Aminoglycoside Dose Calculator — Gentamicin, Tobramycin, Amikacin

How to Use This Calculator

1. Select drug (gentamicin, tobramycin, amikacin) and enter dosing weight (IBW or AdjBW).
2. Enter CrCl and select regimen (once-daily, conventional, or synergy).
3. Dose, interval, and monitoring targets appear instantly.
4. Use the Once-Daily tab for Hartford nomogram guidance.
5. Use the Professional tier for obesity weight selection and toxicity notes.

Formula

Example

Frequently Asked Questions

• How are aminoglycosides dosed? ▼
  Aminoglycosides (gentamicin, tobramycin, amikacin) exhibit concentration-dependent killing — the higher the peak concentration relative to the MIC (Cmax/MIC ratio), the greater the bactericidal effect and the more pronounced the post-antibiotic effect (PAE), allowing continued bacterial suppression after drug concentrations fall below the MIC. This pharmacodynamic property supports once-daily dosing. Dosing weight uses ideal body weight (IBW) for non-obese patients, calculated as 50 kg + 2.3 kg per inch over 5 feet (males) or 45.5 kg + 2.3 kg per inch over 5 feet (females). For obese patients (ABW > 120% of IBW), adjusted body weight (AdjBW = IBW + 0.4 × [ABW − IBW]) is used. For underweight patients, actual body weight is used if less than IBW. Once-daily dosing uses 5–7 mg/kg (gentamicin/tobramycin) or 15 mg/kg (amikacin) as a single daily dose, with interval adjustment for renal impairment. Conventional dosing uses 1.5–2 mg/kg every 8 hours. Synergy dosing for endocarditis uses 1 mg/kg every 8–12 hours.
• What is the difference between once-daily and conventional aminoglycoside dosing? ▼
  Once-daily (extended-interval) dosing, also called high-dose extended-interval or Hartford dosing, administers the full daily dose as a single large bolus. It exploits the concentration-dependent bactericidal mechanism by achieving a very high Cmax/MIC ratio (target > 8–10:1), maximising the PAE and allowing drug-free intervals during which tubular cells can recover from aminoglycoside uptake, reducing nephrotoxicity. Multiple meta-analyses and randomised trials show once-daily dosing has equivalent or superior efficacy and significantly lower nephrotoxicity compared to conventional dosing for most indications. Conventional multiple-daily dosing (1.5–2 mg/kg every 8 hours) uses time-averaged lower concentrations monitored by peak and trough levels. Exceptions where once-daily dosing is not recommended include: endocarditis (synergy requires continuous low-level drug; data from endocarditis trials used conventional dosing); pregnancy (altered pharmacokinetics require conventional dosing with levels); severe renal impairment (CrCl < 20 mL/min, where accumulation in once-daily dosing is difficult to manage); and paediatrics under 1 month of age (neonates).
• Why monitor aminoglycoside levels? ▼
  Aminoglycoside monitoring serves two purposes: to confirm efficacy and to prevent toxicity. For once-daily dosing, the Hartford Nomogram approach draws a random serum level 6–14 hours post-dose and plots it on the nomogram to confirm the selected interval (q24h, q36h, or q48h) achieves a drug-free period long enough for tubular recovery. A trough < 1 mg/L before the next once-daily dose confirms adequate washout. For conventional dosing, a peak of 5–10 mcg/mL (gentamicin/tobramycin) or 20–30 mcg/mL (amikacin) confirms adequate bactericidal exposure, while a trough < 2 mcg/mL (gentamicin/tobramycin) or < 5 mcg/mL (amikacin) minimises accumulation in renal tubular cells (which take up drug via the megalin receptor in the brush border). Elevated troughs predict nephrotoxicity. Serial monitoring (every 3–5 days for conventional dosing) is required during prolonged courses because renal function can change. Baseline audiometry should be considered for courses exceeding 14 days, given the irreversible nature of aminoglycoside ototoxicity.
• When are aminoglycosides preferred over other antibiotics? ▼
  Aminoglycosides are most valued for their concentration-dependent, rapidly bactericidal activity against aerobic gram-negative bacilli, including Pseudomonas aeruginosa, Klebsiella, E. coli, and Acinetobacter. Primary indications include: serious gram-negative infections where beta-lactam resistance is a concern, particularly hospital-acquired or ventilator-associated pneumonia; in combination with beta-lactams for synergistic bactericidal activity against Enterococcus or Streptococcal endocarditis (synergy dosing protocol); as part of empiric sepsis regimens where broad gram-negative coverage is needed while awaiting cultures; Pseudomonas infections (gentamicin or tobramycin); Mycobacterium avium complex (amikacin in combination regimens for MAC lung disease); inhaled tobramycin for chronic Pseudomonas suppression in cystic fibrosis; and tularaemia and plague (gentamicin preferred). They are not appropriate for gram-positive coverage alone, anaerobic infections (aminoglycosides are inactive in anaerobic environments — the drug transport mechanism requires oxygen), or as monotherapy when bacteriostatic activity is sufficient.
• How does renal function affect aminoglycoside dosing? ▼
  Aminoglycosides are eliminated almost entirely unchanged by glomerular filtration, making renal function the primary determinant of drug clearance. As creatinine clearance (CrCl) decreases, drug half-life increases proportionally. For once-daily dosing, normal renal function (CrCl ≥ 60 mL/min) supports q24h dosing; CrCl 40–60 extends to q36h; CrCl 20–40 requires q48h. Below CrCl 20 mL/min, once-daily dosing is not recommended due to unpredictable accumulation — conventional dosing with aggressive level monitoring or alternative antibiotics should be used. Patients on haemodialysis should receive a loading dose after each dialysis session (aminoglycosides are partially dialysed, approximately 50% removed per session); levels must be checked before each dose. Peritoneal dialysis patients require special dosing — aminoglycosides can be given intraperitoneally for peritonitis. Acute kidney injury requires recalculating CrCl at least daily, as rapidly declining renal function makes fixed-interval regimens dangerous without corresponding level monitoring to detect accumulation before toxicity occurs.

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