What is the recommended dose of cloxacillin (amoxicillin) for a 9.4 kg pediatric patient with impetigo suspected to be caused by methicillin-susceptible Staphylococcus aureus (MSSA) using a 250 mg/5 ml suspension?

Cloxacillin Dosing for 9.4 kg Pediatric Patient with MSSA Impetigo

For a 9.4 kg child with impetigo suspected to be MSSA, administer cloxacillin (or dicloxacillin) at 25-40 mg/kg/day divided into 3-4 doses, which translates to 235-376 mg per day total, or approximately 60-95 mg per dose given 4 times daily using your 250 mg/5 mL suspension (1.2-1.9 mL per dose). 1

Dosing Calculation

• Weight-based calculation: 9.4 kg × 30 mg/kg/day (mid-range dosing) = 282 mg/day total 1
• Divided into 4 doses: 282 mg ÷ 4 = approximately 70 mg per dose 1
• Volume per dose: 70 mg ÷ 50 mg/mL = 1.4 mL four times daily 1
• Alternative 3-times-daily regimen: 94 mg per dose = 1.9 mL three times daily 1

Clinical Context for MSSA Impetigo

• For minor skin infections like impetigo in children, topical mupirocin 2% ointment is the first-line recommendation and may be sufficient for localized disease 2
• Oral antibiotics are indicated when topical therapy is impractical or when there are multiple lesions, bullous impetigo, or systemic signs 3
• Penicillin alone is NOT appropriate for impetigo because most S. aureus strains (94.5%) are penicillin-resistant due to beta-lactamase production 3, 4
• Penicillinase-resistant penicillins (flucloxacillin, dicloxacillin) remain the antibiotics of choice for MSSA skin infections 5

Alternative Oral Antibiotic Options

If cloxacillin/dicloxacillin is unavailable or the patient has penicillin allergy:

• Cephalexin: First-generation cephalosporin, effective for MSSA impetigo 3, 6
• Clindamycin: 25-40 mg/kg/day divided into 3 doses (78-125 mg per dose for 9.4 kg child), but only if local clindamycin resistance rates are <10% 2, 1
  • Critical caveat: Clindamycin resistance in S. aureus causing impetigo can be as high as 69.6-90.9% in some regions 4, 6
• Amoxicillin-clavulanate: 45 mg/kg/day of amoxicillin component for MSSA coverage (requires beta-lactamase inhibitor) 3

Treatment Duration and Monitoring

• Duration: 5-10 days based on clinical response, with 7 days being typical for uncomplicated impetigo 2
• Expected response: Clinical improvement should be evident within 48-72 hours of starting appropriate antibiotic therapy 1, 7
• If no improvement by 48-72 hours: Consider inadequate coverage (possible CA-MRSA), poor adherence, or alternative diagnosis 2, 1

Important Clinical Pitfalls

• Do not use penicillin V or amoxicillin alone for impetigo, as they lack activity against beta-lactamase-producing S. aureus 3, 4
• Verify local resistance patterns: If your region has high rates of CA-MRSA (>10%), empiric coverage with TMP-SMX or clindamycin may be needed instead 2
• Bullous impetigo requires systemic therapy: Topical agents alone are insufficient for bullous variants 8, 6
• Screen for nasal carriage: Consider nasal swabs of patient and household contacts to identify asymptomatic S. aureus carriers who may be the source of recurrent infections 8