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

Differential Diagnosis of Abdominal Pain

Approach by Pain Location

The differential diagnosis for abdominal pain should be systematically organized by anatomic location, with right upper quadrant pain suggesting cholecystitis or hepatobiliary pathology, right lower quadrant pain indicating appendicitis or ectopic pregnancy, left lower quadrant pain pointing to diverticulitis or sigmoid volvulus, and epigastric pain suggesting peptic ulcer disease or pancreatitis. 1, 2

Right Upper Quadrant Pain

• Acute cholecystitis is the primary diagnostic consideration 2
• Hepatobiliary pathology including choledocholithiasis and cholangitis 2
• Liver function tests and hepatobiliary markers are particularly important for this location 3

Right Lower Quadrant Pain

• Appendicitis is the most critical diagnosis to exclude, with a frequency of 15.9-28.1% of cases requiring surgery 1, 2
• Ectopic pregnancy must be considered in all women of reproductive age 2
• Non-appendiceal gastrointestinal, genitourinary, and gynecologic conditions remain equally possible 4
• Ovarian torsion and pelvic inflammatory disease in women 1

Left Lower Quadrant Pain

• Diverticulitis is the leading consideration in older adults 2
• Sigmoid volvulus, especially with history of chronic constipation 2
• The classic patient for sigmoid volvulus is elderly, institutionalized, and on psychotropic medications 1

Epigastric or Diffuse Pain

• Peptic ulcer disease or gastritis 2
• Acute pancreatitis 2
• Gastroenteritis (most common cause overall) 5
• Nonspecific abdominal pain (second most common) 5

Approach by Mechanism

Obstructive Causes

• Adhesions account for 55-75% of small bowel obstructions, particularly in patients with prior laparotomy 2
• Hernias cause 15-25% of small bowel obstructions 2
• Colorectal cancer causes 60% of large bowel obstructions 2
• Volvulus causes 15-20% of large bowel obstructions 2
• Bowel obstruction accounts for 15% of acute abdominal pain admissions 1

Inflammatory/Infectious Causes

• Appendicitis, cholecystitis, diverticulitis 2
• Pancreatitis 2
• Pelvic inflammatory disease 2
• Colitis and inflammatory bowel disease 4

Vascular Causes

• Mesenteric ischemia (arterial or venous thrombosis) 2
• Abdominal aortic aneurysm 6
• Elderly patients have higher likelihood of vascular causes 1

Gynecologic Causes (Women of Reproductive Age)

• Ectopic pregnancy 2
• Ovarian torsion 1
• Pelvic inflammatory disease 2
• Benign adnexal mass 4
• Pelvic congestion syndrome 4

Urologic Causes

• Urolithiasis 5
• Urinary tract infection 3

Extra-Abdominal Causes

• Respiratory infections 5
• Acute myocardial infarction 6
• Abdominal wall pain 5

Critical Red Flags Requiring Urgent Evaluation

Hemodynamic Instability

• Tachycardia is the most sensitive early warning sign of surgical complications and should trigger urgent investigation 1
• Hypotension suggests bleeding or sepsis 2
• The combination of fever, tachycardia, and tachypnea predicts serious complications including anastomotic leak, perforation, or sepsis 1

Peritoneal Signs

• Rigid abdomen or rebound tenderness indicates perforation or ischemia 2
• Signs of peritonitis require urgent evaluation 1

Pain Characteristics

• Pain out of proportion to physical findings strongly suggests mesenteric ischemia and should be assumed to be acute mesenteric ischemia until disproven 1, 2
• Abrupt or instantaneous onset of severe pain suggests vascular catastrophe, particularly aortic dissection or mesenteric ischemia 1
• Ripping, tearing, stabbing, or sharp quality pain suggests aortic dissection 1

Associated Symptoms

• Abdominal distension with vomiting indicates bowel obstruction 2
• Fever with severe pain suggests infection, abscess, or perforation 2
• Syncope with abdominal pain warrants evaluation for pericardial tamponade or neurologic injury from aortic dissection 1
• The triad of abdominal pain, fever, and hemocult-positive stools occurs in approximately one-third of acute mesenteric ischemia patients 1

High-Risk Patient Populations

Cardiovascular Disease

• Patients with cardiovascular disease presenting with acute abdominal pain should be suspected of having acute intestinal ischemia 1
• Atrial fibrillation is present in nearly 50% of patients with embolic acute mesenteric ischemia 1
• Recent myocardial infarction predisposes to acute mesenteric arterial thrombosis 1

Post-Bariatric Surgery

• These patients often present with atypical symptoms 1
• Tachycardia is the most critical warning sign 1
• Classic peritoneal signs are often absent, and internal herniation should be considered even with normal lactate 1

Elderly Patients

• Higher likelihood of malignancy, diverticulitis, and vascular causes 1
• Symptoms may be atypical and require more thorough evaluation, even if laboratory tests are normal 1

Common Pitfalls to Avoid

Pregnancy Testing

• Failing to obtain β-hCG testing in all women of reproductive age before imaging can delay diagnosis of ectopic pregnancy 2, 3

Physical Examination

• Overlooking hernia orifices and surgical scars during examination can miss incarcerated hernias 2
• The absence of peritonitis on examination does not exclude bowel ischemia—patients with sigmoid volvulus often lack peritoneal signs despite having established ischemia 1

Laboratory Interpretation

• Over-relying on normal laboratory values early in disease can lead to missed diagnoses 2
• Normal lactate does not exclude internal herniation or early ischemia 1
• Normal C-reactive protein does not exclude complications 1

Clinical Assessment

• Dismissing atypical presentations in elderly patients can lead to missed diagnoses 2
• Classic peritoneal signs are often absent in post-bariatric surgery patients 1