Colistin Sulphate - Dosing Guide

Key Distinction: Two Forms of Colistin

Dose Conversion (Critical)

• 12,500 IU = 1 mg colistimethate (CMS)
• 2.67 mg CMS = 1 mg colistin base activity (CBA)
• 2 million IU = 160 mg CMS = ~60 mg colistin base

1. Colistin Sulphate - Oral (Gut Decontamination / Selective Digestive Decontamination)

• Adults: 1.5-3 million IU (approx. 120-240 mg) 3 times daily orally
• Used for selective digestive decontamination (SDD) in ICU patients
• Not systemically absorbed; acts only in GI tract

2. Colistin Sulphate - Topical

• Applied as powder or solution to wounds, burns, skin infections
• No fixed systemic dose (applied locally to affected area)
• Absorption from intact skin is minimal; take care over large burn surfaces where some absorption may occur

3. IV / Parenteral (via Colistimethate Sodium - CMS)

• Loading dose: 300 mg colistin base IV (given as single dose)
• Maintenance: Start 12 hours after loading dose
  • CrCl >90: 180 mg q12h
  • CrCl 80-90: 170 mg q12h
  • CrCl 70-80: 150 mg q12h

• CrCl ≤40 mL/min: loading dose 300 mg, then start maintenance at 24 hours (not 12h)

• Hemodialysis: 130 mg/day + 50 mg supplemental on dialysis days (4-hr session)
• CRRT: 220 mg q12h

4. Inhaled (CMS - adjunct for pneumonia/VAP)

• Adults: 75 mg or 150 mg colistin base inhaled q12h
  • For VAP: 150 mg q8h may be used
• Children <13 years: 30 mg or 75 mg colistin base inhaled q12h
• Should NOT be used as monotherapy for pneumonia - combine with IV antimicrobial

5. Pediatric IV Dosing (CMS)

• Loading dose: 4-5 mg CBA/kg (~118,000-150,000 IU/kg)
• Maintenance: 2.5 mg CBA/kg every 12 hours
• Note: Standard 5 mg/kg/day without a loading dose produces subtherapeutic levels - a loading dose is now recommended

Toxicity Monitoring

• Nephrotoxicity - most significant; monitor creatinine, urine output
• Neurotoxicity - paraesthesias, dizziness, neuromuscular blockade
• Avoid concurrent nephrotoxins (aminoglycosides, NSAIDs, contrast) where possible

Clinical Notes

• Colistin is reserved for MDR/XDR Gram-negative infections (carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae)
• The IDSA 2024 AMR Guidance found colistin + meropenem combination showed no mortality benefit over colistin monotherapy for CRAB infections
• Expert ID consultation is recommended given dosing complexity - Goodman & Gilman's 14th ed. explicitly states this

Walamycin Drop / Suspension

Composition

Uses

1. Gastrointestinal infections - diarrhea in infants and children caused by susceptible Gram-negative organisms (especially E. coli, Shigella)
2. Bowel sterilization - pre-operative gut decontamination before GI surgery
3. Superficial infections of the external ear canal - otitis externa (as a topical drop)
4. Mastoidectomy and fenestration cavities - post-surgical ear care

Dosage

• Adults: 1.5-3 million IU (approx. 120-240 mg colistin base) 3 times daily
• Children: based on body weight - typically 15,000-25,000 IU/kg/day in 3 divided doses orally
  • Practical dose: ~5-10 mL of the 12.5 mg/5 mL suspension every 8 hours (as directed by physician)

• 3-4 drops instilled into the affected ear 3-4 times daily
• Duration: 5-7 days (as prescribed)

Key Points

• Colistin sulphate is not absorbed orally - it acts only within the GI lumen; safe for gut decontamination without systemic effects
• Absorption from the ear canal is minimal in intact tympanic membrane; do not use if eardrum is perforated
• Prescription (Rx) required in most settings
• Avoid in: myasthenia gravis, known colistin hypersensitivity, impaired renal function (systemic forms), colitis
• Drug interactions: tacrolimus, vancomycin, aminoglycosides (additive nephrotoxicity/neurotoxicity), live cholera vaccine

Note: One online source (Medicoverhospitals) incorrectly describes Walamycin as an erythromycin/macrolide - this is inaccurate. The verified composition from MediBuddy and Lybrate confirms it contains Colistin Sulphate 12.5 mg.

Walamycin (Colistin Sulphate) - Dose for 6-Month-Old Infant

Average weight of a 6-month infant: ~7-8 kg

Oral Dose (GI infections / diarrhea)

50,000 IU/kg/day divided into 3 doses (every 8 hours)

• Daily dose = 7 × 50,000 = 350,000 IU/day
• Per dose (TDS) = ~116,000 IU every 8 hours

• 12.5 mg = ~400,000 IU colistin sulphate (1 mg colistin sulphate ≈ 32,000 IU)
• Typical practical dose for a 6-month infant: ~1.25 mL (= 3.125 mg) three times daily

Always confirm the exact dose with the prescribing doctor as it depends on the child's actual weight and indication.

Simple Weight-Based Guide

Important Notes for Infants

• Give orally - acts only in the gut (not absorbed systemically)
• Can be given with or without food
• Shake the suspension well before each dose
• Complete the full course even if symptoms improve
• Do not use if the infant has known hypersensitivity to colistin or myasthenia gravis
• Monitor for: nausea, vomiting, loose stools
• Store between 10-25°C, away from sunlight

The Medicoverhospitals source quoted erythromycin doses (30-50 mg/kg/day) which are incorrect for Walamycin - those figures do not apply here. The correct active ingredient is Colistin Sulphate, and the doses above reflect that.