Aminoglycoside Dosing Calculator
Calculate extended-interval aminoglycoside doses and monitoring from weight and CrCl. Enter values for instant results with step-by-step formulas.
Reviewed by Rahul Singh, Health & Wellness Specialist
Formula
Dose = DosePerKg x DosingWeight | CrCl = ((140-Age) x Weight) / (72 x SCr) [x 0.85 if female] | Ke = 0.00293 x CrCl + 0.014
Where DosePerKg is the mg/kg dose (7 for gentamicin/tobramycin, 15 for amikacin), DosingWeight is the lesser of actual or adjusted body weight, CrCl is creatinine clearance via Cockcroft-Gault, SCr is serum creatinine, and Ke is the elimination rate constant.
Worked Examples
Example 1: Extended-Interval Gentamicin Dosing
Problem:A 70 kg male, 170 cm tall, age 45, with serum creatinine 1.0 mg/dL needs gentamicin for a gram-negative infection.
Solution:IBW = 50 + 2.3 x ((170/2.54) - 60) = 50 + 2.3 x 6.93 = 65.9 kg\nActual wt (70) < 120% IBW (79.1), so use actual weight = 70 kg\nCrCl = ((140-45) x 70) / (72 x 1.0) = 92.4 mL/min\nVd = 0.25 x 70 = 17.5 L\nKe = 0.00293 x 92.4 + 0.014 = 0.285 hr-1\nDose = 7 mg/kg x 70 = 490 mg\nPredicted peak = 490/17.5 = 28.0 mcg/mL\nInterval = 24 hours (CrCl > 60)
Result:Dose: 490 mg IV q24h | Peak: 28.0 mcg/mL | CrCl: 92.4 mL/min
Example 2: Amikacin Dosing in Obese Patient
Problem:A 110 kg female, 160 cm tall, age 60, with serum creatinine 1.2 mg/dL needs amikacin.
Solution:IBW = 45.5 + 2.3 x ((160/2.54) - 60) = 45.5 + 2.3 x 2.99 = 52.4 kg\n120% IBW = 62.9 kg, actual (110) > 120% IBW, use ABW\nABW = 52.4 + 0.4 x (110 - 52.4) = 52.4 + 23.0 = 75.4 kg\nCrCl = ((140-60) x 75.4) / (72 x 1.2) x 0.85 = 59.4 mL/min\nVd = 0.25 x 75.4 = 18.9 L\nDose = 15 mg/kg x 75.4 = 1131 mg (round to 1100 mg)\nPredicted peak = 1100/18.9 = 58.2 mcg/mL
Result:Dose: 1100 mg IV q36h | ABW: 75.4 kg | CrCl: 59.4 mL/min
Frequently Asked Questions
What is extended-interval aminoglycoside dosing and why is it preferred?
Extended-interval dosing, also called once-daily or high-dose extended-interval administration, involves giving a larger dose less frequently, typically every 24 to 48 hours rather than every 8 hours. This approach takes advantage of the concentration-dependent killing properties and post-antibiotic effect of aminoglycosides. Studies have shown that extended-interval dosing produces equivalent or superior efficacy compared to traditional multiple daily dosing while reducing nephrotoxicity. The prolonged drug-free interval allows renal cortical cells to recover between doses, reducing accumulation and toxicity risk. Most clinical guidelines now recommend extended-interval dosing as the standard approach for most patients.
How is creatinine clearance used in aminoglycoside dosing calculations?
Creatinine clearance is a critical parameter because aminoglycosides are eliminated almost entirely by glomerular filtration in the kidneys. The Cockcroft-Gault equation estimates creatinine clearance using the patient age, weight, serum creatinine, and sex. Lower creatinine clearance indicates reduced renal function, which means the drug is eliminated more slowly, leading to higher trough levels and increased toxicity risk. Patients with creatinine clearance below 60 mL/min typically require extended dosing intervals of 36 to 48 hours. Critically ill patients may have rapidly changing renal function, requiring frequent reassessment and therapeutic drug monitoring to ensure safe and effective dosing.
What is the significance of peak and trough levels in aminoglycoside monitoring?
Peak levels represent the maximum drug concentration achieved after a dose and correlate with antimicrobial efficacy. For extended-interval gentamicin dosing, target peak levels are typically 15 to 25 mcg/mL. Trough levels represent the minimum concentration before the next dose and correlate with toxicity risk, particularly nephrotoxicity and ototoxicity. Target trough levels should generally be below 1 mcg/mL for extended-interval dosing. If trough levels remain elevated, the dosing interval should be extended. Monitoring both peak and trough levels helps clinicians optimize the balance between therapeutic efficacy and minimizing adverse effects throughout the treatment course.
How does body weight affect aminoglycoside dosing calculations?
Aminoglycosides distribute primarily into extracellular fluid, so dosing weight selection is critical for accurate calculations. For patients within 20 percent of their ideal body weight, actual body weight is typically used. For obese patients exceeding 120 percent of ideal body weight, adjusted body weight is calculated using the formula IBW plus 0.4 times the difference between actual and ideal weight. This correction factor of 0.4 accounts for the fact that aminoglycosides partially distribute into adipose tissue but not proportionally to total body weight. Using actual body weight in obese patients would result in excessive doses, while using ideal body weight alone would underdose them.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy