What is the appropriate piperacillin‑tazobactam dose for a 5‑month‑old infant weighing 5.8 kg?

Piperacillin-Tazobactam Dosing for a 5-Month-Old Infant Weighing 5.8 kg

For a 5-month-old infant weighing 5.8 kg, administer piperacillin-tazobactam 80 mg/kg (of the piperacillin component) every 8 hours, infused over 30 minutes, which equals 464 mg per dose.

FDA-Approved Dosing Framework

• The FDA label specifies that for infants aged 2 to 9 months with appendicitis/peritonitis, the dose is 90 mg/kg (80 mg piperacillin + 10 mg tazobactam) every 8 hours administered as a 30-minute infusion. 1

• For nosocomial pneumonia in this same age group (2 to 9 months), the FDA recommends 90 mg/kg every 6 hours to provide more aggressive coverage. 1

• Your 5-month-old patient falls squarely within the 2–9 month age bracket, so the FDA-approved regimen of 80 mg/kg piperacillin component every 8 hours is the appropriate starting point for most serious infections. 1

Weight-Based Dose Calculation

• For a 5.8 kg infant, the calculation is:
  80 mg/kg × 5.8 kg = 464 mg of piperacillin per dose
  (This corresponds to approximately 58 mg of tazobactam per dose, maintaining the 8:1 ratio.) 1

• Administer this dose every 8 hours (three times daily) via 30-minute intravenous infusion. 1

Indication-Specific Adjustments

• For appendicitis, peritonitis, or intra-abdominal infections: the standard FDA regimen of 80 mg/kg every 8 hours is appropriate. 1

• For nosocomial pneumonia or more severe hospital-acquired infections: consider increasing the frequency to every 6 hours (same 80 mg/kg dose, but given four times daily instead of three), as endorsed by the FDA label for this age group. 1

• For critically ill infants or infections caused by organisms with higher MICs (e.g., Pseudomonas aeruginosa with MIC ≥16 mg/L): research data suggest that extended infusions (2–4 hours) or higher total daily doses may be needed to achieve optimal pharmacodynamic targets, though these regimens are not yet FDA-approved for routine use in this age group. 2, 3, 4, 5

Developmental Pharmacokinetic Considerations

• Infants aged 2–6 months have immature renal function, which reduces piperacillin clearance compared to older children; this is why the FDA dose for 2–9 month-olds is lower (80 mg/kg) than for children >9 months (100 mg/kg). 1, 4, 6

• Population pharmacokinetic studies confirm that postmenstrual age (PMA) and body weight are the primary determinants of piperacillin clearance in young infants, supporting the FDA's age-stratified dosing approach. 4, 6

• A study in infants <61 days old found that PMA-based dosing (100 mg/kg every 8 hours for PMA ≤30 weeks, 80 mg/kg every 6 hours for PMA 30–35 weeks, and 80 mg/kg every 4 hours for PMA 35–49 weeks) achieved therapeutic targets in 90% of simulated infants against organisms with MIC ≤32 mg/L. 4 However, your 5-month-old patient is well beyond the neonatal period, so the FDA's 2–9 month dosing (80 mg/kg every 8 hours) is more applicable. 1

Reconstitution and Administration

• Reconstitute a 2.25 gram vial with 10 mL of compatible diluent (0.9% sodium chloride or sterile water for injection) to achieve a concentration of 202.5 mg/mL (180 mg/mL piperacillin + 22.5 mg/mL tazobactam). 1

• Withdraw the required volume for the calculated dose:
  For 464 mg piperacillin, you need approximately 2.6 mL of the reconstituted solution (464 mg ÷ 180 mg/mL = 2.58 mL). 1

• Further dilute this volume in a compatible IV solution (0.9% sodium chloride, D5W, or sterile water) to a final piperacillin concentration of 20–80 mg/mL (tazobactam 2.5–10 mg/mL), typically in a 10–25 mL total volume for an infant. 1

• Infuse over 30 minutes using a programmable syringe pump or infusion pump; during infusion, discontinue any primary IV solution. 1

Monitoring and Safety

• Assess clinical response within 48–72 hours: fever should decrease, and signs of infection (e.g., tachycardia, hypotension, leukocytosis) should improve. 2, 3

• Monitor renal function (serum creatinine, urine output) because piperacillin is primarily renally eliminated; the FDA label does not provide specific dosing adjustments for pediatric renal impairment, so consult nephrology if creatinine clearance is reduced. 1

• Watch for infusion-site reactions: in one study, 3 of 79 children (3.8%) experienced site infiltrations considered definitely related to extended infusions, though these were the only adverse events directly attributed to the infusion method. 2

• Common adverse events in pediatric studies included diarrhea, rash, and transient transaminase elevations, occurring in approximately 49% of children, though most were mild and not directly attributed to the drug. 2

Common Pitfalls to Avoid

• Do not use the >9 month dosing (100 mg/kg every 8 hours) in a 5-month-old: the FDA explicitly stratifies dosing by age because younger infants have lower clearance and require the 80 mg/kg dose to avoid underdosing. 1, 6

• Do not mix piperacillin-tazobactam with lactated Ringer's solution: it is incompatible and will precipitate. 1

• Do not add piperacillin-tazobactam to solutions containing only sodium bicarbonate or solutions that significantly alter pH, as this causes chemical instability. 1

• Do not freeze reconstituted vials: use immediately or store at room temperature for up to 24 hours or refrigerated for up to 48 hours. 1

• Do not underdose severe infections: if your patient has nosocomial pneumonia or sepsis, consider the every-6-hour regimen (same 80 mg/kg dose, but four times daily) to maximize time above MIC. 1, 3, 5