What is the appropriate colistimethate sodium (CMS) dosing regimen for a patient with chronic kidney disease?

Colistin Dosing in Chronic Kidney Disease

In patients with chronic kidney disease, administer a full loading dose of 9 MU (million units) of colistimethate sodium (CMS) regardless of renal function, followed by maintenance doses adjusted according to creatinine clearance using the formula: 4.5 MU × [2.5 mg/(1.5 × CrCl + 30)] twice daily. 1

Loading Dose Strategy

The loading dose should always be given at full strength (9 MU or 5 mg/kg) even in patients with severe CKD 1, 2. This is critical because:

• CMS is a prodrug that must convert to active colistin, creating a delay in achieving therapeutic levels
• Subtherapeutic concentrations during the first 2-3 days are associated with higher mortality 2
• The loading dose primarily fills the distribution volume, not dependent on renal clearance

Maintenance Dose Adjustment by Renal Function

After the loading dose, adjust maintenance dosing based on creatinine clearance (CrCl):

For CrCl ≥50 mL/min:

• 4.5 MU every 12 hours 1, 2

For CrCl 30-49 mL/min:

• 2.5 mg/kg once daily or divided into 2 doses 3
• Alternatively: Use the formula-based approach from guidelines 1

For CrCl 10-29 mL/min:

• 1.5 mg/kg every 36 hours 3

For patients on hemodialysis:

• 2 MU every 12 hours with normal loading dose 2
• Schedule dialysis toward the end of the CMS dosing interval

For continuous renal replacement therapy (CRRT):

• At least 9 MU per day 2
• No dose adjustment needed as extracorporeal clearance can account for up to 67% of total drug clearance 4

Critical Safety Considerations

Monitor renal function closely throughout treatment 1. This is a strong recommendation because:

• Acute kidney injury during colistin treatment is a major factor related to clinical failure and mortality 1
• Patients with pre-existing CKD are at higher risk for further kidney injury 1
• Nephrotoxicity occurred in 33.9% of CKD patients in one cohort but was reversible in 52.6% 5

Common Pitfalls to Avoid

1. Underdosing is prevalent: Studies show clinicians use almost half the recommended doses in CKD patients (3.36 vs 6.07 MU daily) 5. This leads to subtherapeutic colistin plasma concentrations (<2 mg/L in 83.3% of patients) 5.

2. Reducing the loading dose in renal impairment: This is incorrect. The loading dose should be full strength regardless of kidney function 1, 2, 6.

3. Using spot urine or single measurements: These can lead to measurement bias and poor adherence assessment 7.

4. Failing to account for augmented renal clearance: Patients with preserved or enhanced renal function may need higher doses (up to 3 MU q8h) to achieve therapeutic targets, though this carries nephrotoxicity risk >33% 8.

Therapeutic Drug Monitoring

Target colistin steady-state plasma concentration (Css) of ≥2 mg/L for optimal efficacy 5. Consider therapeutic drug monitoring in:

• Patients with fluctuating renal function
• Those with high MIC organisms (≥1 mg/L)
• Critically ill patients with augmented renal clearance

Alternative Consideration

Polymyxin B may be preferable in CKD patients 2. Unlike CMS, polymyxin B:

• Does not require dose adjustment for renal function
• Has lower incidence of nephrotoxicity compared to colistin 2
• Recommended dose: 1.5-3 mg/kg/day with loading dose of 2-2.5 mg/kg 2