What is the antibiotic of choice for a patient with suspected Methicillin-resistant Staphylococcus aureus (MRSA) skin infection, considering their allergy history and renal function?

Antibiotic of Choice for Suspected MRSA Skin Infection

For suspected MRSA skin infections, trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 double-strength tablets (160mg/800mg) twice daily is the first-line oral antibiotic, with doxycycline 100 mg twice daily or clindamycin 300-450 mg every 6 hours as alternatives. 1, 2

Treatment Algorithm Based on Clinical Presentation

Purulent Infections (Abscesses, Furuncles, Carbuncles)

• Incision and drainage is mandatory and the primary treatment—antibiotics play only a subsidiary role 1, 2, 3
• Add antibiotics only when specific features are present: systemic signs (fever, tachycardia), extensive surrounding cellulitis (>5 cm erythema), multiple lesions, immunocompromise, or failed drainage alone 1
• Obtain culture from purulent drainage before starting antibiotics to confirm MRSA and guide definitive therapy 2, 3

Oral Antibiotic Options for MRSA Coverage

First-line choices:

• TMP-SMX 1-2 double-strength tablets (160mg/800mg) twice daily for 7-10 days is the most effective oral option based on clinical cure rates 1, 2, 3
• Doxycycline 100 mg twice daily for 7-10 days is an alternative when TMP-SMX fails or is contraindicated 1, 2
• Clindamycin 300-450 mg every 6 hours for 7-10 days is preferred when coverage for both MRSA and β-hemolytic streptococci is needed, but only if local clindamycin resistance is <10% 1, 2

Advanced oral options:

• Linezolid 600 mg twice daily is highly effective but expensive, reserved for complicated cases or treatment failures 1, 4
• Minocycline 200 mg loading dose, then 100 mg twice daily may be more reliable than doxycycline for community-acquired MRSA 5, 6

Intravenous Options for Severe or Complicated Infections

Hospitalize if any of the following are present: systemic inflammatory response syndrome (SIRS), hypotension, altered mental status, severe immunocompromise, or concern for deeper/necrotizing infection 1, 7

IV antibiotic choices (all with A-I level evidence):

• Vancomycin 15-20 mg/kg IV every 8-12 hours remains first-line for hospitalized patients 1, 7, 3
• Linezolid 600 mg IV twice daily is equally effective and may be superior for complicated skin infections 1, 4
• Daptomycin 4 mg/kg IV once daily (increase to 6-10 mg/kg for bacteremia/endocarditis) 1, 7, 8
• Ceftaroline 600 mg IV every 12 hours is a broad-spectrum cephalosporin with MRSA activity 1, 8

Critical Decision Points

When to add MRSA coverage to typical cellulitis treatment:

• Penetrating trauma or injection drug use 1, 7
• Purulent drainage or exudate present 1, 7
• Known MRSA colonization or previous MRSA infection 7
• Systemic inflammatory response syndrome (SIRS) 7
• Do NOT add MRSA coverage reflexively for typical nonpurulent cellulitis—beta-lactam monotherapy succeeds in 96% of cases 7

For nonpurulent cellulitis with MRSA risk factors:

• Combine TMP-SMX or doxycycline with a beta-lactam (cephalexin, amoxicillin) to ensure streptococcal coverage 1, 7
• Never use TMP-SMX or doxycycline as monotherapy for typical cellulitis—their activity against β-hemolytic streptococci is unreliable 1, 7, 2
• Clindamycin monotherapy covers both organisms, avoiding the need for combination therapy 1, 7, 2

Treatment Duration

• 5 days if clinical improvement occurs for uncomplicated infections, extending only if symptoms have not improved 1, 7
• 7-14 days for complicated skin and soft tissue infections with systemic signs or requiring hospitalization 1, 3
• Reassess clinically within 48-72 hours to ensure appropriate response 3

Special Populations

Pediatric patients:

• Clindamycin 10-13 mg/kg/dose every 6-8 hours is preferred for children with MRSA infections 1, 2
• Avoid doxycycline in children <8 years old due to tooth discoloration and bone growth effects 7, 2
• Linezolid dosing: 10 mg/kg every 8 hours for children <12 years, 600 mg twice daily for ≥12 years 1, 4

Pregnancy:

• Avoid doxycycline (pregnancy category D) 7
• Clindamycin or TMP-SMX (after first trimester) are safer alternatives 2

Renal impairment:

• Adjust vancomycin dosing based on creatinine clearance and trough levels 1
• Linezolid requires no renal dose adjustment 4

Common Pitfalls to Avoid

• Do not treat simple abscesses with antibiotics alone—drainage is the definitive treatment 1, 2
• Do not use TMP-SMX or doxycycline as monotherapy for cellulitis without purulent features—add a beta-lactam for streptococcal coverage 1, 7, 2
• Do not reflexively add MRSA coverage to all cellulitis cases—MRSA is uncommon in typical nonpurulent cellulitis even in high-prevalence settings 7
• Do not continue ineffective antibiotics beyond 48 hours—progression despite appropriate therapy indicates resistant organisms or deeper infection 7
• Clindamycin resistance in MRSA can be inducible (D-test positive)—verify local resistance patterns before use 1, 2, 9