Colistin Dosing in a 17-Year-Old with Renal Impairment
Yes, use the adult Cockcroft-Gault formula to calculate creatinine clearance, then apply the FDA-approved colistin renal dosing adjustments from the package insert—there are no separate "pediatric renal adjustment charts" for colistin, as the FDA label provides weight-based dosing that applies to both adults and pediatric patients with renal impairment. 1
Why Use Adult Cockcroft-Gault Formula
For medication dosing decisions, the Cockcroft-Gault formula is the standard because drug manufacturers and pharmacokinetic studies (including those establishing colistin dosing) have historically used this formula to create renal dosing guidelines. 2
The FDA colistin label explicitly states dosing for "Adults and Pediatric Patients" without age-based distinctions, using weight-based dosing (2.5-5 mg/kg/day of colistin base) adjusted for creatinine clearance. 1
At 17 years old and 35 kg, this patient should be treated as an adult for creatinine clearance calculation purposes, as guideline societies recommend using adult formulas (Cockcroft-Gault) for patients 18 years and older, and this patient is essentially at that threshold. 3
Calculating Creatinine Clearance
Apply the Cockcroft-Gault formula: CrCl (mL/min) = [(140 - age) × weight (kg)] / [72 × serum creatinine (mg/dL)] × (0.85 if female). 2
Critical consideration for low body weight: In a 35 kg patient, serum creatinine may appear "near normal" but actually represent significant renal impairment—never rely on serum creatinine alone. 4, 5
For this underweight patient, use actual body weight (35 kg) in the Cockcroft-Gault formula, not ideal body weight, as the formula was designed for actual weight and ideal body weight would underestimate clearance. 2, 6
Applying FDA Colistin Renal Dosing
Once you calculate the creatinine clearance, use Table 1 from the FDA label 1:
CrCl ≥80 mL/min (Normal): 2.5-5 mg/kg divided into 2-4 doses per day
CrCl 50-79 mL/min (Mild impairment): 2.5-3.8 mg/kg divided into 2 doses per day
CrCl 30-49 mL/min (Moderate impairment): 2.5 mg/kg once daily or divided into 2 doses per day
CrCl 10-29 mL/min (Severe impairment): 1.5 mg/kg every 36 hours
The maximum daily dose should not exceed 5 mg/kg/day with normal renal function, and this must be reduced proportionally with declining renal function. 1
Critical Pitfalls to Avoid
Do not use ideal body weight for colistin dosing in this underweight patient—the FDA label states "in obese individuals, dosage should be based on ideal body weight," but this 35 kg patient is underweight, not obese, so use actual body weight. 1
Colistin nephrotoxicity is dose-dependent and reversible, but overdosing can cause renal insufficiency, muscle weakness, and apnea—strict adherence to renal-adjusted dosing is essential. 1
Monitor renal function closely during therapy, as colistin itself can cause acute kidney injury, particularly in patients with normal baseline renal function (paradoxically). 7
The Cockcroft-Gault formula may underestimate true GFR in young, low-weight patients, potentially leading to underdosing—if clinical response is inadequate and the patient tolerates therapy well, consider that actual renal function may be better than estimated. 4, 8
There Is No Separate Pediatric Renal Chart
The FDA label provides a single unified dosing table (Table 1) that applies to "Adults and Pediatric Patients" without age-based distinctions—the adjustments are based solely on creatinine clearance and weight. 1
Research confirms that colistin dosing in clinical practice often deviates from manufacturer recommendations, particularly in patients with renal impairment, which is associated with worse outcomes—follow the FDA table strictly. 7