Appendicitis is one of the most common surgical emergencies in the United States, but its exact cause in any individual patient often remains unclear. What is consistent is the diagnostic approach: a thorough clinical evaluation, targeted blood tests, and imaging (typically ultrasound or CT scan) leads to confirmation and prompt treatment in most US emergency departments. This article walks through what is known about causes, who is at higher risk, and how appendicitis is evaluated and diagnosed.
If you are new to appendicitis, the cluster overview article is a useful starting place.
What Causes Appendicitis?
The exact cause in any individual case is often not identifiable. Several contributing mechanisms are well-recognized:
Obstruction of the appendiceal lumen. The most commonly cited mechanism. A blockage can be caused by:
- Fecaliths — hardened pieces of stool
- Lymphoid hyperplasia — swelling of lymph tissue in the appendix wall, often after viral or bacterial infection
- Tumors (rare, especially in adults)
- Parasites (uncommon in the US)
- Foreign bodies (very rare)
Bacterial infection. Once the lumen is obstructed, normal gut bacteria multiply, producing inflammation, swelling, and pus. The bacteria involved are usually E. coli, Bacteroides, and others normally found in the gut.
Vascular compromise. As the appendix swells, blood flow is reduced. Tissue death can occur, eventually leading to perforation.
Familial and genetic factors. Modest familial clustering is seen.
Dietary patterns. Western diets low in fiber are associated with higher appendicitis rates, though the exact mechanism is unclear.
For most patients, no specific cause can be identified — appendicitis “just happens.”
Major Risk Factors for Appendicitis
Several factors influence appendicitis risk:
Age. Most common in ages 10 to 30. Less common but more often atypical in very young children and elderly adults.
Sex. Slightly more common in males (lifetime risk approximately 9 percent in men vs 7 percent in women).
Family history. Modest increase in risk in first-degree relatives.
Geographic and lifestyle factors. Higher rates in Western developed countries, possibly related to diet.
Recent infection. Viral or bacterial infections may trigger lymphoid hyperplasia.
Cystic fibrosis. Slightly higher risk in some patient groups.
Most appendicitis cases occur in otherwise healthy people without identifiable risk factors.
Common Symptoms That Suggest Appendicitis
Classic symptom pattern:
- Pain that starts near the belly button (periumbilical) and migrates to the lower right abdomen (right lower quadrant, RLQ)
- Pain that worsens with movement or coughing
- Loss of appetite
- Nausea and vomiting
- Low-grade fever
- Inability to pass gas
- Sometimes diarrhea or constipation
Atypical presentations are common, especially in:
- Children (more variable symptoms, fast progression)
- Pregnant women (pain may be in the upper abdomen due to displaced appendix)
- Elderly patients (less classic, more often perforated at diagnosis)
- Patients with retrocecal appendix (back or flank pain)
- Patients with pelvic appendix (pelvic pain, urinary symptoms)
How US Clinicians Evaluate Suspected Appendicitis
The standard workup combines clinical evaluation, blood tests, and imaging.
Clinical history
Questions about:
- Pain location, timing, and progression
- Migration pattern of pain
- Loss of appetite
- Nausea and vomiting
- Fever
- Bowel and urinary patterns
- Last menstrual period (in women)
- Prior abdominal surgery
- Medical conditions
Physical examination
Includes:
- Vital signs (heart rate, blood pressure, temperature)
- Abdominal palpation for tenderness, especially at McBurney’s point (a third of the way from the right hip bone to the navel)
- Rebound tenderness — pain when pressure is released
- Rovsing’s sign — pain in the RLQ when pressing on the left lower abdomen
- Psoas sign — pain when extending the right hip (suggests retrocecal appendix)
- Obturator sign — pain when rotating the right hip
- Rectal exam in some cases
- Pelvic exam in women to rule out gynecological causes
Blood tests
- Complete blood count — elevated white blood cells (usually 10,000-18,000) in most cases
- C-reactive protein (CRP) — often elevated
- Comprehensive metabolic panel — assesses kidneys, electrolytes
- Pregnancy test — for women of reproductive age (mandatory before imaging or surgery)
- Lactic acid — sometimes, in suspected severe cases
Imaging
Abdominal ultrasound. Often the first-line imaging test, especially in children and pregnant women (no radiation). Can show an enlarged, non-compressible appendix. Operator-dependent.
Computed tomography (CT) scan of the abdomen and pelvis. The most accurate imaging for appendicitis in adults. Shows the appendix, surrounding inflammation, perforation, and abscess. Often with IV contrast. Standard in most US emergency departments.
Magnetic resonance imaging (MRI). Used in pregnant patients when ultrasound is inconclusive (no radiation). Less commonly used otherwise.
Clinical scoring systems
Several validated scoring tools help estimate appendicitis probability and guide decisions:
- Alvarado score
- Appendicitis Inflammatory Response (AIR) score
- Pediatric Appendicitis Score
These combine history, exam, and lab findings to risk-stratify patients.
Differential Considerations
Not every abdominal pain is appendicitis. The workup helps identify or rule out:
- Gastroenteritis (viral or bacterial)
- Ovarian cyst rupture or torsion (women)
- Ectopic pregnancy (women)
- Pelvic inflammatory disease (women)
- Mittelschmerz (ovulation pain)
- Kidney stones or pyelonephritis
- Diverticulitis (especially on the right in some populations)
- Mesenteric adenitis
- Crohn’s disease
- Constipation
- Urinary tract infection
- Testicular torsion (men)
- Right-sided pneumonia (can refer to abdomen)
When to Seek Urgent or Emergency Care
Seek emergency department care promptly for:
- New severe abdominal pain that worsens over hours
- Pain shifting to the lower right abdomen
- Pain with fever, nausea, vomiting
- Pain that prevents normal daily activity
- Severe pain in pregnancy
- Children with new abdominal pain plus fever or irritability
Call 911 immediately for:
- Severe sudden abdominal pain
- Rigid, hard abdomen
- Confusion or fainting
- Signs of shock (rapid heart rate, low blood pressure)
How the Diagnosis Is Made
The diagnosis of appendicitis typically follows this process:
- Clinical evaluation by a primary care doctor, urgent care, or emergency physician
- Blood work assesses inflammation and other contributing conditions
- Imaging (ultrasound or CT) confirms appendicitis or identifies alternatives
- Surgical consultation for appendectomy planning
- Pregnancy testing in women before imaging
- Severity assessment — uncomplicated vs complicated
If imaging is inconclusive but clinical suspicion is high, observation with repeat exam may be appropriate. If perforation or abscess is suspected, additional steps may be needed.
What Happens After Diagnosis
Once appendicitis is confirmed:
- Uncomplicated appendicitis: Usually treated with laparoscopic appendectomy within hours
- Complicated appendicitis (perforation, abscess): May require drainage, antibiotics, and delayed surgery, or open surgery
- Pregnancy: Specialized care with obstetric coordination
- Pediatric cases: Pediatric surgery team
- Antibiotics-only approach: Considered in select adult cases of uncomplicated appendicitis; surgery remains the standard
The full procedure overview, recovery, and what to expect are covered in our appendectomy cluster and appendectomy procedure and recovery article.
Continue Reading the Appendicitis Cluster
- Appendicitis: Overview, Types, and Symptoms
- Appendicitis: Treatment, Prevention, and Management
- Appendicitis: FAQs, Statistics, and Patient Stories
- Appendectomy: Causes, Diagnosis, and When to Consider Surgery
Sources
- American College of Surgeons (ACS). Appendicitis diagnosis. https://www.facs.org/
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Appendicitis diagnosis. https://www.niddk.nih.gov/health-information/digestive-diseases/appendicitis
- Society of American Gastrointestinal and Endoscopic Surgeons (SAGES). Appendectomy guidelines. https://www.sages.org/
- American College of Radiology (ACR). Appropriateness criteria for right lower quadrant pain. https://www.acr.org/
- Mayo Clinic. Appendicitis diagnosis. https://www.mayoclinic.org/diseases-conditions/appendicitis/diagnosis-treatment/drc-20369549
- Cleveland Clinic. Appendicitis. https://my.clevelandclinic.org/health/diseases/8095-appendicitis
- NIH MedlinePlus. Appendicitis. https://medlineplus.gov/appendicitis.html
Medical Disclaimer
The information in this article is for general education and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek prompt medical care for new severe abdominal pain, especially if accompanied by fever, vomiting, or worsening symptoms.
