What is the recommended antibiotic dose for a patient with a history of Methicillin-resistant Staphylococcus aureus (MRSA) and an open wound prior to surgery, considering Keflex (Cephalexin) for 7 days orally?

Keflex (Cephalexin) is NOT Appropriate for MRSA Wound Infections

Do not use Keflex (cephalexin) for this patient—it has no activity against MRSA and will fail to treat the infection. A patient with a history of MRSA colonization or infection requires MRSA-active antibiotics, not a first-generation cephalosporin like cephalexin, which is only effective against methicillin-susceptible Staphylococcus aureus (MSSA) 1, 2, 3.

Recommended Treatment Algorithm for MRSA Wound Infections

Step 1: Surgical Management First

• Perform incision and drainage immediately for any purulent collection or abscess—this is the cornerstone of therapy and must precede or occur concurrent with antibiotic administration 1, 3.
• Obtain wound cultures from purulent drainage before starting antibiotics to confirm MRSA and guide definitive therapy 1.

Step 2: Oral Antibiotic Selection for MRSA

First-line oral therapy:

• Trimethoprim-sulfamethoxazole (Bactrim) 1-2 double-strength tablets (160mg/800mg) PO twice daily for 7-10 days is the preferred oral agent for uncomplicated MRSA wound infections after adequate drainage 1, 2.

Alternative oral options if Bactrim is contraindicated or fails:

• Doxycycline 100 mg PO twice daily for 7-10 days 1, 2.
• Clindamycin 300-450 mg PO three times daily for 7-10 days if coverage for both MRSA and beta-hemolytic streptococci is needed, though local resistance patterns should be checked 1, 3.

Step 3: When to Use IV Therapy Instead

Use vancomycin 15-20 mg/kg IV every 8-12 hours if:

• Systemic signs of infection are present (fever >38.5°C, heart rate >110 bpm, WBC >12,000/µL, or erythema extending >5 cm from wound edge) 4.
• The patient requires hospitalization or has complicated infection 2, 3.
• Transition to oral Bactrim once systemic signs resolve 2.

Critical Pitfalls to Avoid

Why Cephalexin Fails in MRSA

• Cephalexin and all first-generation cephalosporins lack activity against MRSA—they are only appropriate for MSSA infections 3, 5.
• The "methicillin-resistant" designation means resistance to all beta-lactams including cephalosporins, penicillins, and carbapenems 5.
• Using cephalexin in a patient with known MRSA history will result in treatment failure and potential progression to serious infection 1, 2.

Surgical Site Infection Context

• For surgical site infections specifically, vancomycin is first-line IV therapy when MRSA risk factors are present (prior MRSA infection, recent hospitalization, recent antibiotics, nasal colonization) 4.
• Prophylactic vancomycin dosing (30 mg/kg IV over 120 minutes, ending 30 minutes before incision) is indicated preoperatively in patients with known MRSA colonization 4.
• However, the question asks about treatment of an established open wound, not prophylaxis, so therapeutic dosing is required 2.

Duration and Monitoring

• 7-10 days total antibiotic duration is adequate after adequate source control 1, 2.
• Do not extend antibiotics beyond 10 days unless there is documented inadequate source control or deep tissue involvement 4, 2.
• Monitor for treatment failure at 48-72 hours: persistent fever, expanding erythema, or worsening pain suggests inadequate drainage rather than antibiotic failure 2.

Special Considerations

Contraindications to Bactrim

• Avoid in third trimester pregnancy, infants <2 months, and polymicrobial abdominal/perineal wounds without adding anaerobic coverage 1.
• Requires dose reduction if creatinine clearance 15-30 mL/min; avoid if CrCl <15 mL/min 2.

Never Use These for MRSA

• Do not use rifampin as monotherapy—resistance develops rapidly 3.
• Do not use beta-lactams (including cephalexin), fluoroquinolones, macrolides, or clindamycin if resistance is confirmed 2.