What colistin dose in international units (IU) should be given to a 1.6 kg infant?

Colistin Dosing for a 1.6 kg Infant

For a 1.6 kg infant, administer 4,000-8,000 IU (0.13-0.27 mg CBA) of colistin intravenously every 12 hours, which corresponds to 2.5-5 mg CBA/kg/day divided into 2 doses.

Dosing Calculation

The 2022 guideline from the Journal of Microbiology, Immunology and Infection provides the most recent pediatric dosing framework 1:

• Recommended dose: 2.5-5 mg CBA/kg/day IV in 2 or 4 divided doses
• For a 1.6 kg infant:
  • Total daily dose: 4-8 mg CBA/day
  • If divided into 2 doses (q12h): 2-4 mg CBA per dose
  • Converting to IU: 1 mg CBA = approximately 30,000 IU 2
  • Therefore: 4,000-8,000 IU per dose every 12 hours

Dosing Unit Conversion (Critical)

Pay careful attention to unit conversion to avoid dosing errors 2:

• 1 million IU = 80 mg CMS (colistimethate sodium mass) = 33 mg CBA (colistin base activity)
• 1 mg CBA ≈ 30,000 IU
• Dosing should be based on colistin base activity (CBA), not CMS mass

Important Clinical Considerations

Loading Dose Controversy

The FDA and EMA recommend a loading dose of 0.15 MU/kg (approximately 150,000 IU/kg) followed by maintenance dosing 1. However, for a 1.6 kg infant, this would be 240,000 IU as a loading dose. Recent evidence suggests this may be inadequate for pathogens with MIC ≥1 mg/L 1. A 2019 randomized trial in critically ill children demonstrated improved efficacy with loading doses 3, though neonatal-specific data remains limited.

Given the very low weight and vulnerability of this patient, I recommend starting with the maintenance dose without a loading dose unless treating a life-threatening infection with documented multidrug-resistant pathogen, as neonatal safety data for loading doses is insufficient 4, 5.

Renal Function Monitoring

Monitor serum creatinine before treatment and on days 3,5, and 7 6. Nephrotoxicity occurs in 17.8-29.8% of pediatric patients, with most episodes occurring between days 3-7 of treatment 7, 8. The nephrotoxicity is typically reversible upon discontinuation 4, 8.

Combination Therapy

Colistin should always be used in combination with at least one other active antimicrobial agent (typically a carbapenem like meropenem) 1, 2. Monotherapy is associated with higher treatment failure rates and mortality.

Duration and Frequency

• Administer every 12 hours (q12h) rather than every 6-8 hours to reduce nephrotoxicity risk while maintaining efficacy 1
• Typical treatment duration: 10-14 days depending on infection site and clinical response
• For neonates, the mean duration in published case series was 13.1 days (range 5-21 days) 4

Common Pitfalls to Avoid

1. Unit confusion: Always verify whether the prescription is in IU, mg CMS, or mg CBA—these are NOT interchangeable
2. Failure to adjust for renal impairment: Even mild renal dysfunction requires dose reduction 6
3. Inadequate monitoring: Nephrotoxicity can develop rapidly; don't wait for routine weekly labs
4. Monotherapy: Never use colistin alone for systemic infections
5. Concurrent nephrotoxins: Avoid aminoglycosides, vancomycin, and amphotericin B when possible 7

Reconstitution for This Patient

Using a 150 mg (4.5 million IU) vial 6:

• Reconstitute with 2 mL sterile water to achieve 75 mg/mL (2.25 million IU/mL)
• For a 4,000 IU dose: withdraw 0.0018 mL (impractical—requires further dilution)
• Practical approach: Dilute reconstituted solution further in compatible IV fluid (0.9% NaCl or D5W) to achieve measurable volumes for accurate dosing

Administer each dose as a slow IV infusion over 3-5 minutes 6.