What is the Hartford extended-interval gentamicin regimen?

The Hartford method gives 5 to 7 mg/kg as a single daily dose, with the interval stretched by renal function: every 24 hours if creatinine clearance is 60 or above, every 36 hours at 40-59, and every 48 hours at 20-39. A single level drawn 6 to 14 hours after the infusion is plotted on the Hartford nomogram to set the interval.

What are the target gentamicin levels?

For extended-interval dosing, the peak should exceed 20 mcg/mL (typically 20-30) and the trough should be below 1 mcg/mL. For traditional multiple-daily dosing, the peak target is 5-10 mcg/mL with a trough below 2 mcg/mL. For synergy dosing in endocarditis, the peak target is 3-4 mcg/mL with a trough below 1.

When should traditional dosing be used instead of extended-interval?

Traditional multiple-daily dosing is preferred in endocarditis synergy regimens, pregnancy, significant renal impairment (creatinine clearance below 20-40), burns, and ascites, and wherever local practice or monitoring capacity favours peak and trough pairs over nomogram-based dosing.

What weight should gentamicin be dosed on?

Use actual body weight unless it exceeds 120% of ideal (Devine) body weight, in which case use adjusted body weight: ideal plus 40% of the excess. Aminoglycosides distribute poorly into adipose tissue, so full actual weight would overdose the obese patient.

What is synergy dosing of gentamicin?

Synergy dosing uses low gentamicin doses (1 mg/kg every 8 hours or 3 mg/kg once daily) alongside a cell-wall-active agent such as a beta-lactam, for organisms like enterococci in endocarditis. The aim is enhanced bacterial killing at exposures low enough to limit toxicity.

How is ototoxicity monitored during gentamicin therapy?

Ask about tinnitus, hearing change and vertigo before each dose, keep courses as short as the infection allows, avoid concurrent ototoxins such as loop diuretics where possible, and consider audiometry for prolonged courses or high-risk patients. Ototoxicity can be irreversible, so symptoms should prompt immediate review.

Gentamicin Dosing Calculator

Calculate gentamicin regimens by the Hartford extended-interval, traditional multiple-daily or synergy methods, with renal interval adjustment, weight selection and Hartford nomogram guidance for a random level.

Gentamicin dosing tool

Dosing method Extended-interval dose (mg/kg) Infection type Actual body weight (kg) Height (cm) Sex Age (years) Serum creatinine (mg/dL) Random level for Hartford nomogram (mcg/mL, optional) Hours after infusion start (optional)

Key definitions

Extended-interval (Hartford) dosing: One large daily gentamicin dose (5 to 7 mg/kg), with the interval set by renal function and a single 6-14 hour level plotted on the Hartford nomogram. Exploits concentration-dependent killing and the post-antibiotic effect.
Traditional dosing: Smaller doses given two to three times daily (1.5 mg/kg per dose), monitored with peak and trough pairs. The older standard, still preferred where extended-interval is unsuitable.
Synergy dosing: Low gentamicin doses (1 mg/kg q8h or 3 mg/kg q24h) combined with a beta-lactam or vancomycin for endocarditis, aiming for enhanced killing of enterococci or streptococci at low, safer exposures.
Hartford nomogram: A published plot of the 6-14 hour gentamicin concentration against time, divided into q24h, q36h and q48h zones, used to choose the dosing interval after the first extended-interval dose.
Post-antibiotic effect: Continued bacterial suppression after the drug concentration falls below the MIC, which is prolonged for aminoglycosides and makes long intervals safe despite low troughs.

Why three methods exist

Gentamicin kills in proportion to its peak concentration: the higher the peak relative to the MIC, the faster and more complete the killing, and the effect persists for hours after the drug has fallen below detectable levels. That pharmacodynamic fact is the entire argument for extended-interval dosing. One large dose drives a very high peak, maximising killing, and the long drug-free interval lets the kidneys and the inner ear recover before the next exposure. Randomised comparisons found extended-interval dosing at least as effective as traditional dosing with no more nephrotoxicity and possibly less, which is why it became the default for most adult Gram-negative infections.

Traditional dosing survives because the conditions for extended-interval dosing are not universal. In endocarditis, where synergy rather than peak killing is the goal, low divided doses are the studied regimen. In pregnancy, the expanded volume of distribution and increased clearance change the pharmacokinetics enough that the nomogram no longer applies. In significant renal impairment the interval stretches beyond what the nomogram was validated for, and in burns and ascites the volume of distribution is so abnormal that population intervals mislead. The calculator offers all three methods so the prescriber matches the method to the patient rather than forcing the patient into one method.

Weight, kidneys and the interval table

Gentamicin distributes into lean tissue and extracellular fluid, not fat, so dosing an obese patient on actual body weight systematically overdoses. The standard correction, applied here, is to use actual weight unless it exceeds 120% of Devine ideal body weight, then switch to adjusted body weight (ideal + 0.4 x the excess). Creatinine clearance is estimated by Cockcroft-Gault on the dosing weight, and the interval follows the Hartford bands: every 24 hours at 60 mL/min or above, every 36 hours at 40 to 59, every 48 hours at 20 to 39, and not recommended below 20, where individualised dosing with levels is required.

Worked example: a 45-year-old man, 70 kg, 175 cm, creatinine 1.0 mg/dL. Ideal weight is 70.6 kg, so actual weight is used; creatinine clearance is 92.4 mL/min. Extended-interval gives 490 mg every 24 hours (7 mg/kg), traditional gives 105 mg every 8 hours, and synergy gives 70 mg every 8 hours or 210 mg once daily. Second example: a 70-year-old woman, 90 kg, 165 cm, creatinine 1.4 mg/dL. Ideal weight is 57.0 kg, so the dosing weight becomes 70.2 kg (adjusted), creatinine clearance 41.4 mL/min, and the extended-interval regimen is 491 mg every 36 hours. Note how the weight correction and the interval stretch compound: without them she would have received a substantially larger, more frequent dose.

Monitoring: levels, nomogram and toxicity

For extended-interval dosing, draw a single level 6 to 14 hours after the start of the first infusion and plot it on the Hartford nomogram; the zone it falls in sets the interval, and a point above the q48h line means holding the dose. For traditional dosing, draw a peak 30 minutes after a 30-minute infusion and a trough just before the next dose, targeting 5 to 10 and below 2 mcg/mL respectively. Recheck levels with any change in renal function, and keep courses as short as the infection allows: every extra day of aminoglycoside is extra ototoxicity and nephrotoxicity risk without extra benefit once the infection is controlled.

Nephrotoxicity is usually reversible and tracks with trough accumulation, which is why the trough targets are set low. Ototoxicity, cochlear and vestibular, can be irreversible, which is why symptoms matter more than numbers: ask about tinnitus, hearing change, fullness and vertigo before every dose, and take new symptoms seriously even when levels look acceptable. Avoid concurrent ototoxins, particularly loop diuretics, where the clinical situation allows any choice in the matter.

Limitations and pitfalls

The Hartford nomogram was validated in adults with stable renal function receiving 7 mg/kg; applying it to different doses, unstable creatinine, dialysis, pregnancy, burns or major fluid shifts extrapolates beyond its evidence. Cockcroft-Gault misleads in the frail elderly with low muscle mass and in acute kidney injury where creatinine lags behind true filtration. The calculator's synergy doses assume a susceptible organism and an appropriate partner drug; synergy is a microbiological claim, not a dosing trick, and it needs microbiology input.

Gentamicin has poor lung penetration, so pneumonia dosing and expectations should be tempered, and it is inactive against streptococci and enterococci alone, which is why synergy regimens always pair it. Finally, local resistance patterns may make empiric gentamicin a poor choice regardless of perfect dosing; the calculator sizes the dose, but the decision to use the drug belongs to the clinical and microbiological picture.

Key takeaways

Default to Hartford extended-interval dosing (5-7 mg/kg) for adult Gram-negative infections with adequate renal function.
Dose on adjusted body weight when actual weight exceeds 120% of ideal; aminoglycosides do not distribute into fat.
Stretch the interval by creatinine clearance: q24h, q36h, q48h; individualise below 20 mL/min.
Use the 6-14 hour level on the Hartford nomogram to set the interval; use peak-trough pairs for traditional dosing.
Reserve synergy dosing (1 mg/kg q8h or 3 mg/kg q24h) for endocarditis with a partner drug.
Ask about hearing and balance before every dose; ototoxicity can be permanent.

Frequently asked questions

What is the Hartford extended-interval gentamicin regimen?: 5 to 7 mg/kg as a single daily dose: every 24 hours if creatinine clearance is 60 or above, every 36 hours at 40-59, every 48 hours at 20-39. A level drawn 6-14 hours after the infusion is plotted on the Hartford nomogram to confirm the interval.
What are the target gentamicin levels?: Extended-interval: peak above 20 mcg/mL (usually 20-30), trough below 1. Traditional: peak 5-10 mcg/mL, trough below 2. Synergy: peak 3-4 mcg/mL, trough below 1.
When should traditional dosing be used instead?: In endocarditis synergy regimens, pregnancy, creatinine clearance below about 20-40 mL/min, burns, ascites, and wherever monitoring favours peak-trough pairs over nomogram dosing.
What weight should gentamicin be dosed on?: Actual body weight, unless it exceeds 120% of ideal body weight, in which case use adjusted body weight (ideal + 40% of the excess).
What is synergy dosing?: Low gentamicin doses with a cell-wall-active partner for endocarditis, aiming for enhanced killing of enterococci or streptococci at low, safer exposures: 1 mg/kg every 8 hours or 3 mg/kg once daily.
How is ototoxicity monitored?: Ask about tinnitus, hearing change and vertigo before each dose, keep courses short, avoid concurrent ototoxins where possible, and arrange audiometry for prolonged courses or high-risk patients.
Can extended-interval dosing be used in renal impairment?: Down to a creatinine clearance of 20 mL/min, with the interval stretched to 48 hours and nomogram confirmation. Below that, use individualised traditional dosing with measured levels.
Why is the trough target so low in extended-interval dosing?: Because the long drug-free interval is the point of the method: troughs below 1 mcg/mL confirm the drug has cleared, giving the kidneys and inner ear a recovery period before the next large dose.

Aminoglycoside Pharmacokinetic Dosing Calculator - individualise from measured peak and trough.
Vancomycin AUC Dosing Calculator - the other nephrotoxic antibiotic dosed by levels.
Nephrology calculators - creatinine clearance and kidney function tools.
Infectious disease calculators - infection-related scores and tools.

References

Nicolau DP, Freeman CD, Belliveau PP, et al. Experience with a once-daily aminoglycoside program administered to 2,184 adult patients. Antimicrob Agents Chemother. 1995;39(3):650-655. - the Hartford programme
Begg EJ, Barclay ML. Aminoglycosides: 50 years on. Br J Clin Pharmacol. 1995;39(6):597-603.

Medical disclaimer

This calculator is an educational tool for clinicians, students and informed readers. It does not provide medical advice. Aminoglycoside dosing should be confirmed with therapeutic drug monitoring and local pharmacy or antimicrobial stewardship guidance, particularly in renal impairment, pregnancy, obesity and critical illness. Never start, stop or change a prescribed medicine on the basis of a calculator result alone; if you are unwell, seek professional care promptly.