What are the clinical, laboratory, and imaging criteria for diagnosing localized (non‑perforated) appendicitis?

Diagnostic Criteria for Localized (Non-Perforated) Appendicitis

Localized appendicitis is diagnosed through a combination of clinical presentation, laboratory findings, and imaging confirmation, with CT abdomen/pelvis with IV contrast serving as the definitive diagnostic test demonstrating 85.7-100% sensitivity and 94.8-100% specificity. 1

Clinical Criteria

Classic Presentation

• Periumbilical pain migrating to the right lower quadrant is the hallmark symptom, though present in only approximately 50% of cases 1, 2, 3
• Anorexia, nausea, and intermittent vomiting typically follow the onset of pain 2, 3, 4
• Low-grade fever may be present, but fever is absent in approximately 50% of appendicitis cases 1, 5
• The sequence matters: vomiting before pain makes appendicitis unlikely 6

Physical Examination Findings

• Right lower quadrant tenderness with guarding indicates peritoneal irritation 2, 7
• Rebound tenderness (Blumberg sign) suggests localized peritonitis 2, 7
• McBurney point tenderness (one-third the distance from anterior superior iliac spine to umbilicus) is highly suggestive 5
• Positive psoas sign increases likelihood of appendicitis 6
• Absence of guarding, rebound, or fever significantly reduces probability but does not exclude the diagnosis 1, 5

Critical Pitfall

• Clinical examination alone misdiagnoses appendicitis in 34-68% of cases, with negative appendectomy rates as high as 25% when imaging is omitted 1, 5

Laboratory Criteria

White Blood Cell Count

• Leukocytosis (typically 10,000-18,000/μL) is common but not universal 7, 3
• Normal WBC does not exclude appendicitis, particularly in early disease 5, 7
• The combination of normal WBC and normal CRP significantly reduces probability when combined with benign clinical presentation (negative likelihood ratio 0.25) 5

Inflammatory Markers

• Elevated C-reactive protein (CRP) supports the diagnosis, especially when combined with elevated WBC 7
• Normal WBC and normal CRP together help exclude acute appendicitis in low-risk patients 7

Clinical Scoring Systems

• AIR score (Appendicitis Inflammatory Response) has the best discriminatory power in men (cutoff ≤2, specificity 24.7%, failure rate 2.4%) 7
• AAS score (Adult Appendicitis Score) performs better in women (cutoff ≤8, specificity 63.1%, failure rate 3.7%) 7
• Alvarado score has mixed results and does not improve diagnostic accuracy sufficiently to replace imaging 1

Imaging Criteria (Definitive Diagnosis)

CT Abdomen/Pelvis with IV Contrast (First-Line in Adults)

This is the gold standard imaging modality for adults with suspected appendicitis. 1, 2, 5

CT Findings of Localized Appendicitis

• Appendiceal diameter ≥7-8.2 mm (maximal outer diameter cutoff 8.2 mm provides optimal sensitivity/specificity) 5, 3
• Periappendiceal fat stranding indicates inflammation extending beyond the appendix 2, 5
• Appendiceal wall enhancement after IV contrast administration 5
• Absence of intraluminal gas within the appendix 2
• Appendicolith (fecalith) present in approximately 25-30% of cases 3

CT Performance Characteristics

• Sensitivity: 85.7-100% 1, 5
• Specificity: 94.8-100% 1, 5
• Identifies alternative diagnoses in 23-45% of patients with right lower quadrant pain 1, 5

Technical Protocol

• IV contrast only (no oral contrast required) allows rapid acquisition without loss of diagnostic accuracy 1, 5, 7
• Coverage from T10-L2 to pubic symphysis is sufficient 1

Ultrasound (First-Line in Children and Pregnant Women)

• Graded compression ultrasound of the right lower quadrant 1, 2
• Sensitivity: 51.8-81.7% 2
• Specificity: 53.9-81.4% 2
• Appendiceal diameter >6 mm with non-compressibility 2
• Hyperechoic periappendiceal fat indicates inflammation 2

Staged Algorithm

• Ultrasound followed by CT if nondiagnostic achieves 99% sensitivity and 91% specificity 1, 5
• This approach is particularly appropriate for children and reproductive-age women to minimize radiation exposure 1, 5

MRI (Special Populations)

• Preferred in pregnant patients 1, 2
• Sensitivity and specificity: 96% 1, 2
• Abbreviated protocols using T2-weighted HASTE and diffusion-weighted imaging (DWI) reduce acquisition time while maintaining accuracy 1
• Lower apparent diffusion coefficient (ADC) values in inflamed appendix compared to normal 1

Distinguishing Localized from Perforated Appendicitis

Imaging Features of Non-Perforated (Localized) Disease

• Intact appendiceal wall without defect 1
• Periappendiceal fat stranding confined to immediate vicinity 2
• Absence of free fluid or abscess formation 2, 5
• No extraluminal air 2
• Appendiceal diameter typically 7-13 mm (>13 mm suggests higher perforation risk) 3

Clinical Indicators of Localized Disease

• Symptom duration <24-48 hours 3, 8
• Absence of diffuse peritonitis on examination 7
• Localized right lower quadrant tenderness without generalized rigidity 7

Diagnostic Algorithm

Step 1: Clinical Risk Stratification

• Apply AIR or AAS scoring system to categorize as low, intermediate, or high risk 7
• Low-risk patients (AIR ≤2 in men, AAS ≤8 in women): discharge with 24-hour follow-up, no imaging required 5, 7
• Intermediate-risk patients: proceed to imaging 7
• High-risk patients: imaging or direct surgical consultation 5

Step 2: Laboratory Evaluation

• Obtain CBC with differential and CRP in all patients 7
• β-hCG in all reproductive-age women to exclude ectopic pregnancy before imaging 5
• Urinalysis to evaluate for alternative diagnoses (note: sterile pyuria may occur with adjacent appendiceal inflammation) 5

Step 3: Imaging Selection

• Adults (non-pregnant): CT abdomen/pelvis with IV contrast 1, 2, 5, 7
• Children and reproductive-age women: ultrasound first, then CT if nondiagnostic 1, 2, 5
• Pregnant women: ultrasound first, then MRI if nondiagnostic 1, 2
• Elderly patients: low threshold for CT given atypical presentations and higher perforation rates (55-70%) 7

Step 4: Interpretation and Management

• Positive imaging with localized findings: surgical consultation for appendectomy or consideration of antibiotic therapy in selected cases 3, 9
• Negative imaging with persistent symptoms: 24-hour observation with serial examinations 5
• Equivocal imaging with high clinical suspicion: diagnostic laparoscopy 5

Common Pitfalls to Avoid

• Do not rely on absence of fever to exclude appendicitis (absent in ~50% of cases) 1, 5
• Do not discharge patients with normal labs alone without imaging if clinical suspicion is intermediate or high 5, 7
• Do not delay imaging in elderly patients due to high perforation rates and atypical presentations 7, 4
• Do not skip β-hCG testing in reproductive-age women before imaging 5
• Do not assume antibiotic failure indicates resistance; persistent symptoms with normal inflammatory markers suggest non-infectious etiology requiring imaging 5
• Do not order plain abdominal radiographs as they have very limited sensitivity for appendicitis 5