Introduction
The acute abdomen differential diagnosis is arguably the most important clinical puzzle you will solve in your daily practice. Defined as the rapid onset of severe symptoms that may indicate life-threatening intra-abdominal pathology, the “Acute Abdomen” often demands immediate surgical or medical intervention.
For the UKMLA candidate, the abdomen presents a unique challenge. Unlike the chest (where pathology is often limited to the heart or lungs), the abdomen houses the gastrointestinal, renal, vascular, and reproductive systems. A single complaint of “tummy pain” can stem from a benign viral infection or a ruptured abdominal aortic aneurysm (AAA).
To navigate this complexity under exam conditions, you cannot rely on guesswork. You need a robust, anatomical framework. By dividing the abdomen into four quadrants—Right Upper, Right Lower, Left Upper, and Left Lower—you can filter the list of differentials from fifty possibilities down to a probable three.
In this comprehensive guide, we will break down the acute abdomen differential diagnosis by quadrant. We will explore the specific “Red Flags” you must not miss, the investigations that confirm your suspicions, and the management steps required for the UKMLA.
Key Takeaways
The Quadrant Approach: Anatomy is your best friend. Mapping pain to the underlying organ is the fastest way to narrow your differential.
Red Flags: Recognizing signs of peritonitis (guarding, rigidity) and haemodynamic instability (hypotension) takes precedence over the exact diagnosis.
RLQ Rules: In the Right Lower Quadrant, Appendicitis is king, but in women of reproductive age, Ectopic Pregnancy is the “killer” you must rule out.
RUQ Essentials: Distinguishing Biliary Colic from Acute Cholecystitis relies on the presence of inflammation (fever, raised white cells) and a positive Murphy’s sign.
LLQ Commonality: Acute Diverticulitis is the primary differential in the elderly, often presenting as “left-sided appendicitis.”
Step 1: Rapid Assessment & Red Flags
Before applying your stethoscope to the abdomen, you must first ensure the patient is safe. In the UKMLA CPSA (clinical skills exam), failing to recognize an unstable patient is an automatic failure, regardless of your diagnostic skills.
The First 60 Seconds: A-E Assessment
Perform a rapid “End of the Bed” assessment.
Airway: Is the patient talking in full sentences?
Breathing: Are they tachypnoeic? Rapid, shallow breathing can be a sign of acidosis (e.g., Diabetic Ketoacidosis or Ischaemic Bowel).
Circulation: Are they pale, clammy, or tachycardic? Hypotension in the context of abdominal pain suggests significant fluid loss (sepsis, pancreatitis) or active bleeding (ruptured ectopic, ruptured AAA).
Disability: confusion may indicate hypoxia or late-stage sepsis.
Red Flag Symptoms
Regardless of where the pain is located, the following signs indicate a “Surgical Emergency” requiring immediate senior involvement.
UKMLA Warning: If you identify a “Rigid Abdomen” (involuntary guarding), this indicates Generalised Peritonitis. The patient likely has a perforated viscus and needs urgent surgical review, not just observation.
| Sign | Possible Diagnosis | Mechanism |
|---|---|---|
| Rigid Abdomen | Perforated Ulcer / Diverticulum | Parietal peritonitis (chemical or faecal irritation). |
| Haematemesis | Upper GI Bleed (Varices/Ulcer) | Significant haemorrhage into the stomach. |
| Pulsatile Mass | Abdominal Aortic Aneurysm (AAA) | Expanding aneurysm >5cm. |
| Pain > Clinical Signs | Mesenteric Ischaemia | Visceral ischaemia causes agony before physical signs appear. |
The “SOCRATES” of Abdominal Pain
When taking a history, pay special attention to Radiation and Character:
Loin to Groin: Renal Colic.
Through to the Back: Pancreatitis or AAA.
Shoulder Tip: Diaphragmatic irritation (e.g., blood from a ruptured ectopic or air from a perforation).
Step 2: Right Upper Quadrant (RUQ)
The Right Upper Quadrant is dominated by the liver and the biliary tree. Pathology here is extremely common in the UKMLA. For a broader understanding of liver conditions, review our guide on Gastroenterology and Hepatology Essentials.
You must be able to distinguish between the three stages of gallstone disease.
| Condition | Pain Character | Fever/WBC | Key Sign |
|---|---|---|---|
| Biliary Colic | Colicky (intermittent), fatty food trigger. | No Fever, Normal WBC. | Murphy’s Sign Negative. |
| Acute Cholecystitis | Constant, severe RUQ pain. | Fever Present, Raised WBC/CRP. | Murphy’s Sign Positive. |
| Ascending Cholangitis | Constant RUQ pain. | High Fever (Rigors). | Charcot’s Triad (Pain + Fever + Jaundice). |
Hepatic Causes
Acute Hepatitis: Viral (A, B, E) or Alcoholic. Look for jaundice, hepatomegaly, and risk factors (travel, alcohol intake).
Hepatic Abscess: Rare, but consider in patients with a history of recent intra-abdominal infection (e.g., diverticulitis) or travel.
Fitz-Hugh-Curtis Syndrome: Peri-hepatitis caused by PID (Gonorrhoea/Chlamydia), presenting with RUQ pain and “violin string” adhesions.
Step 3: Right Lower Quadrant (RLQ)
The RLQ contains the appendix, the caecum, and the right ovary/fallopian tube.
Acute Appendicitis
Appendicitis is the most common surgical emergency you will encounter. It is a key topic in our Surgery Essentials for UKMLA.
Pathophysiology: Obstruction of the appendiceal lumen (by a faecolith or lymphoid hyperplasia) leads to distension, ischaemia, and infection.
Classic History:
Early: Periumbilical pain (visceral pain referred to the T10 dermatome).
Late (24 hours): Pain migrates to the Right Iliac Fossa (somatic pain as the parietal peritoneum becomes inflamed).
Examination Signs:
McBurney’s Point Tenderness: Maximum tenderness at the junction of the lateral and middle thirds of the line joining the ASIS and umbilicus.
Rovsing’s Sign: Palpation of the Left Lower Quadrant causes pain in the Right Lower Quadrant (due to peritoneal shift).
Gynaecological Causes (The “Gynae” Sieve)
In any female of reproductive age, gynaecological pathology is the primary differential until proven otherwise. For more details, see our Obstetrics & Gynaecology Essentials.
The Golden Rule: Every female of childbearing age presenting with abdominal pain requires a Urinary Pregnancy Test (UPT). You must never assume “it’s just appendicitis” without ruling out pregnancy.
1. Ectopic Pregnancy
Presentation: Missed period (amenorrhoea), sudden onset sharp iliac fossa pain, vaginal bleeding.
Rupture: Signs of shock (tachycardia/hypotension) and shoulder tip pain indicate a ruptured ectopic, which is a surgical emergency.
2. Ovarian Torsion
Presentation: Sudden, severe, deep pain. Often associated with vomiting.
Risk Factors: Previous ovarian cysts (especially dermoid cysts).
Renal Causes
Ureteric Colic: A stone in the right ureter can mimic appendicitis. However, the patient will typically be restless (unable to find a comfortable position), and the pain radiates from “loin to groin.”
Step 4: Left Lower Quadrant (LLQ)
The LLQ houses the sigmoid and descending colon.
Acute Diverticulitis
This is the most common cause of LLQ pain in adults, especially the elderly.
Pathology: Inflammation of diverticula (mucosal outpouchings) in the sigmoid colon.
Presentation: Constant LLQ pain, fever, and a change in bowel habit (often constipation, sometimes loose stools).
Complications: Perforation (peritonitis), Abscess formation, or Fistula (e.g., colovesical fistula presenting with pneumaturia/bubbles in urine).
Investigation: CT Abdomen is the gold standard.
Caution: Colonoscopy is contraindicated in the acute phase of diverticulitis due to the high risk of perforation.
Other Causes
Ulcerative Colitis Flare: Bloody diarrhoea, mucus, tenesmus, and LLQ pain.
Ischaemic Colitis: Often seen in elderly patients with vascular disease. Presents with pain and bloody diarrhoea, often at the “watershed areas” of the colon (splenic flexure).
Step 5: Left Upper Quadrant (LUQ) & Epigastric
The stomach, pancreas, and spleen reside here.
Epigastric Pain
1. Peptic Ulcer Disease (PUD)
Gastric Ulcer: Pain is typically worsened by eating (acid secretion).
Duodenal Ulcer: Pain is typically relieved by eating (pyloric closure).
Red Flag: Sudden, severe generalized pain suggests Perforation, leading to peritonitis and free air under the diaphragm on Chest X-Ray.
2. Acute Pancreatitis
Causes (GET SMASHED): Gallstones and Alcohol account for the vast majority.
Presentation: Severe epigastric pain radiating through to the back. Vomiting is a prominent feature.
Investigation: Serum Amylase or Lipase (Lipase is more specific). A level >3x the upper limit of normal is diagnostic.
LUQ Pain (Rare)
1. Splenic Rupture
History: Usually follows trauma (e.g., handlebar injury, RTA).
Medical Cause: Spontaneous rupture can occur in splenomegaly caused by EBV (Glandular Fever) or Malaria.
Sign: Kehr’s Sign (referred pain to the left shoulder tip due to diaphragmatic irritation).
Step 6: Diffuse / Central Pain
Some conditions affect the midgut or the entire peritoneal cavity.
Bowel Obstruction
Distinguishing between Small and Large Bowel Obstruction is crucial. Detailed radiological signs can be found in our guide on Decoding the AXR.
Small Bowel Obstruction (SBO):
Causes: Adhesions (previous surgery) are #1. Hernias are #2.
Symptoms: Early vomiting, central colicky pain, distension.
Large Bowel Obstruction (LBO):
Causes: Malignancy (Colorectal Cancer) or Volvulus.
Symptoms: Absolute constipation (no wind/stool passed), late vomiting, significant distension.
Abdominal Aortic Aneurysm (AAA)
Demographics: Male, >65, Smoker, Hypertension.
Presentation: Central abdominal pain radiating to the back.
Rupture: Sudden collapse, severe back pain, hypotension. This requires immediate vascular intervention.
Step 7: Investigation Strategy
In the UKMLA, choosing the correct investigation is as important as the diagnosis itself.
Bedside Tests
Urine Dipstick: Essential for every abdominal pain patient. It rules out UTI (Nitrites/Leukocytes) and Renal Colic (Haematuria). For interpretation, see Mastering the Urine Dipstick.
Beta-HCG: Mandatory for females of childbearing age.
ECG: An Inferior MI can mimic epigastric pain (indigestion). Always perform an ECG in older patients with upper abdominal pain.
VBG: Check the Lactate. A high lactate is a sensitive marker for ischaemia (bowel) or sepsis.
Imaging Choices
| Suspected Diagnosis | First-Line Imaging | Reasoning |
|---|---|---|
| Gallstones / Biliary Colic | Ultrasound Abdomen | Best sensitivity for stones in the gallbladder. |
| Appendicitis (Adult Male) | CT Abdomen (Contrast) | Gold standard for visualization and ruling out mimics. |
| Appendicitis (Child/Pregnant) | Ultrasound | Avoids ionizing radiation. |
| Diverticulitis | CT Abdomen (Contrast) | Confirms wall thickening and perforation risk. |
| Bowel Obstruction | Abdominal X-Ray (AXR) | Quick screen for dilated loops and air-fluid levels. |
Clinical Scenarios: Putting it into Practice
Scenario 1: The “Gynae” Trap
Patient: A 24-year-old female presents with sudden onset Right Iliac Fossa pain and syncope. She is pale, anxious, and tachycardic (HR 120).
Assessment: The location mimics appendicitis, but the suddenness and syncope are red flags for haemorrhage.
Diagnosis: Ruptured Ectopic Pregnancy.
Action: Do not wait for imaging. Insert two large-bore cannulas, cross-match blood, and call the Gynaecology registrar immediately. This is a life-threatening emergency.
Scenario 2: The Silent Ischaemia
Patient: A 72-year-old male with a history of Atrial Fibrillation presents with severe, generalized abdominal pain. On examination, his abdomen is soft and non-tender.
Assessment: The mismatch between the severity of his pain and the “benign” examination is the hallmark of Mesenteric Ischaemia. His AF is the source of the embolus.
Diagnosis: Acute Mesenteric Ischaemia.
Action: Urgent VBG (expect high lactate) and CT Angiogram.
Practice Exercise: Test Your Knowledge
Question 1: A 45-year-old female presents with fever (39°C), jaundice, and RUQ pain. She is confused and hypotensive (BP 85/50). What is the diagnosis and the most appropriate management step?
A) Acute Cholecystitis – Urgent Cholecystectomy
B) Biliary Colic – Analgesia
C) Ascending Cholangitis – Urgent ERCP
D) Hepatitis A – Supportive Care
E) Pancreatitis – IV Fluids
Correct Answer: C. This patient has Reynolds’ Pentad (Charcot’s Triad + Confusion and Hypotension), which indicates severe Ascending Cholangitis. The definitive treatment is biliary decompression via ERCP.
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Question 2: An 8-year-old boy presents with RIF pain and a temperature of 38.5°C. He had a sore throat last week. His abdomen is tender in the RIF but he is hungry and asks for food. What is the most likely diagnosis?
A) Acute Appendicitis
B) Mesenteric Adenitis
C) Testicular Torsion
D) Constipation
E) Inguinal Hernia
Correct Answer: B. While appendicitis must be excluded, the high fever, recent URTI history, and preserved appetite (anorexia is almost universal in appendicitis) point towards Mesenteric Adenitis. Active observation is the strategy.
Frequently Asked Questions (FAQ) about Acute Abdomen
Both are markers of inflammation, but neither is perfectly specific. However, they have a high negative predictive value. This means that if a patient has had symptoms for over 24 hours and both the CRP and WBC count are completely normal, appendicitis is highly unlikely. Conversely, a raised CRP/WBC supports the diagnosis but can also be seen in PID, Adenitis, or UTI.
Ultrasound is the investigation of choice for children (to avoid radiation), pregnant women, and focused biliary or pelvic pathology. For almost all other adult presentations—especially elderly patients or those with undifferentiated pain—CT with contrast is superior. It visualizes the bowel, pancreas, aorta, and mesentery, which ultrasound often misses due to bowel gas.
Yes, absolutely. There is an outdated myth that giving painkillers “masks the signs” of peritonitis. Evidence shows that analgesia (including morphine) makes the patient more comfortable and actually makes the physical examination more reliable because the patient is not guarding purely from distress. Never leave a patient in pain.
As the uterus expands during pregnancy, the appendix is displaced upwards and laterally. By the third trimester, the appendix may be in the Right Upper Quadrant (RUQ). This can lead to diagnostic confusion with Cholecystitis or Pyelonephritis. Ultrasound is the first-line imaging modality, but MRI can be used if the diagnosis remains unclear.
The diaphragm is innervated by the Phrenic nerve (roots C3, C4, C5). Irritation of the diaphragm—usually by blood (from a ruptured spleen or ectopic pregnancy) or air (from a perforation)—is interpreted by the brain as pain in the dermatomes supplied by those nerve roots, which corresponds to the shoulder tip. This is a crucial sign of intra-abdominal pathology.
While not routine for every tummy ache, a PR exam is vital if you suspect specific pathologies. It is used to assess for melena (upper GI bleed), rectal tumours, or severe constipation/impaction. In women, it (or a vaginal exam) can elicit “cervical excitation tenderness,” which is a sign of Pelvic Inflammatory Disease (PID).
Yes, and missing them can lead to unnecessary surgery. The two most common “medical mimics” are Diabetic Ketoacidosis (DKA) and Myocardial Infarction (MI). DKA often presents with generalized abdominal pain and vomiting due to ketosis/acidosis, especially in children; always check a capillary blood glucose and ketones. An Inferior MI can present as epigastric pain due to vagal stimulation; always perform an ECG in adult patients with upper abdominal pain. Other mimics include Pneumonia (referred pain), Hypercalcaemia, and Porphyria.
Elderly patients often present with “atypical” symptoms due to physiological changes and polypharmacy. They may have a reduced inflammatory response, meaning they might not mount a fever or raise their white cell count even with severe infection (e.g., cholecystitis). Furthermore, lax abdominal muscles mean they may not exhibit classic “guarding” or rigidity even in the presence of peritonitis. This “silent abdomen” can mask catastrophic pathology like mesenteric ischaemia or a ruptured AAA, leading to delayed diagnosis and higher mortality.
A raised lactate on a Venous Blood Gas (VBG) is a major red flag. It indicates anaerobic metabolism, usually due to tissue hypoperfusion or ischaemia. In the context of an acute abdomen, a lactate >2 mmol/L (and especially >4 mmol/L) strongly suggests ischaemic bowel (mesenteric ischaemia), severe sepsis, or necrotic bowel (e.g., strangulated hernia or volvulus). It is a marker of severity that warrants immediate senior surgical review and often CT imaging.
Surprisingly, the Abdominal X-Ray (AXR) is not the best test for perforation. The gold standard plain film is an Erect Chest X-Ray (CXR). Air rises, so in an upright patient, free gas will accumulate under the diaphragm (pneumoperitoneum), appearing as a black crescent. An AXR is taken supine (lying flat), so the air disperses across the belly and is much harder to see (requiring signs like Rigler’s sign). If the patient cannot sit up, a reported CT scan is the safer choice.
Conclusion
The acute abdomen differential diagnosis is a test of your anatomical knowledge and your safety behaviours. By strictly mapping organs to the 4 quadrants and applying the “Red Flag” filters, you can navigate even the most complex UKMLA stations.
Remember the golden rule: Safety first. Always rule out the “killers”—AAA, Ectopic Pregnancy, Perforation, and Ischaemic Bowel—before diagnosing a benign condition.
Your Next Steps
Review Anatomy: Open an anatomy atlas and strictly map organs to the 4 quadrants.
Practice the “Sieve”: For your next 5 patients with abdominal pain, force yourself to write down 3 differentials based purely on the quadrant before checking the scan results.
Learn the Guidelines: Read the TeachMeSurgery Acute Abdomen page for a deeper dive into the surgical management of these conditions.
Bedside Reference: Bookmark the Patient.info Abdominal Pain Pro article for a quick check of differentials when you are on the ward.
