Is it normal for a furuncle (boil) to heal leaving a subcutaneous nodule that is mildly tender without overlying infection, and what other diagnoses should be considered?

Residual Subcutaneous Nodule After Boil Resolution

Yes, it is relatively common for a healed furuncle (boil) to leave behind a palpable subcutaneous nodule with mild tenderness and no overlying infection, though this warrants evaluation for alternative diagnoses.

Why This Occurs After Furuncle Healing

• Furuncles extend through the dermis into subcutaneous tissue where a small abscess forms, and the healing process can leave residual fibrosis, scar tissue, or organizing inflammatory tissue that presents as a firm nodule 1
• The subcutaneous abscess cavity may not completely resolve even after surface healing, leaving behind a palpable mass of organizing tissue or residual encapsulated material 1
• This residual nodule typically becomes less tender and gradually softens over weeks to months as the inflammatory process fully resolves 1

Alternative Diagnoses to Consider

Epidermoid Cyst (Most Common Alternative)

• Epidermoid cysts (often mislabeled "sebaceous cysts") are the most likely alternative diagnosis, containing cheesy keratinous material that can become inflamed and mimic a furuncle 1
• These cysts contain normal skin flora even when uninflamed, and inflammation occurs as a reaction to rupture of the cyst wall rather than true infection 1
• After acute inflammation subsides, the cyst wall remains and presents as a persistent subcutaneous nodule that may be mildly tender 1

Hidradenitis Suppurativa

• Consider hidradenitis suppurativa if the lesion is in typical locations (axillae, groin, anogenital regions, inframammary areas) and there is a history of recurrent inflammatory nodules 2
• This chronic inflammatory condition presents with painful inflamed nodules that can leave behind depressed scars, nodules, and cysts after healing 2
• Look for additional features: multiple lesions, comedones, sinus tracts, or scarring in characteristic locations 2

Incompletely Drained Abscess

• A residual abscess cavity that was not adequately drained during initial treatment can persist as a fluctuant or firm nodule 1
• This requires re-evaluation for fluctuance and may need incision, thorough evacuation, and probing to break up loculations 1

Recurrent Staphylococcal Colonization

• Patients with recurrent furunculosis often have nasal colonization with Staphylococcus aureus (20-40% of general population), which can lead to repeated infections at the same site 1, 3
• The nodule may represent early recurrence rather than residual tissue from the original infection 1

Clinical Approach

Examination Findings to Assess

• Fluctuance: If present, suggests residual abscess requiring drainage 1
• Central punctum or pore: Suggests epidermoid cyst rather than simple post-furuncle scarring 1
• Skin changes: Look for orange peel appearance (peau d'orange), which would suggest active cellulitis rather than simple residual nodule 1
• Location and pattern: Multiple lesions or lesions in axillae/groin suggest hidradenitis suppurativa 2

Management Strategy

• If the nodule is firm, non-fluctuant, and gradually improving: Observe for 4-6 weeks as residual inflammatory tissue typically resolves spontaneously 1
• If fluctuance develops or tenderness worsens: Perform incision and drainage with probing to break up loculations 1
• If an epidermoid cyst is suspected: Incision and thorough evacuation of contents is required; simple drainage without removing cyst wall contents leads to recurrence 1
• Systemic antibiotics are not indicated unless there is extensive surrounding cellulitis, fever, or systemic manifestations 1, 4

Prevention of Recurrence

• Evaluate for nasal S. aureus colonization and consider intranasal mupirocin twice daily for 5 days each month, which reduces recurrences by approximately 50% 1, 4
• Daily chlorhexidine washes and thorough laundering of clothing, towels, and bedding help prevent reinfection 4, 5
• For patients with persistent recurrences, oral clindamycin 150 mg daily for 3 months decreases subsequent infections by approximately 80% (for susceptible strains) 1

Common Pitfalls to Avoid

• Do not prescribe antibiotics for a non-fluctuant residual nodule without signs of active infection, as this represents organizing scar tissue rather than active bacterial infection 1, 4
• Do not assume all post-furuncle nodules are benign: Failure to recognize an incompletely drained abscess or epidermoid cyst can lead to recurrence 1
• Do not overlook hidradenitis suppurativa in patients with recurrent lesions in characteristic locations, as this requires different long-term management 2