Vancomycin Dose Adjustment Calculator
Vancomycin Dose Adjustment Calculator
Adjust a vancomycin dose proportionally from a measured level, using linear pharmacokinetics to reach a target AUC24 or trough.
Vancomycin Dose Adjustment
Proportional dosingCurrent dose 2000 mg/day, measured AUC24 340, target AUC24 500
Formula
- current
- current total daily dose, mg
- measured
- measured AUC24 or trough at the current dose, drawn at steady state
- target
- target AUC24 or trough — must use the same measure as 'measured'
Worked example
Current dose 2000 mg/day, measured AUC24 340, target AUC24 500
2000 × 500 ÷ 340 = 2941 mg/day
Round to a practical dose, for example 3000 mg/day split across the existing interval, and recheck a level once the new dose has reached steady state
Rounding to a practical regimen
| Calculated dose | Practical dose |
|---|---|
| 2941 mg/day | 3000 mg/day, e.g. 1500 mg every 12 hours |
| 1620 mg/day | 1750 mg/day, e.g. 875 mg every 12 hours |
| 3800 mg/day | 4000 mg/day, e.g. 2000 mg every 12 hours |
A proportional adjustment, and where it breaks down
Vancomycin follows linear (first-order) pharmacokinetics within the clinical dose range, meaning a proportional change in dose produces a proportional change in exposure. That makes a simple rule of three valid: multiply the current dose by the ratio of the target level to the measured level to get the dose expected to achieve the target, whether the level is an AUC24 or a trough — the arithmetic is identical for either measure, provided the same measure is used on both sides.
The result should be rounded to a dose that can actually be prescribed and prepared — typically the nearest 250 mg — and fitted to a practical dosing interval rather than left as a precise but unworkable number. This is an adjustment calculation, not a starting-dose calculation: it assumes the current regimen has already run long enough to reach steady state, which takes four to five half-lives. A level drawn before that point reflects accumulation still in progress, and any adjustment calculated from it will overshoot.
The proportional method also assumes clearance is stable between the old dose and the new one. In a patient whose renal function is changing — acute kidney injury, recovery from it, or a new nephrotoxic drug — the clearance that produced the measured level will not be the clearance the new dose experiences, and no proportional calculation holds. Recheck renal function and consider a fresh estimate, or move to Bayesian dosing software, rather than trusting the arithmetic in that setting.
Frequently asked questions
Why can vancomycin dose be adjusted proportionally?
Because it follows linear pharmacokinetics at clinical doses — exposure scales directly with dose, so multiplying by the ratio of target to measured level gives the dose expected to reach the target.
How precisely should the calculated dose be prescribed?
Round to a practical, deliverable dose — typically the nearest 250 mg — and fit it to a sensible interval rather than prescribing the raw calculated figure.
When is this calculation not valid?
When the level was drawn before steady state (fewer than four to five half-lives on the current dose), or when renal function is changing, since the clearance that produced the measured level will differ from the clearance the new dose sees.
Does it matter whether I use AUC24 or trough?
No, the arithmetic is the same either way, but ‘measured’ and ‘target’ must use the same measure — do not mix a measured trough with a target AUC24.
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References
- Rybak MJ et al. Therapeutic monitoring of vancomycin for serious MRSA infections: a revised consensus guideline. Am J Health-Syst Pharm. 2020;77(11):835-864.
- Winter ME. Basic Clinical Pharmacokinetics, 6th ed. Lippincott Williams & Wilkins.