A 28-year-old woman presents with acute onset right lower quadrant pain for 18 hours. On examination, she is afebrile, and there is localized tenderness at McBurney's point without rebound or guarding. WBC is 11,200/μL. Abdominal ultrasound shows an appendix measuring 6.5 mm with mild periappendiceal echogenicity but no free fluid. The sonographer notes the appendix is compressible. What is the most likely diagnosis, and what is the most appropriate next step?

Explanation

## Diagnostic Dilemma: Borderline Ultrasound Findings **Key Point:** This patient has equivocal ultrasound findings: - Appendix 6.5 mm (borderline; >6 mm is abnormal, but <8 mm is mild) - **Compressible** (suggests early/mild inflammation, not acute) - Mild periappendiceal echogenicity (not marked) - No free fluid - Afebrile, no rebound/guarding (suggests early stage) **High-Yield:** When ultrasound findings are borderline and clinical presentation is mild, CT abdomen-pelvis is the next step to: 1. Confirm or exclude appendicitis 2. Identify alternative diagnoses (mesenteric lymphadenitis, Crohn's disease, gynecologic pathology) 3. Reduce unnecessary appendectomy rate ## Diagnostic Algorithm for Borderline Appendicitis ```mermaid flowchart TD A[RLQ pain + clinical exam]:::outcome --> B{Ultrasound findings}:::decision B -->|Clear acute appendicitis<br/>Non-compressible, >8mm, free fluid| C[Immediate appendectomy]:::action B -->|Borderline findings<br/>Compressible, 6-7mm, mild echo| D[CT abdomen-pelvis]:::action D --> E{CT confirms appendicitis?}:::decision E -->|Yes| F[Appendectomy]:::action E -->|No| G[Conservative management<br/>or alternative diagnosis]:::action B -->|Normal appendix| H[Observe, alternative diagnosis]:::action ``` ## Ultrasound Criteria for Acute Appendicitis | Feature | Acute Appendicitis | Borderline/Early | Normal | |---------|-------------------|------------------|--------| | **Diameter** | >8 mm | 6–7.9 mm | <6 mm | | **Compressibility** | Non-compressible | Compressible | Compressible | | **Periappendiceal fluid** | Yes (marked) | Minimal/absent | Absent | | **Wall echogenicity** | Markedly increased | Mildly increased | Normal | | **Clinical signs** | Fever, rebound, guarding | Mild tenderness | Minimal | **Clinical Pearl:** A compressible appendix on ultrasound is a reassuring sign and suggests early inflammation or resolving process, not fulminant appendicitis. In this context, CT provides superior sensitivity and specificity. **Warning:** Do not perform appendectomy based on borderline ultrasound alone — this increases negative appendectomy rate and morbidity. CT clarifies diagnosis in equivocal cases. ## Why Not the Other Options? | Option | Why Incorrect | |--------|---------------| | Immediate appendectomy | Unjustified with borderline ultrasound and mild clinical signs. Risks negative appendectomy (removal of normal appendix). | | Conservative management without imaging | Dangerous if appendicitis is present. CT is needed to confirm or exclude diagnosis before conservative approach. | | Discharge without follow-up | Appendicitis can progress rapidly. Patient requires imaging confirmation and close follow-up, not outpatient discharge. | **Mnemonic:** **BORDERLINE = Better to CT before appendectomy** — when ultrasound is equivocal, CT is the next step. [cite:Sabiston Textbook of Surgery 21e Ch 49; Radiology Fundamentals]