How to differentiate and treat an infected sebaceous cyst versus an abscess?

Differentiating Infected Sebaceous Cyst from Abscess

The key distinction is that infected sebaceous cysts contain a cyst wall with cheesy keratinous material and skin flora, where inflammation results from rupture of the cyst wall rather than primary infection, while abscesses are simple collections of pus without a cyst structure—however, both require incision and drainage as primary treatment. 1, 2

Clinical Differentiation

Infected Sebaceous Cyst (Epidermoid Cyst)

• Pre-existing history: Patient typically reports a longstanding painless nodule that recently became inflamed, painful, and enlarged 1, 2
• Palpable cyst wall: You can often feel a distinct capsule or wall structure beneath the skin 2
• Central punctum: May have a visible dark central opening (blocked follicle) on the surface 1
• Cheesy material: When ruptured or drained, contains thick white-yellow keratinous debris mixed with pus, rather than pure liquid pus 1, 2
• Pathophysiology: Inflammation occurs as a reaction to cyst wall rupture and extrusion of contents into dermis, not true bacterial infection 1, 2

Simple Abscess

• Acute onset: Develops over days without pre-existing mass 1, 2
• No cyst wall: Purely a collection of pus within dermis and deeper tissues without encapsulation 1, 2
• Fluctuant: Typically more uniformly fluctuant throughout 1
• Pure purulent drainage: Contains liquid pus without keratinous material 1, 2
• Polymicrobial: Usually contains normal skin flora; S. aureus present as single pathogen in only ~25% of cases 1

Treatment Approach

Primary Management (Both Conditions)

Incision and drainage is the cornerstone treatment for both infected sebaceous cysts and abscesses. 1, 3, 2

• Technique: Make adequate incision, thoroughly evacuate all contents, probe cavity to break up loculations 1, 3, 2
• Wound care: Simply covering with dry dressing is usually sufficient; packing is optional 1, 3
• For infected cysts specifically: The entire cyst wall should ideally be excised to prevent recurrence, though this can be done in the same sitting under appropriate anesthesia 4

When Antibiotics Are NOT Needed

For both infected cysts and simple abscesses, antibiotics are generally unnecessary after adequate drainage if: 1, 3

• Erythema extends <5 cm from the lesion 1
• Temperature <38.5°C 1
• Heart rate <110 beats/minute 1
• WBC count <12,000 cells/µL 1
• No systemic signs of infection 1, 3
• Adequate source control achieved 3

When Antibiotics ARE Indicated

Add systemic antibiotics to incision and drainage when: 1, 5, 3

• Temperature ≥38.5°C or systemic inflammatory response syndrome present 1, 5, 3
• Heart rate >110 beats/minute 1
• Erythema extending >5 cm from margins 1, 3
• Severely immunocompromised host 1, 3
• Incomplete source control after drainage 3
• Multiple lesions or extensive surrounding cellulitis 1, 3

Antibiotic Selection (When Indicated)

For MRSA coverage (most common in community-acquired infections): 1, 5, 3

• Trimethoprim-sulfamethoxazole 1, 5, 3
• Clindamycin 1, 5, 3
• Doxycycline (avoid in children <8 years) 1, 3

Duration: 5-10 days based on clinical response 5, 3

Critical Distinction in Culture Recommendations

Gram stain and culture of pus from inflamed epidermoid cysts are NOT recommended, as they typically contain normal skin flora and inflammation is not primarily infectious 1

Gram stain and culture ARE reasonable for abscesses and carbuncles, though treatment without these studies is acceptable in typical cases 1

Common Pitfalls to Avoid

• Failing to perform adequate incision and drainage: This is the most critical error—antibiotics alone are insufficient for either condition 1, 5, 3, 2
• Prescribing antibiotics unnecessarily: Studies show no significant benefit when added to adequate drainage in simple cases (88.1% vs 86.0% cure rate, OR 1.17) 6
• Inadequate drainage: Ensure complete evacuation of purulent material and break up all loculations 1, 5, 3
• Missing systemic signs: Always check vital signs and extent of erythema to determine if antibiotics are truly needed 1, 3
• Not considering recurrence: For recurrent lesions, consider decolonization strategies including intranasal mupirocin and chlorhexidine washes 3