What is the appropriate cefazolin dose for a 15‑year‑old, 35‑kg patient with methicillin‑susceptible Staphylococcus aureus skin‑and‑soft‑tissue infection, normal renal function (estimated creatinine clearance 168 mL/min), and a history of ulcerative colitis?

Cefazolin Dosing for MSSA SSTI in a 15-Year-Old Patient

For this 15-year-old, 35-kg patient with MSSA skin and soft tissue infection and normal renal function, administer cefazolin 1,155 mg (approximately 33 mg/kg/dose) IV every 8 hours.

Dosing Calculation and Rationale

The FDA-approved pediatric dosing for moderate to severe infections is 25-50 mg/kg/day divided into 3-4 doses, with severe infections requiring up to 100 mg/kg/day 1. For MSSA specifically, the IDSA guidelines recommend cefazolin 33 mg/kg/dose every 8 hours for staphylococcal infections 2.

Specific Dose Calculation:

• Weight: 35 kg
• Dose: 33 mg/kg × 35 kg = 1,155 mg every 8 hours
• Daily total: 3,465 mg/day (approximately 99 mg/kg/day)

This falls within the severe infection range and aligns with IDSA recommendations for staphylococcal SSTI 2.

Renal Function Considerations

With a creatinine clearance of 168 mL/min (supranormal, likely due to young age and hyperfiltration), no dose adjustment is required. The FDA label specifies full doses for CrCl ≥55 mL/min 1. This patient's excellent renal function actually supports using the higher end of the dosing spectrum.

Ulcerative Colitis Consideration

The presence of ulcerative colitis does not require dose modification for cefazolin. There are no specific contraindications or dose adjustments for inflammatory bowel disease with cefazolin therapy 1. The primary concern would be monitoring for antibiotic-associated diarrhea or C. difficile, but this doesn't alter the initial dosing strategy.

Pharmacokinetic Support

Recent pharmacokinetic data demonstrate that cefazolin achieves excellent tissue penetration in skin and soft tissue infections, with tissue/serum ratios around 1.06 3. For MSSA with typical MIC90 of 1 mg/L, a 1 g dose every 8 hours achieves >90% probability of target attainment (30% fT>MIC) in tissue 4. Scaling this to pediatric dosing at 33 mg/kg provides comparable exposure.

Practical Administration

• Reconstitute with sterile water per FDA guidelines 1
• Administer IV over 3-5 minutes as direct injection or via intermittent infusion over 30 minutes
• Duration: Typically 5-10 days for uncomplicated SSTI, adjusted based on clinical response
• Monitor: Clinical improvement (reduced erythema, swelling, pain), renal function (though risk is minimal with cefazolin), and signs of treatment failure

Key Clinical Pitfalls to Avoid

1. Underdosing: Using adult fixed doses (1 g q8h) would provide only 28.6 mg/kg/day—inadequate for this weight
2. Ignoring weight-based dosing: Pediatric patients require mg/kg calculations, not adult fixed doses
3. Unnecessary dose reduction: The ulcerative colitis and supranormal CrCl do not warrant dose modification
4. Delayed source control: Ensure any abscesses are drained; antibiotics alone are insufficient for undrained collections

Alternative if Severe or Complicated

If the infection proves more severe than initially apparent (necrotizing features, systemic toxicity, bacteremia), consider escalating to 50 mg/kg/dose every 8 hours (1,750 mg q8h), which remains within FDA-approved limits for severe pediatric infections 1.