The gentamicin once-daily dosing calculator provides an extended-interval aminoglycoside starting dose and suggests a dosing interval based on renal function, following the Hartford-style nomogram used in antimicrobial stewardship.
How it works
The dose is simply the target per kilogram times the dosing weight:
Dose (mg) = target(mg/kg) × dosing weight(kg)Common targets are 5–7 mg/kg, with 7 mg/kg used for serious Gram-negative
sepsis. The result is usually rounded to a practical increment (for example to
the nearest 20 mg).
The interval is chosen from creatinine clearance using the Hartford nomogram bands:
- CrCl > 60 mL/min → every 24 hours.
- CrCl 40–60 mL/min → every 36 hours.
- CrCl 20–40 mL/min → every 48 hours.
- CrCl < 20 mL/min → avoid extended interval; use conventional dosing with levels.
A drug level drawn 6–14 hours after the dose is plotted on the nomogram to confirm or lengthen the interval.
Example and notes
A 70 kg patient with a CrCl of 75 mL/min dosed at 7 mg/kg receives
7 × 70 = 490 mg, rounded to about 480–500 mg, every 24 hours.
Extended-interval dosing is generally avoided in pregnancy, major burns, ascites, endocarditis, cystic fibrosis, and severe renal impairment, where conventional dosing with peak and trough monitoring is preferred. This tool gives only a starting dose and suggested interval — always confirm with therapeutic drug monitoring and follow local protocol.
Why extended-interval dosing is used
Gentamicin, like all aminoglycosides, kills bacteria concentration-dependently: a single high peak concentration is more effective than the same total dose spread across smaller, more frequent doses. Extended-interval dosing exploits this by delivering the full daily dose as one large hit, maximizing the peak-to-MIC ratio. There is also a “post-antibiotic effect” — suppression of bacterial regrowth for several hours after the drug concentration falls below the MIC — which creates a natural window before the next dose without losing efficacy.
Choosing the dosing weight in practice
Weight choice has a real impact on the calculated dose, so getting it right matters:
- Normal body weight (within about 20% of ideal): use actual weight.
- Underweight: use actual weight, since ideal body weight would be higher.
- Obese patients: use adjusted body weight, commonly defined as ideal body weight plus 0.4 × (actual − ideal), because aminoglycosides do not distribute well into adipose tissue.
- Ascites or significant fluid overload: actual weight overstates the lean mass, and volume of distribution is altered. Conventional dosing with levels is usually preferred in this situation.
Ideal body weight (IBW) is typically estimated with a height-based formula: for illustration, a 5’10” male IBW is approximately 78 kg. Most clinical pharmacy resources and electronic health records provide IBW calculators.
Following up with the Hartford nomogram
The starting dose from this calculator is a first-pass estimate. The definitive clinical step is to draw a random gentamicin level between 6 and 14 hours after the first dose and plot it on the Hartford nomogram diagram. Points in the “24h” zone confirm the interval; points in the “36h” or “48h” zones indicate the kidneys are clearing the drug more slowly and require a longer gap before the next dose. Points above the “48h” line suggest avoiding extended-interval dosing entirely for this patient and switching to conventional dosing with trough monitoring.