Aminoglycoside Dosing Calculator
Calculate extended-interval aminoglycoside (gentamicin/tobramycin) dosing based on adjusted body weight and renal function.
About this calculator
This calculator implements extended-interval (once-daily) gentamicin/tobramycin dosing per the Hartford nomogram protocol from Nicolau et al.'s 1995 study of 2,184 patients (Antimicrobial Agents and Chemotherapy): a fixed 7 mg/kg dose of adjusted body weight, given at an interval set by creatinine clearance -- every 24 hours at CrCl 60 mL/min or above, every 36 hours at CrCl 40-59, every 48 hours at CrCl 20-39. Actual Body Weight drives the calculated Dose directly: at typical (non-obese) body compositions, adjusted body weight simply equals actual weight, so the dose scales with weight almost one-to-one. Serum Creatinine and Age have NO effect on the Dose figure at all -- they only feed into Creatinine Clearance, which determines the Dosing Interval, a completely separate output.
Height affects the dose only indirectly, through the Devine ideal-body-weight formula, and only once actual weight crosses 120% of ideal body weight, which triggers an adjusted-weight calculation instead of using actual weight directly -- but because that 120%-of-IBW threshold is itself a function of height, a height change that pushes the patient across the threshold can move the calculated dose by MORE than the equivalent percentage change in weight itself, especially for female patients, whose Devine formula produces a lower ideal body weight (and therefore an easier threshold to cross) than the male formula does. The Hartford protocol's published validation covers creatinine clearance of 20 mL/min and above; below that threshold, or in dialysis patients, this fixed-interval lookup is outside its validated range and individualized pharmacy dosing is needed instead.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
This tool estimates a starting point only and is not a validated clinical decision-support system. It does not account for every patient-specific factor (organ function, drug interactions, allergies, comorbidities, or current clinical status). Any dose, rate, or setting must be independently verified by the treating clinician and pharmacist or respiratory therapist against current institutional protocols, product labeling, and the patient's full clinical picture before being administered or applied.
Inputs
Extended-interval dosing protocol validated for CrCl >= 20 mL/min; below that, or on dialysis, use individualized dosing instead
Results
Recommended Dose (7 mg/kg)
490 mg
Dosing Interval
24 hours
Below Validated CrCl Range (<20 mL/min)
No
Figures current as of 1995. Source: Nicolau DP, Freeman CD, Belliveau PP, Nightingale CH, Ross JW, Quintiliani R. Experience with a once-daily aminoglycoside program administered to 2,184 adult patients. Antimicrob Agents Chemother. 1995;39(3):650-655.
How to Use This Calculator
- Enter the patient's Actual Body Weight in kg, Height in cm, and Age in years.
- Enter the most recent Serum Creatinine in mg/dL and select the patient's Sex.
- The calculator computes Ideal Body Weight (Devine formula) and Adjusted Body Weight for obese patients, then estimates creatinine clearance (Cockcroft-Gault).
- Review the Recommended Dose at 7 mg/kg adjusted body weight and Dosing Interval (24h, 36h, or 48h based on CrCl).
- Check the Estimated Peak and Trough concentrations — confirm trough < 1 mcg/mL per Hartford nomogram and monitor drug levels after the first dose.
- Below CrCl 20 mL/min or in dialysis patients, this fixed 24/36/48-hour interval lookup is outside its validated range — consult pharmacy for individualized dosing instead.
How the result changes with Actual Body Weight
| Actual Body Weight | Recommended Dose (7 mg/kg) | Dosing Interval | Below Validated CrCl Range (<20 mL/min) |
|---|---|---|---|
| 35 | 245 mg | 36 hours | No |
| 53 | 371 mg | 24 hours | No |
| 105 | 571 mg | 24 hours | No |
| 175 | 767 mg | 24 hours | No |
What each input means
- Actual Body Weight
- Patient's actual body weight in kilograms.
- Height
- Patient's height in centimeters (used for ideal body weight calculation).
- Age
- Patient's age in years.
- Serum Creatinine
- Most recent serum creatinine level in mg/dL. The extended-interval q24/36/48h protocol below was validated for creatinine clearance of 20 mL/min or higher (Nicolau et al. 1995); below CrCl 20 mL/min, or in dialysis patients, this fixed-interval lookup no longer applies and dosing needs individualized pharmacy consultation.
- Sex
- Biological sex. Adjusts the Cockcroft-Gault CrCl gender factor and the Devine ideal body weight formula.
What each result means
- Below Validated CrCl Range (<20 mL/min)
- True when CrCl is under 20 mL/min (includes dialysis patients). The Hartford nomogram was not validated below CrCl 20 — the 48-hour interval shown above is a rough floor, not a validated recommendation, and individualized pharmacy dosing is needed instead of this table lookup.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersActual Body Weight = 70, Height = 170, Age = 40, Serum Creatinine = 1 = 5 input(s) provided
- Calculate Recommended DoseRecommended Dose490 = 490
- Calculate Dosing IntervalDosing Interval24 = 24
- Calculate Estimated PeakEstimated Peak28 = 28
- Calculate Estimated TroughEstimated Trough0.02 = 0.02
Figures and sources
- Hartford nomogram (extended-interval aminoglycoside dosing) (1995) — Nicolau DP, Freeman CD, Belliveau PP, Nightingale CH, Ross JW, Quintiliani R. Experience with a once-daily aminoglycoside program administered to 2,184 adult patients. Antimicrob Agents Chemother. 1995;39(3):650-655.
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Do serum creatinine and age affect the calculated dose in milligrams?
No. The Dose output depends only on Adjusted Body Weight -- it's simply 7 mg/kg of adjusted body weight, with no creatinine or age term anywhere in that calculation. Serum Creatinine and Age instead feed into Creatinine Clearance, which determines the Dosing Interval (24, 36, or 48 hours) -- a completely separate output from the dose in milligrams.
Why does the recommended dosing interval depend on kidney function instead of the dose itself?
Extended-interval aminoglycoside dosing keeps the mg/kg dose fixed at 7 mg/kg regardless of renal function, but STRETCHES the time between doses as creatinine clearance falls, because slower renal clearance means the drug takes longer to fall to a safe trough level between doses. This is the Hartford nomogram's approach: a constant dose with a variable interval (24/36/48 hours based on CrCl), rather than a constant interval with a variable dose.
Is this dosing protocol valid for a patient in kidney failure or on dialysis?
No, not without modification. The Hartford nomogram protocol this calculator implements was validated for creatinine clearance of 20 mL/min and above. Below that threshold, or for patients on dialysis, this fixed 24/36/48-hour interval lookup falls outside its validated range, and dosing needs to be individualized with pharmacy input rather than read off this calculator.
Why does the calculator ask for height if the dose is based on weight?
Height is used to calculate Ideal Body Weight via the Devine formula, which the calculator compares against Actual Body Weight. If actual weight exceeds 120% of ideal body weight, the calculator switches to an Adjusted Body Weight formula (partway between ideal and actual) instead of dosing directly off actual weight, since dosing an obese patient purely by actual weight can substantially overestimate the correct aminoglycoside dose.
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