What is the diagnosis for a patient with a recurrent boil and suspected Methicillin-resistant Staphylococcus aureus (MRSA) infection, who has a history of recent antibiotic exposure and recurrent infections?

Diagnosis of Recurrent Boil with Suspected MRSA Infection

The diagnosis is recurrent skin abscess (furuncle/carbuncle) with suspected community-acquired MRSA infection, requiring culture confirmation and evaluation for underlying predisposing factors.

Diagnostic Approach

Immediate Clinical Assessment

Obtain Gram stain and culture of pus from the abscess to confirm MRSA and guide antibiotic therapy 1. While treatment without these studies is reasonable in typical cases, culture is strongly recommended for recurrent infections 1.

Key clinical features to document:

• Presence of systemic inflammatory response syndrome (SIRS): temperature >38°C or <36°C, tachycardia >90 beats/min, tachypnea >24 breaths/min, or WBC >12,000 or <400 cells/µL 1
• Size and depth of lesion (furuncles vs. carbuncles—the latter are larger, deeper, and involve multiple adjacent follicles) 1
• Evidence of surrounding cellulitis or induration 1
• History of recent antibiotic exposure (increases MRSA likelihood) 1

Evaluation for Recurrent Disease

A recurrent abscess at the same site should prompt a search for local causes such as pilonidal cyst, hidradenitis suppurativa, or foreign material 1. This is critical as these conditions require different management approaches beyond antibiotics.

For patients with recurrent abscesses at different sites:

• Evaluate for neutrophil disorders only if recurrent abscesses began in early childhood 1
• Adult-onset recurrent abscesses do not require neutrophil function testing 1
• Consider risk factors for MRSA colonization: injection drug use, nasal colonization, penetrating trauma, or evidence of MRSA infection elsewhere 1

Culture Interpretation

Blood cultures are recommended if systemic signs of infection are present 1. The prevalence of MRSA in anorectal and skin abscesses can be as high as 35% 1, making culture particularly important in:

• High-risk patients (HIV, immunocompromised) 1
• Recurrent infections or non-healing wounds 1
• Cases with risk factors for multidrug-resistant organisms 1

Important caveat: MRSA isolates increasingly demonstrate high-level mupirocin resistance (≥512 mcg/mL), particularly in methicillin-resistant strains 2. Testing MRSA populations for mupirocin susceptibility prior to decolonization is appropriate 2.

Distinguishing Features of Severe MRSA Infection

Be alert for Panton-Valentine Leukocidin (PVL)-positive CA-MRSA, which can cause life-threatening invasive infection 1. While PVL-positive S. aureus is usually associated with recurrent cutaneous infection, severe presentations include:

• Short influenza-like prodrome with high fever (>39°C) 1
• Marked leukopenia with lymphopenia 1
• Rapid progression to septic shock 1
• Multilobar pulmonary infiltrates if pneumonia develops 1

Additional Diagnostic Considerations

Do not culture inflamed epidermoid cysts, as this is not recommended 1. These are distinct from true abscesses and require different management.

For patients with diabetes, carbuncles most commonly develop on the back of the neck 1, making this a key anatomic location to examine in diabetic patients with recurrent infections.

Clinical Pitfalls to Avoid

• Do not assume simple folliculitis: Furuncles extend through the dermis into subcutaneous tissue with abscess formation, unlike folliculitis where pus is limited to the epidermis 1
• Do not delay surgical evaluation if signs suggest deeper infection: pain disproportionate to findings, violaceous bullae, cutaneous hemorrhage, skin sloughing, skin anesthesia, rapid progression, or gas in tissue 1
• Screening cultures prior to decolonization are not routinely recommended if at least one prior infection was documented as MRSA 1
• Surveillance cultures following decolonization are not routinely recommended in the absence of active infection 1