What is the diagnosis and treatment for a painful pustule with hard white sebum, not pus, on the skin of an adult patient?

Diagnosis: Inflamed Epidermoid Cyst (Not Sebaceous Cyst)

The most likely diagnosis is an inflamed epidermoid cyst, which requires incision and drainage as primary treatment, with antibiotics reserved only for specific indications such as fever, extensive surrounding cellulitis, or systemic signs of infection. 1

Key Diagnostic Features

The description of "hard white sebum" is pathognomonic for an epidermoid cyst rather than a simple abscess or pustule:

• Epidermoid cysts contain thick white-yellow keratinous debris (often called "cheesy material") mixed with pus, rather than pure liquid pus 1
• The inflammation occurs as a reaction to rupture of the cyst wall and extrusion of its contents into the dermis, not from primary bacterial infection 2, 1
• Patients typically report a longstanding painless nodule that recently became inflamed, painful, and enlarged 1
• A distinct capsule or wall structure can often be felt beneath the skin 1
• The cyst may have a visible dark central opening (punctum) on the surface 1

Treatment Algorithm

Primary Treatment: Incision and Drainage

Incision and drainage is mandatory and the cornerstone of treatment 2, 3, 1:

• Perform thorough evacuation of the keratinous material and pus 2
• Probe the cavity to break up all loculations 2, 3
• Ideally excise the entire cyst wall to prevent recurrence, though this can be done in the same sitting under appropriate anesthesia 1
• Simply cover the surgical site with a dry dressing after drainage 2

When to Add Antibiotics

Antibiotics are generally unnecessary after adequate drainage unless specific criteria are met 3, 1:

Add systemic antibiotics ONLY if:

• Temperature ≥38.5°C or systemic inflammatory response syndrome present 1
• Heart rate >110 beats/minute 1
• Erythema extending >5 cm from the lesion margins 1
• Multiple lesions present 2, 3
• Severely immunocompromised host 2, 1
• Extensive surrounding cellulitis 2, 3
• Incomplete source control after drainage 1

Antibiotic Selection (If Indicated)

For community-acquired infections with MRSA coverage 1:

• Trimethoprim-sulfamethoxazole (first-line)
• Clindamycin (alternative)
• Doxycycline (alternative)

Duration: 5-10 days based on clinical response 1

Critical Pitfalls to Avoid

• Do NOT treat with antibiotics alone without drainage—this will fail 3, 1
• Do NOT routinely obtain Gram stain and culture from inflamed epidermoid cysts, as they typically contain normal skin flora and inflammation is not primarily infectious 1
• Do NOT prescribe antibiotics unnecessarily, as they show no significant benefit when added to adequate drainage in simple cases without systemic signs 1
• Ensure complete evacuation of all purulent and keratinous material—inadequate drainage leads to treatment failure 1
• Always check vital signs and measure extent of erythema to avoid missing systemic signs that would warrant antibiotics 1

Prevention of Recurrence

For patients with recurrent lesions 1:

• Consider decolonization strategies including intranasal mupirocin
• Daily chlorhexidine washes
• Complete cyst wall excision during initial drainage when feasible