What is the appropriate dose, dosing interval, and duration of cloxacillin (250 mg/5 mL oral suspension) for a 35‑kg child?

Cloxacillin Dosing for a 35 kg Pediatric Patient

For a 35 kg child, cloxacillin 250 mg/5 mL oral suspension should be dosed at 50 mg/kg/day divided into four doses (approximately 440 mg or 8.8 mL every 6 hours) for a typical duration of 7–10 days, depending on the specific infection being treated.

Weight-Based Dose Calculation

• The standard pediatric dosing for cloxacillin is 50 mg/kg/day divided every 6 hours for most staphylococcal infections 1.
• For a 35 kg child, this equals 1,750 mg total daily dose, administered as 437.5 mg (approximately 8.8 mL of 250 mg/5 mL suspension) every 6 hours 1.
• Round to a practical volume of 9 mL every 6 hours for ease of administration.

Dosing Frequency Rationale

• Cloxacillin is a time-dependent antibiotic requiring the serum concentration to remain above the minimum inhibitory concentration (MIC) for at least 40% of the dosing interval to achieve optimal bactericidal activity 2.
• Four times daily dosing (every 6 hours) is essential to maintain therapeutic concentrations throughout the day, as cloxacillin has a relatively short half-life 2.
• Do not use twice-daily or three-times-daily dosing, as this will result in subtherapeutic trough levels and treatment failure 2.

Treatment Duration by Indication

Skin and Soft Tissue Infections

• 7 days is the standard duration for uncomplicated cellulitis, impetigo, or wound infections caused by methicillin-sensitive Staphylococcus aureus (MSSA) 3.
• Extend to 10 days if there is slow clinical response or if the infection involves deeper structures 3.

Bone and Joint Infections

• For osteomyelitis or septic arthritis, intravenous therapy is preferred initially, but if oral cloxacillin is used for step-down therapy, continue for a minimum of 3–4 weeks total (IV + oral combined) 2.

Bacteremia

• Uncomplicated MSSA bacteremia requires at least 14 days of therapy; complicated bacteremia (e.g., with endocarditis or deep-seated infection) requires 4–6 weeks 2.

Clinical Monitoring and Expected Response

• Clinical improvement should be evident within 48–72 hours of initiating therapy: reduction in fever, decreased erythema and swelling, and improved pain 3.
• If no improvement or worsening occurs after 72 hours, reassess the diagnosis, obtain cultures if not already done, and consider:
  • Inadequate source control (abscess requiring drainage)
  • Methicillin-resistant S. aureus (MRSA) requiring a switch to clindamycin or trimethoprim-sulfamethoxazole
  • Alternative diagnosis (e.g., viral infection, inflammatory condition) 3.

Administration and Absorption Considerations

• Administer on an empty stomach (1 hour before or 2 hours after meals) to maximize absorption, as food significantly reduces bioavailability 1.
• Oral bioavailability in children is approximately 60–80%, with younger children (<6 months) showing lower and more variable absorption 4.
• For a 35 kg child (likely school-age), absorption should be reliable if taken on an empty stomach 4.

When Oral Cloxacillin Is Inappropriate

• Do not use oral cloxacillin for severe, life-threatening infections (e.g., septic shock, necrotizing fasciitis, severe osteomyelitis) where intravenous therapy is mandatory 2.
• Switch to intravenous cloxacillin (or an alternative IV anti-staphylococcal agent) if the child cannot tolerate oral intake, has malabsorption, or shows clinical deterioration 2.
• For MRSA infections, cloxacillin is ineffective; use clindamycin, trimethoprim-sulfamethoxazole, or vancomycin depending on severity 2.

Common Pitfalls to Avoid

• Do not underdose: Using 25 mg/kg/day (half the recommended dose) will result in subtherapeutic levels and treatment failure 5, 2.
• Do not extend dosing intervals beyond 6 hours: Twice-daily or three-times-daily dosing fails to maintain adequate time above MIC 2.
• Do not administer with food: This reduces absorption by up to 50% 1.
• Do not use for empiric therapy without culture confirmation: Cloxacillin has a narrow spectrum and is only effective against MSSA; if MRSA is suspected or confirmed, an alternative agent is required 2.

Practical Dosing Summary for a 35 kg Child

Parameter	Recommendation
Total daily dose	1,750 mg (50 mg/kg/day)
Dose per administration	440 mg (≈9 mL of 250 mg/5 mL suspension)
Frequency	Every 6 hours (four times daily)
Timing	1 hour before or 2 hours after meals
Duration	7–10 days for skin/soft tissue infections; longer for bone/joint or bacteremia

5, 2, 4, 3, 1