain

• Primary differential: Ectopic pregnancy, ovarian torsion, pelvic inflammatory disease, appendicitis 2
• Initial imaging: CT abdomen/pelvis with contrast after negative pregnancy test 2

Epigastric Pain

• Primary differential: Peptic ulcer disease, pancreatitis, gastritis, GERD, early appendicitis 2
• Initial imaging: Consider upper GI series if gastritis or peptic ulcer suspected; CT if pancreatitis or perforation suspected 2

Non-Specific or Diffuse Abdominal Pain

• Primary differential: Non-specific abdominal pain (most common at 30-41%), bowel obstruction, mesenteric ischemia, peritonitis 1, 6, 5
• Initial imaging: CT abdomen/pelvis with contrast, especially if fever or serious illness suspected 2

Condition-Specific Considerations

Bowel Obstruction (15% of acute abdominal pain admissions)

• Small bowel: Adhesions (55-75% of cases), hernias, neoplasms account for 90% 1
• Large bowel: Cancer (60%), volvulus and diverticular disease (30%) 1
• Imaging: CT abdomen/pelvis with contrast 2
• Clinical clues: Constipation and abdominal distension strongly suggest obstruction 4

Mesenteric Ischemia

• Imaging: CT angiography of the abdomen 2
• Key clinical feature: Pain out of proportion to examination findings 2
• Laboratory: Lactate and D-dimer may be helpful but lack specificity 2, 3

Acute Appendicitis (8-30% of cases; 15.9-28.1% requiring surgery)

• Imaging: CT abdomen/pelvis with contrast is most accurate 2
• Alternative: Ultrasound first in young patients to minimize radiation, followed by CT if non-diagnostic 2
• Note: Anorexia has little predictive value despite traditional teaching 4

Acute Cholecystitis

• Imaging: Abdominal ultrasound is first-line 2, 4

Nephrolithiasis

• Imaging: Non-contrast CT abdomen/pelvis 2

Critical Pitfalls to Avoid

• Never skip pregnancy testing in women of childbearing age before imaging, as this delays diagnosis of ectopic pregnancy 3
• Do not rely on normal CRP or lactate alone to exclude surgical pathology 1
• Avoid conventional radiography as routine initial imaging—it has limited diagnostic value except possibly in bowel obstruction, where CT remains superior 3, 7
• Do not withhold appropriate analgesia while awaiting diagnosis—early pain control does not impair diagnostic accuracy 5
• Recognize that elderly patients may present atypically with normal laboratory values despite serious pathology 2
• Consider diagnostic laparoscopy when clinical suspicion remains high despite negative imaging, particularly in ICU patients with sepsis of unknown origin 5

Special Population Considerations

Women of Childbearing Age

• Always consider gynecologic causes: ectopic pregnancy, ovarian torsion, pelvic inflammatory disease 2
• Mandatory beta-hCG before imaging 2, 3

Elderly Patients

• Symptoms may be atypical with normal laboratory values 2
• Higher likelihood of malignancy, diverticulitis, and vascular causes 1

Post-Bariatric Surgery Patients

• Tachycardia is the most critical warning sign 1
• Classic peritoneal signs often absent 1
• Consider internal herniation even with normal lactate 1

Immunocompromised Patients

• Require more extensive workup 3
• May lack typical inflammatory responses 3

Imaging Strategy to Minimize Radiation

• Use ultrasound first when appropriate (right upper quadrant pain, right lower quadrant pain in young patients, pelvic pain in women) 2
• Reserve CT for non-diagnostic ultrasound or when high clinical suspicion warrants immediate definitive imaging 2, 7
• Consider MRI as alternative in patients with contraindications to CT or significant radiation concerns, though data remain limited 7