Colistin Dose Conversion
2.25 million international units (MIU) of colistin is equivalent to 180 mg of colistimethate sodium (CMS) or 74.25 mg of colistin base activity (CBA).
Conversion Formula
The conversion between colistin units follows this standardized relationship 1:
- 1 million IU = 80 mg CMS = 33 mg CBA 2
Therefore, for 2.25 MIU:
- CMS mass: 2.25 × 80 = 180 mg
- Colistin base activity: 2.25 × 33 = 74.25 mg CBA
Critical Dosing Context
This dose (2.25 MIU) represents a relatively low dose in modern practice. Current guidelines recommend significantly higher dosing for critically ill patients 1:
- Loading dose: 6-9 million IU 1
- Maintenance dose: 4.5 million IU every 12 hours (9 million IU/day total) 1
The 2.25 MIU dose would only be appropriate in specific scenarios:
- Patients with severe renal impairment (CrCl 10-29 mL/min) receiving maintenance dosing 3
- Patients on intermittent hemodialysis (2 million IU every 12 hours) 1
Important Conversion Warnings
Multiple dosing nomenclatures exist for colistin, which frequently causes medication errors 1. Always verify:
- Whether the dose is expressed as international units (IU), CMS mass (mg), or colistin base activity (CBA in mg)
- The specific formulation being used (colistimethate sodium vs. colistin sulfate)
For colistin sulfate (different formulation):
- 1 mg = 22,700 IU 4
For polymyxin B sulfate (different drug entirely):
- 1 mg = 10,000 IU 4
Never confuse these formulations—they are not interchangeable and use different conversion factors.