Gentamicin Dosing Calculator (Extended-Interval Aminoglycoside)
Calculate extended-interval (once-daily) gentamicin, tobramycin or amikacin doses: 7 or 15 mg/kg on the right dosing weight, with the Hartford interval from Cockcroft-Gault creatinine clearance.
How it works
IBW (kg) = 50 (men) or 45.5 (women) + 2.3 × (height in inches − 60) · dosing weight = actual weight, or IBW + 0.4 × (actual − IBW) if actual > 120% of IBW · dose = 7 mg/kg (gentamicin, tobramycin) or 15 mg/kg (amikacin) × dosing weight · CrCl = (140 − age) × weight ÷ (72 × creatinine mg/dL) [× 0.85 if female] · interval: CrCl ≥ 60 → q24h, 40–59 → q36h, 20–39 → q48h
Aminoglycosides kill bacteria in proportion to their peak concentration and have a post-antibiotic effect, while their kidney and ear toxicity depends on sustained trough exposure. Extended-interval (“once-daily”) dosing therefore gives a large dose — 7 mg/kg of gentamicin or tobramycin in the Hartford Hospital program — and stretches the interval when kidney function is reduced. Fat holds little drug, so obese patients are dosed on an adjusted body weight. The Hartford protocol then checks a single random level 6–14 h after the dose against its nomogram to confirm the interval. Extended-interval dosing is generally avoided in pregnancy, burns over 20%, ascites, endocarditis, dialysis and creatinine clearance below 20 mL/min.
Worked example
A 50-year-old man, 175 cm and 80 kg, creatinine 90 µmol/L (1.02 mg/dL), gentamicin: ideal body weight = 50 + 2.3 × (68.9 − 60) = 70.5 kg; 80 kg is under 120% of ideal, so actual weight is used and the dose is 7 × 80 = 560 mg. Creatinine clearance with ideal weight is (140 − 50) × 70.5 ÷ (72 × 1.02) = 87 mL/min, so the interval is every 24 h.
Sources: Nicolau DP et al. Experience with a once-daily aminoglycoside program administered to 2,184 adult patients. Antimicrob Agents Chemother 1995;39:650–5 · Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron 1976;16:31–41
Frequently asked questions
What is the once-daily dose of gentamicin?
In the Hartford Hospital extended-interval program, 7 mg/kg of dosing weight for gentamicin and tobramycin, given every 24, 36 or 48 hours according to creatinine clearance. Some protocols use 5 mg/kg; amikacin is usually 15 mg/kg. Follow your institution’s protocol.
Which weight should be used for aminoglycoside dosing?
Actual body weight, unless the patient weighs more than 120% of ideal body weight — then an adjusted weight, IBW + 0.4 × (actual − IBW), because aminoglycosides distribute poorly into fat.
How is the level monitored with extended-interval dosing?
For gentamicin and tobramycin, one random level 6–14 hours after the start of the first dose is plotted on the Hartford nomogram, which confirms or changes the interval (q24h, q36h or q48h). If the level is above the q48h line, the drug is held and dosed by levels.
Who should not receive extended-interval dosing?
Typically excluded: creatinine clearance below 20 mL/min or dialysis, burns over 20% of body surface, ascites, pregnancy, endocarditis (where gentamicin is used for synergy at low doses) and some pediatric and cystic fibrosis patients, who use specific protocols.
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Educational calculation for health professionals only, not medical advice or a prescription. Check every dose against the current product label, local protocols and a pharmacist, and adjust it for the individual patient.