Tobramycin Unit Converter
Tobramycin Unit Converter
Convert tobramycin between mg/L, µg/mL and µmol/L, with trough and peak targets separated by regimen — and with the timing rule that decides whether a level means anything at all.
Tobramycin converter
Mass ⇄ molarPre-dose trough 0.5 mg/L on an extended-interval regimen
The conversion
mg/L = µmol/L ÷ 2.13899
because 2.13899 = 1 mg/L ÷ 467.51 g/mol, the molecular weight of tobramycin
- mg/L = µg/mL
- the same concentration written two ways, so no arithmetic is needed between them. Tobramycin is reported as a mass concentration everywhere, and every dosing nomogram is written in mg/L
- MW 467.51
- tobramycin, C₁₈H₃₇N₅O₉. Unlike gentamicin, it is a single defined compound rather than a mixture of congeners, so the molar conversion is exact rather than approximate
- the trough
- taken immediately before the next dose is due. It is the sample that predicts toxicity, because it measures whether the drug has been cleared
- the one-hour peak
- taken one hour after the end of the infusion, once distribution into tissue is complete. It measures whether the dose suits the patient's volume of distribution, and it is not part of routine once-daily monitoring
- the two times
- the dose time and the sampling time. Without both on the request, neither target above can be applied to the number, and the level should be repeated rather than interpreted
Worked example
Pre-dose trough 0.5 mg/L on an extended-interval regimen
0.5 mg/L = 0.5 µg/mL — the same concentration
0.5 × 2.13899 = 1.07 µmol/L
0.5 mg/L is below the 1 mg/L trough ceiling for extended-interval dosing, so the drug is clearing between doses
Read against the other regimen the meaning shifts: 1.6 mg/L would be an acceptable trough on multiple-daily dosing, where the ceiling is 2 mg/L, and unacceptable on a once-daily regimen
Back the other way: 1.07 ÷ 2.13899 = 0.50 mg/L
And the arithmetic is still the least important part. A trough taken three hours before the dose was due is not a trough, and it reads high; the level has to be repeated rather than reinterpreted
Targets by regimen
| Regimen | Trough (pre-dose) | One-hour peak | µmol/L equivalents |
|---|---|---|---|
| Once-daily / extended-interval | under 1 mg/L | set by local protocol; not part of routine monitoring | trough under 2.14 µmol/L |
| Multiple-daily dosing | under 2 mg/L | should not exceed 10 mg/L; usually 5–10 mg/L | trough under 4.28; peak 10.69–21.39 µmol/L |
| Cystic fibrosis | under 1 mg/L on once-daily regimens | unit protocol — often an AUC or a timed mid-interval level instead | trough under 2.14 µmol/L |
Trough versus peak
| Trough (pre-dose) | One-hour peak | |
|---|---|---|
| The question | Is the drug cleared before the next dose? | Is the dose big enough for this volume of distribution? |
| What it predicts | Toxicity — cumulative tissue exposure | Efficacy, through the peak-to-MIC ratio |
| What a bad value changes | The dosing interval | The dose size |
| How it goes wrong | Sampled too early, so it reads high | Sampled late, so it reads low |
Toxicity, and what shifts the target
| Nephrotoxicity | Ototoxicity | |
|---|---|---|
| Site | Proximal tubular cells | Cochlear and vestibular hair cells |
| Course | Non-oliguric acute kidney injury, usually reversible | High-frequency hearing loss, tinnitus, vestibular failure — often irreversible |
| Risk multipliers | Duration, raised troughs, hypovolaemia, other nephrotoxins, existing chronic kidney disease | Duration, cumulative dose, loop diuretics, existing hearing loss, mitochondrial 12S rRNA variants |
| Effect on the target | Falling glomerular filtration means a longer interval; in significant impairment extended-interval dosing may be inappropriate | No change to the number, but a strong reason to stop early and to audiometer patients on prolonged or repeated courses |
A single molecule, two regimens, and the two times that make a level readable
Tobramycin is reported in milligrams per litre, which is the same number as micrograms per millilitre. The molar conversion uses 467.51 grams per mole, so one milligram per litre is 2.13899 micromoles per litre. Unlike gentamicin, tobramycin is a single defined compound rather than a mixture of congeners, so that conversion is exact — but it is also of little practical use, because every target, nomogram and local protocol for aminoglycoside dosing is written in mg/L. Convert for comparison with a molar report if you need to, and then work in mass units.
What actually determines whether a tobramycin level can be used is timing. The request must state when the dose was given and when the blood was taken, because the interpretation of a concentration depends entirely on where in the dosing interval it sits. A pre-dose trough drawn three hours early is not a trough and reads high; a one-hour peak drawn two hours late is not a peak and reads low. In both cases the right response is to repeat the level with the times recorded, not to reinterpret the one you have. This is the commonest reason an aminoglycoside level is reported without an interpretation.
The trough is the sample that matters most for safety. Aminoglycosides are concentration-dependent bactericidal agents, so the peak drives efficacy through the peak-to-MIC ratio; toxicity, though, follows the time the proximal tubule and the inner-ear hair cells spend exposed to drug. A trough that fails to fall says the drug is accumulating, and the correction is to lengthen the interval rather than to shave the dose, because the interval is what governs how far the concentration falls before the next dose. The two toxicities are not equivalent: aminoglycoside nephrotoxicity is usually a non-oliguric acute kidney injury that recovers when the drug stops, while the ototoxicity — high-frequency hearing loss, tinnitus, unsteadiness — is often permanent and may only declare itself after the course has finished. Loop diuretics, cumulative dose, prolonged or repeated courses, and mitochondrial 12S rRNA variants all increase that risk.
Targets depend on the regimen, so this page offers the regimen as a choice rather than quoting one range. On the older multiple-daily schedules the one-hour peak should not exceed 10 mg/L and the trough should be under 2 mg/L; on once-daily or extended-interval dosing the trough should be under 1 mg/L and a peak is not part of routine monitoring. In cystic fibrosis, where clearance is faster, the volume of distribution larger per kilogram and courses are repeated over decades, monitoring follows the unit’s own protocol and is often based on area under the curve rather than a peak and trough pair. Renal function shifts everything, because clearance is almost entirely glomerular. None of these numbers decides a dose on its own: they feed a decision that belongs to the prescriber and the pharmacist, together with the indication, the duration of treatment and whether the aminoglycoside is still needed — and the aminoglycoside half-life calculator is what turns two timed levels into the interval they imply.
Frequently asked questions
How do you convert tobramycin from mg/L to µmol/L?
Multiply by 2.13899, which is one milligram per litre divided by tobramycin’s molecular weight of 467.51 g/mol. A trough of 0.5 mg/L is 1.07 µmol/L, and dividing by the same factor goes back the other way. mg/L and µg/mL are numerically identical. Because tobramycin is a single compound rather than a congener mixture, the molar figure is exact — but dosing targets are all written in mg/L.
What trough should tobramycin be?
It depends on the regimen. On once-daily or extended-interval dosing the pre-dose trough should be under 1 mg/L. On multiple-daily dosing the BNF sets a trough under 2 mg/L with a one-hour peak that should not exceed 10 mg/L. In cystic fibrosis, monitoring follows local protocol and is often based on area under the curve. There is no single tobramycin range, and averaging the regimens would produce a figure that is wrong for each of them.
Why does the request need the dose time and the sampling time?
Because the same concentration is reassuring or alarming depending on when in the dosing interval it was taken. A sample drawn hours before the next dose is due is not a trough and will read high; one drawn well after the infusion has finished is not a peak and will read low. Without both times the laboratory cannot say which sample it has, and the level should be repeated rather than interpreted.
Is tobramycin ototoxicity reversible?
Frequently not. Tobramycin accumulates in cochlear and vestibular hair cells and the resulting high-frequency hearing loss or vestibular impairment is often permanent, sometimes becoming apparent only after treatment has ended. Duration of therapy, cumulative dose, concurrent loop diuretics, pre-existing hearing loss and mitochondrial 12S rRNA variants all raise the risk, which is why patients receiving repeated courses — particularly in cystic fibrosis — need baseline and follow-up audiometry rather than levels alone.
Why is tobramycin dosed differently in cystic fibrosis?
Clearance is higher and the volume of distribution is larger per kilogram, so standard doses produce lower concentrations than expected and higher milligram-per-kilogram doses are used. Monitoring follows the treating unit’s protocol, commonly an area-under-the-curve target or a single timed level rather than a conventional peak and trough, with a trough under 1 mg/L on once-daily regimens. Because courses are repeated over many years, cumulative exposure and audiometric surveillance matter as much as any individual level.
Related calculators
References
- Joint Formulary Committee. Tobramycin — Monitoring requirements. British National Formulary. London: BMJ Group and Pharmaceutical Press. One-hour peak concentration should not exceed 10 mg/L; pre-dose trough below 2 mg/L.
- Smyth AR, Bhatt J. Once-daily versus multiple-daily dosing with intravenous aminoglycosides for cystic fibrosis. Cochrane Database Syst Rev. 2014;(2):CD002009.
- Prescott WA Jr, Nagel JL. Extended-interval once-daily dosing of aminoglycosides in adult and pediatric patients with cystic fibrosis. Pharmacotherapy. 2010;30(1):95–108.
- 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.
- Selimoglu E. Aminoglycoside-induced ototoxicity. Curr Pharm Des. 2007;13(1):119–126.
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.