Gentamicin Unit Converter
Gentamicin Unit Converter
Convert gentamicin between mg/L, µg/mL and µmol/L, with the trough and peak targets separated by regimen — because a gentamicin level without a documented dose time and sampling time cannot be interpreted at all.
Gentamicin converter
Mass ⇄ molarPre-dose trough 0.4 mg/L on a once-daily regimen
The conversion, and why the molar figure is approximate
mg/L = µmol/L ÷ 2.0938
because 2.0938 = 1 mg/L ÷ 477.6 g/mol, the molecular weight of gentamicin C1
- mg/L = µg/mL
- a milligram per litre and a microgram per millilitre are the same concentration, so nothing needs calculating between them. Every laboratory that measures gentamicin reports one or the other
- MW 477.6 — and why it is only part of the story
- gentamicin is not one molecule. The clinical product is a mixture of the closely related congeners C1, C1a and C2 (with C2a and C2b), which differ in methylation and therefore in mass. 477.6 is C1; C1a is lighter. A µmol/L figure derived from any single congener is an approximation for a mixture whose exact composition varies between batches, which is why laboratories report gentamicin in mg/L and why dosing nomograms are written in mg/L
- the trough
- the pre-dose sample, taken immediately before the next dose is due. It is the one that predicts toxicity, because it measures whether the drug is being cleared
- the one-hour peak
- taken one hour after the end of the infusion, once distribution is complete. It measures whether the dose is big enough for this patient's volume of distribution, and it is not used routinely on once-daily regimens
Worked example
Pre-dose trough 0.4 mg/L on a once-daily regimen
0.4 mg/L = 0.4 µg/mL — the same concentration written the other way
0.4 × 2.0938 = 0.84 µmol/L, using gentamicin C1's mass of 477.6 — approximate, because the drug is a congener mixture
0.4 mg/L sits under the 1 mg/L trough ceiling for extended-interval dosing, so the drug is clearing between doses
The same number read against the wrong target changes meaning entirely: 1.8 mg/L is an acceptable trough on multiple-daily dosing (ceiling 2 mg/L) and unacceptable on once-daily dosing (ceiling 1 mg/L)
And none of it means anything without two times on the request form. A "trough" drawn four hours before the next dose is not a trough, and it will read high; a "peak" drawn three hours after the infusion is not a peak, and it will read low
Targets by regimen — the reason this page has no single range
| Regimen | Trough (pre-dose) | One-hour peak | µmol/L equivalents |
|---|---|---|---|
| Once-daily / extended-interval (e.g. 5–7 mg/kg) | under 1 mg/L | not routinely measured | trough under 2.09 µmol/L |
| Multiple-daily dosing (e.g. 3–5 mg/kg/day in divided doses) | under 2 mg/L | 5–10 mg/L | trough under 4.19; peak 10.47–20.94 µmol/L |
| Synergy dosing for endocarditis (e.g. 1 mg/kg 8–12 hourly) | under 1 mg/L | 3–5 mg/L | trough under 2.09; peak 6.28–10.47 µmol/L |
Trough versus peak: what each one is actually telling you
| Trough (pre-dose) | One-hour peak | |
|---|---|---|
| The question it answers | Is the drug being cleared before the next dose? | Is the dose large enough for this patient’s volume of distribution? |
| What it predicts | Toxicity — sustained exposure is what damages the proximal tubule and the cochlea | Efficacy against the organism, via the peak-to-MIC ratio |
| What a bad result should change | The dosing interval — lengthen it, or stop | The dose size |
| Commonest reason it is uninterpretable | Drawn too early, mid-interval, so it reads high | Drawn late, after distribution and some elimination, so it reads low |
The two toxicities, and what changes the target
| Nephrotoxicity | Ototoxicity | |
|---|---|---|
| Mechanism | Accumulation in proximal tubular cells | Accumulation in cochlear and vestibular hair cells |
| Typical course | Non-oliguric acute kidney injury after several days, usually reversible | High-frequency hearing loss or vestibular failure — often permanent |
| Warning signs | Rising creatinine, falling urine output, a rising trough | Tinnitus, high-tone hearing loss, unsteadiness, oscillopsia — often noticed late or after discharge |
| What raises the risk | Duration of therapy, a persistently raised trough, hypovolaemia, other nephrotoxins, pre-existing chronic kidney disease | Duration of therapy, cumulative dose, loop diuretics, pre-existing hearing loss, and mitochondrial 12S rRNA variants such as m.1555A>G |
Two times on the form, a trough that matters more than the peak, and a mixture rather than a molecule
Gentamicin is reported in milligrams per litre, or identically in micrograms per millilitre. The molar conversion uses 477.6 grams per mole, the mass of gentamicin C1, so one milligram per litre is about 2.09 micromoles per litre — but that figure deserves a caveat that most converters omit. Gentamicin is not a single molecule. The drug is a mixture of the congeners C1, C1a and C2, which differ in methylation and therefore in mass, in proportions that vary between manufacturing batches. A molar concentration calculated from any one congener is an approximation for the mixture, which is why laboratories report gentamicin as a mass concentration and why every dosing nomogram is written in mg/L. If you are acting on a gentamicin level, act on the mg/L figure.
The far more important point is that a gentamicin level is uninterpretable without two times. The request has to state when the last dose was given and when the sample was taken, because the same number means different things at different points in a dosing interval. A pre-dose trough drawn four hours early is not a trough and will read high; a one-hour peak drawn three hours after the infusion finished is not a peak and will read low. Laboratories and pharmacy teams cancel or caveat aminoglycoside levels for exactly this reason, and a level reported without the timings should be repeated rather than acted upon.
Of the two samples, the trough is the one that matters for safety. Aminoglycosides kill in a concentration-dependent way, so the peak drives efficacy through the peak-to-MIC ratio; toxicity, by contrast, tracks the time the proximal tubule and the cochlear hair cells spend exposed to the drug. A high trough says the drug is not clearing between doses and is accumulating in those tissues. The response to a high trough is to lengthen the interval rather than to shave the dose, because it is the interval that determines how low the concentration falls before the next dose arrives. Both toxicities are real and they are not symmetrical: the acute kidney injury is usually non-oliguric and usually recovers, while the ototoxicity — high-frequency hearing loss, tinnitus, vestibular failure — is frequently irreversible, can appear after the course has finished, and is made more likely by loop diuretics, by cumulative dose, and in people carrying mitochondrial 12S rRNA variants such as m.1555A>G.
Because the targets depend on the regimen, this page offers five rather than one. Modern once-daily or extended-interval dosing gives a large dose at a long interval and is monitored chiefly by a trough, which should be under 1 mg/L; the older multiple-daily regimens are monitored by a one-hour peak of 5–10 mg/L with a trough under 2 mg/L; and low-dose synergy regimens for endocarditis aim at a peak of 3–5 mg/L with a trough under 1 mg/L. Renal function shifts all of it, because clearance is almost entirely glomerular. What none of these numbers can do is decide a dose. They inform a decision that belongs to the prescribing team and to pharmacy, alongside the indication, the duration of therapy so far, the renal trend and whether the drug is still needed at all — and the aminoglycoside half-life calculator is the tool for turning two timed levels into the interval that follows from them.
Frequently asked questions
How do you convert gentamicin from mg/L to µmol/L?
Multiply by 2.0938, which is one milligram per litre divided by 477.6 g/mol, the molecular weight of gentamicin C1. A trough of 0.4 mg/L is 0.84 µmol/L. Divide by the same factor to go back, and note that mg/L and µg/mL are the same number. Treat the molar figure as approximate: gentamicin is a mixture of congeners of differing mass, so laboratories report and nomograms use mg/L.
Why does the trough matter more than the peak?
Because they answer different questions. The peak reflects whether the dose is large enough for the patient’s volume of distribution, and drives efficacy through the peak-to-MIC ratio. The trough reflects whether the drug is being cleared before the next dose, and it is sustained exposure — not peak height — that accumulates in the renal proximal tubule and in cochlear hair cells. A rising trough is the early warning of toxicity, and the response to it is to lengthen the dosing interval rather than to reduce the dose.
What is the target gentamicin trough?
It depends on the regimen, which is why this page will not quote one number. On once-daily or extended-interval dosing the trough should be under 1 mg/L. On older multiple-daily regimens it should be under 2 mg/L, alongside a one-hour peak of 5–10 mg/L. On low-dose synergy regimens for endocarditis it should be under 1 mg/L with a peak of 3–5 mg/L. Averaging these gives a band that is wrong for all three. Follow your local protocol.
Why was my gentamicin level rejected or reported without an interpretation?
Almost always because the dose time, the sampling time or both were missing from the request. The same concentration is acceptable or dangerous depending on where in the dosing interval it was taken, so without those two times the laboratory cannot say which sample it is. A “trough” drawn mid-interval reads high and a “peak” drawn late reads low. Record both times and repeat the level rather than acting on an untimed one.
Does gentamicin ototoxicity recover?
Often it does not. Aminoglycosides accumulate in cochlear and vestibular hair cells, and the resulting high-frequency hearing loss or vestibular failure is frequently permanent; it can also become apparent only after the course has finished. Risk rises with the duration of therapy and the cumulative dose, with concurrent loop diuretics, with pre-existing hearing loss, and in people carrying mitochondrial 12S rRNA variants such as m.1555A>G. This asymmetry — reversible kidneys, irreversible ears — is why a persistently raised trough is treated as urgent rather than watched.
Related calculators
References
- Joint Formulary Committee. Gentamicin — Monitoring requirements. British National Formulary. London: BMJ Group and Pharmaceutical Press. Multiple daily dosing: one-hour peak 5–10 mg/L (3–5 mg/L for endocarditis); pre-dose trough below 2 mg/L (below 1 mg/L for endocarditis).
- 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.
- Rybak MJ, Abate BJ, Kang SL, Ruffing MJ, Lerner SA, Drusano GL. Prospective evaluation of the effect of an aminoglycoside dosing regimen on rates of observed nephrotoxicity and ototoxicity. Antimicrob Agents Chemother. 1999;43(7):1549–1555.
- Selimoglu E. Aminoglycoside-induced ototoxicity. Curr Pharm Des. 2007;13(1):119–126.
- Gentamicin: dose regimens and monitoring. The Pharmaceutical Journal. Targets by regimen — multiple daily dosing trough under 2 mg/L and peak 5–10 mg/L; once-daily trough under 1 mg/L; synergy trough under 1 mg/L and optional peak 3–5 mg/L.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.