What is the diagnostic approach for a suspected abscess?

Diagnostic Approach for Abscess

Most abscesses can be diagnosed by clinical examination alone, with imaging reserved for atypical presentations, deep or complex abscesses, or when clinical examination is equivocal. 1, 2, 3

Clinical Examination

Perform a focused history and complete physical examination, including digital rectal examination (DRE) for suspected anorectal abscesses. 1, 2

Key History Elements

• Perianal pain, swelling, fever, or purulent discharge for anorectal abscesses 1
• Symptoms of inflammatory bowel disease (particularly Crohn's disease, which causes anorectal abscess in one-third of patients) 1, 4
• Immunosuppression status, diabetes mellitus, or recurrent infections 1, 4
• Deep abscesses may present with referred pain to perineum, low back, buttocks, or urinary retention 1

Physical Examination Findings

• Localized collection of purulent material with surrounding inflammation and fluctuance 3
• DRE identifies occult supralevator abscesses not apparent on external examination and helps differentiate abscess types (perianal, ischiorectal, intersphincteric, supralevator) 1, 2
• Severe pain may limit adequate DRE without anesthesia 2

Point-of-Care Ultrasound (POCUS)

When clinical examination is equivocal, POCUS is highly accurate for differentiating abscess from cellulitis, with 97% sensitivity and 83% specificity. 5

POCUS Indications

• Equivocal physical examination findings 5, 6
• Suspected deep soft tissue abscesses 1
• Determining abscess size and depth to guide management 7
• Guiding aspiration or drainage procedures 1

POCUS Advantages

• Detects fluid collections, joint effusions, and infected tendon sheaths 1
• Superior to CT for superficial abscesses (96.7% sensitivity vs 76.7%) 1
• Evaluates internal characteristics (necrosis, debris, foreign bodies, fistulas) 1

POCUS Limitations

• Limited for deeper structures and adjacent bone involvement compared to MRI/CT 1
• Operator-dependent 1
• Less accurate with significant bowel gas or extensive surgical dressings 1

Advanced Imaging

When to Order Imaging

Imaging is indicated for: 1, 2

• Atypical presentations or suspected deep/occult abscesses 2
• Recurrent abscesses or suspected fistula 2, 8
• Suspected inflammatory bowel disease 2, 8
• Complex anatomy or failed initial drainage 1, 2
• Immunocompromised patients (though CT sensitivity drops to 77% in this population) 2

MRI (Preferred Advanced Imaging)

MRI is the preferred imaging modality for complex or recurrent abscesses due to superior soft tissue resolution. 1, 2

• For anorectal abscesses: MRI pelvis without and with IV contrast is first-line 2, 8
• For brain abscesses: MRI with DWI/ADC and T1-weighted imaging with/without gadolinium (92% sensitivity, 91% specificity) 1, 4
• For intra-abdominal abscesses: MR enterography yields 85% sensitivity and 94% specificity 1
• MRI correctly classifies fistulas in 82% of cases vs only 24% with CT 8

CT Imaging (Alternative)

CT with IV contrast is acceptable when MRI is unavailable, contraindicated, or emergency assessment is needed. 1, 2

• For intra-abdominal abscesses: CT is suggested as initial imaging for adults/adolescents with acceptable diagnostic accuracy 1
• CT maintains accuracy with dressings, stomas, or drains present 1
• Less operator-dependent than ultrasound 1
• Major limitation: Only 77% sensitivity for perirectal abscesses, with poorer spatial resolution in pelvis and difficulty differentiating fistula tracts from inflammation 1, 2, 8

Laboratory Investigations

Routine Testing

• Check serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus 1
• For systemic infection/sepsis: complete blood count, serum creatinine, inflammatory markers (CRP, procalcitonin, lactate) 1

Specialized Testing

• Amoebic serology (>90% sensitivity) for suspected amoebic liver abscess with appropriate travel history 4
• Consider hydatid serology before aspiration in endemic regions 4

Critical Differential Diagnoses to Exclude

• Fournier's gangrene/necrotizing fasciitis: Requires urgent recognition and aggressive surgical debridement, not simple drainage 4
• Crohn's disease: Mandatory to exclude in recurrent anorectal abscesses 4
• Malignancy: Consider anal cancer in anorectal presentations 4
• Tuberculosis: Can mimic chronic perianal infection 4

Common Pitfalls

• Ordering imaging when clinical examination is adequate delays definitive treatment (incision and drainage remains primary therapy regardless of imaging findings) 2
• Relying solely on CT may miss small abscesses or fistulous tracts due to limited soft tissue resolution 8
• Inadequate initial drainage is a major risk factor for recurrence (up to 44% recurrence rate) 2
• Abscesses >0.4 cm depth from skin surface typically require drainage; those <0.4 cm may be treated with antibiotics alone 7