Nephrotoxic Drug Adjustment Calculator
Dose adjustment for nephrotoxic drugs based on renal function (GFR).
About this calculator
This calculator estimates renal function with the Cockcroft-Gault equation, the 1976 formula still widely used at the bedside for drug-dosing decisions: subtract the patient's age from 140, multiply by their dosing weight (and by 0.85 if female), then divide that whole product by 72 times the serum creatinine. It maps the resulting creatinine clearance onto commonly used dose-reduction bands: full dose above a creatinine clearance of 60 mL/min, roughly 50-100% (interpolated) between 30 and 60, roughly 25-50% between 15 and 30, and a 25% floor below 15, where many nephrotoxic drugs are avoided outright rather than merely reduced.
Because obesity distorts creatinine clearance estimates based on actual body weight, the calculator first computes ideal body weight using the Devine formula (50 + 2.3 x inches over 5 feet for men, 45.5 + 2.3 x inches over 5 feet for women) and, when actual weight exceeds 120% of ideal, an adjusted body weight (ideal weight plus 40% of the excess) -- using that adjusted weight rather than actual weight in the Cockcroft-Gault formula, which is standard practice for dosing decisions in patients who carry significant excess weight. The specific dose-reduction percentages and the aminoglycoside extended-interval schedule shown here are general tiers meant to illustrate the shape of renal dose adjustment; actual prescribing should always be checked against the specific drug's package insert or a pharmacist, since different nephrotoxic medications reduce dose very differently even at the same creatinine clearance.
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.
Inputs
Results
Creatinine Clearance (CG)
36.5 mL/min
Figures current as of 1976. Source: Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41.
How to Use This Calculator
- Enter patient age (years), actual weight (kg), and height (cm).
- Enter serum creatinine (mg/dL) and biological sex.
- Enter the normal drug dose (mg) for a patient with full renal function.
- Read creatinine clearance (Cockcroft-Gault, mL/min) and the recommended dose adjustment (%).
- Use adjusted dose (mg) and aminoglycoside dosing interval (hours) for prescribing decisions.
How the result changes with Serum Creatinine (mg/dL)
| Serum Creatinine (mg/dL) | Creatinine Clearance (CG) |
|---|---|
| 1 | 72.9 mL/min |
| 1.5 | 48.6 mL/min |
| 3 | 24.3 mL/min |
| 5 | 14.6 mL/min |
What each input means
- Age (years)
- Patient age.
- Actual Weight
- Actual body weight in kg.
- Height
- Height for ideal/adjusted body weight calculation.
- Serum Creatinine (mg/dL)
- Current serum creatinine.
- Sex
- Biological sex.
- Normal Drug Dose (mg)
- Standard dose for the medication in a patient with normal renal function.
What each result means
- Creatinine Clearance (CG)
- Cockcroft-Gault creatinine clearance.
- Dose Adjustment
- Percentage of normal dose to administer.
- Adjusted Dose
- Recommended dose based on renal function.
- Aminoglycoside Interval
- Extended dosing interval for aminoglycosides.
- Adjusted Body Weight
- ABW used for dosing in obese patients.
- Ideal Body Weight
- IBW from Devine formula.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersAge (years) = 65, Actual Weight (kg) = 70, Height (cm) = 170, Serum Creatinine (mg/dL) = 2 = 6 input(s) provided
- Calculate Creatinine Clearance36.5 = 36.5
- Calculate Dose AdjustmentDose Adjustment61 = 61
- Calculate Adjusted DoseAdjusted Dose = normalDose * (doseAdjustmentPct / 100)607.6 = 607.6
Figures and sources
- Cockcroft-Gault equation for estimating creatinine clearance (1976) — Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41.
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does creatinine move the estimated creatinine clearance more than age or weight does?
Creatinine sits in the denominator of the Cockcroft-Gault formula, so its relationship to creatinine clearance is inverse rather than linear -- at low creatinine values, a given absolute change in creatinine produces a disproportionately large swing in the calculated clearance. Age and weight, by contrast, enter the formula's numerator roughly linearly, so equivalent percentage changes in those inputs move the result by a comparatively smaller and steadier amount.
Why does this calculator use adjusted body weight instead of actual weight for obese patients?
Creatinine production scales roughly with lean muscle mass, not total body weight, so using a heavily obese patient's actual weight in the Cockcroft-Gault formula tends to overestimate their true creatinine clearance. Once actual weight exceeds 120% of ideal body weight, this calculator switches to an adjusted body weight (ideal weight plus 40% of the excess above ideal) as a partial correction, which is standard practice for dosing decisions in this population.
Does the prescribed drug dose affect the calculated creatinine clearance?
No -- the Normal Drug Dose input is used only afterward, to scale the adjusted dose output once the creatinine clearance and dose-adjustment percentage are already known. Creatinine clearance itself depends only on age, weight, height, sex, and serum creatinine; changing the prescribed dose changes the recommended adjusted dose but has no effect on the kidney-function estimate underneath it.
What does the Aminoglycoside Interval output represent?
It's an extended dosing interval used for aminoglycoside antibiotics like gentamicin, which are dosed less frequently (rather than at a smaller per-dose amount) as renal function declines, because they clear the body more slowly when the kidneys are impaired. This is a different dose-adjustment strategy from the percentage-based reduction this calculator applies to other nephrotoxic drugs, reflecting how aminoglycoside dosing is conventionally handled in renal impairment.
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