What is the appropriate dosing regimen of colistimethate sodium (CMS) for a patient on maintenance slow low‑efficiency dialysis (SLED) with acute kidney injury?

Colistimethate Sodium Dosing for Patients on SLED

For patients on maintenance SLED, administer a loading dose of 6-9 million IU CMS followed by 4.5 million IU every 12 hours on SLED days, with dose reduction to 2.5 million IU every 12 hours on non-SLED days.

Dosing Algorithm for SLED Patients

Loading Dose (All Patients)

• Give 6-9 million IU CMS regardless of renal function or dialysis status 1
• Administer this even in patients with severe renal impairment
• This is critical because colistin has a long half-life and takes 2-3 days to reach steady state without loading 1

Maintenance Dosing Strategy

On SLED Days:

• 4.5 million IU CMS every 12 hours 1
• SLED removes approximately 67% of total colistin clearance 2
• The extracorporeal clearance during SLED (3.49 L/h) substantially exceeds non-dialysis clearance (1.69 L/h) 2
• Time the dose so dialysis occurs toward the end of the dosing interval when possible 1

On Non-SLED Days:

• Reduce to 2.5 million IU CMS every 12 hours (equivalent to the intermittent hemodialysis recommendation) 1
• This accounts for the patient's underlying severe renal impairment when not receiving dialysis

Critical Pharmacokinetic Considerations

The evidence is clear that SLED dramatically increases colistin clearance and must be accounted for in dosing:

• Standard dosing regimens designed for continuous renal replacement therapy (CRRT) frequently fail to achieve pharmacodynamic targets (fAUC/MIC ≥12) in SLED patients 3
• SLED clearance accounts for up to 67% of total drug elimination on dialysis days 2
• The apparent volume of distribution is 50.2 L, requiring adequate loading to achieve therapeutic levels 2

Monitoring Requirements

Renal Function Monitoring:

• Monitor serum creatinine closely during therapy 4
• Colistin-associated AKI is a major concern, though the incidence may be lower than historically reported (14.7-20.4% in recent studies) 5, 6
• Patients with pre-existing chronic kidney disease have significantly higher AKI risk 5

Clinical Response Assessment:

• Evaluate clinical response at 48-72 hours
• Consider therapeutic drug monitoring if available, though this is not routinely accessible 7, 8

Common Pitfalls to Avoid

1. Underdosing on SLED days: The most critical error is failing to increase the dose on dialysis days. The 2015 ICU guidelines recommend "at least 9 million IU/day" for CRRT 1, and SLED has similar clearance characteristics requiring higher dosing than non-dialysis days.

2. Omitting the loading dose: Without a loading dose, therapeutic levels take 2-3 days to achieve, during which time the patient has suboptimal drug exposure 1

3. Using the same dose on dialysis and non-dialysis days: This leads to either toxicity on non-dialysis days or subtherapeutic levels on dialysis days 2

4. Ignoring concomitant nephrotoxins: Over 50% of patients receive other nephrotoxic agents; minimize these when possible 5

Alternative Consideration

Polymyxin B may be preferable in SLED patients because:

• Dosing is 1.5-3 mg/kg/day (loading dose 2-2.5 mg/kg) with no adjustment needed for renal replacement therapy 1
• Lower incidence of nephrotoxicity compared to colistin 1
• Not administered as a prodrug, avoiding the complex pharmacokinetics of CMS conversion 1

However, if colistin is chosen (often due to availability or formulary restrictions), the above SLED-specific dosing strategy is essential for optimizing the balance between efficacy and nephrotoxicity.