Tobramycin (Trough)
The lowest tobramycin level in your blood, taken just before your next dose, used to check the antibiotic clears well and does not build up.
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What it is
Tobramycin is an antibiotic given by IV or injection for serious infections caused by certain bacteria, such as Pseudomonas. A trough level is drawn right before the next dose, when the amount in the blood is lowest. Your care team reads it together with a peak level and your kidney function. A low trough is expected and is a good sign that the drug is clearing between doses.
Why it matters
A trough that stays high or keeps rising is an early signal that tobramycin is building up, usually because the kidneys are clearing it more slowly. FDA labeling says rising troughs above 2 mcg/mL may indicate accumulation, and that kidney injury is more likely when troughs do not fall below 2. Catching this early lets your team adjust the dose before harm to the kidneys or the inner ear (hearing and balance).
If it’s low
A low trough is usually expected and desired with tobramycin, because it means the drug is clearing between doses. It is only a question when the peak is also low, which can mean the dose is smaller than your body needs.
Common causes
- Normal or fast kidney clearance
- A once-daily (extended-interval) schedule, which is designed to leave a very low level before the next dose
- A sample drawn long after the previous dose
What can help
- Ask your team to read the trough together with the peak and your dosing schedule
If it’s high
A trough above the usual range means tobramycin is not clearing as fast as planned, or the sample was not a true trough. Kidney function is the most common reason. Your care team will usually look at your creatinine and may change the dose or the time between doses. A high trough is linked to a higher chance of kidney and hearing harm if it continues.
Common causes
- Reduced kidney function or dehydration
- A sample drawn too early, before the dose had time to clear
- A sample drawn from the IV line that the drug ran through
- Other medicines that can strain the kidneys or hearing, such as vancomycin, polymyxin B, cisplatin or strong water pills
- Older age, which often lowers kidney clearance
What can help
- Ask your care team how often your kidney function is being checked
- Make sure the blood draw was timed right before your dose and not taken from the infusion line
- Tell your team about every medicine you take, including other antibiotics and water pills
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: up to 2 µg/mL. Source: Quest Diagnostics, DailyMed, National Library of Medicine. Labs treat above 2 µg/mL as urgent.
- Adults (this one applies)up to 2 µg/mL
Sources: Quest Diagnostics, DailyMed, National Library of Medicine
What can affect your result
- Time between the last dose and the blood draw
- Dosing schedule: conventional (several times a day) or extended-interval (once a day)
- Kidney function, hydration and age
- Other medicines that affect the kidneys or hearing
- Whether the sample came from the infusion line
- A sample drawn after the dose has started or well before the next dose does not give a true trough.
- Drawing blood from the same IV line used for the infusion can falsely raise the result.
- Quest rejects serum separator (gel) tubes for this test; it asks for a red-top tube without gel (plasma in EDTA or heparin tubes is also accepted).
Just before the next dose (ARUP asks for collection about 30 minutes before the dose). Timing matters more than time of day. Tobramycin has a serum half-life of about 2 hours with normal kidney function, so a trough reflects how well the last dose cleared. Reduced kidney function makes it last longer (FDA labeling).
When to talk to a clinician
Tell your care team promptly about hearing loss, ringing in the ears, dizziness or balance trouble, much less urine, or new swelling while you are on tobramycin. Many hospitals ask the care team to call about troughs above 2 µg/mL. Call 911 or your local emergency number for trouble breathing, swelling of the face or throat, or fainting.
Questions you could ask
- How often will my levels and kidney tests be checked while I am on this antibiotic?
- Which signs of hearing or kidney problems should I report right away?
- Was this sample drawn at the right time, and not from the infusion line?
- Should the dose or the time between doses change based on my kidney function?
- Should we check my hearing and kidney tests?
- Is my trough this low because of my dosing schedule?
Questions people ask
My report says the trough range is 1.0–2.0. Is a result under 1.0 a problem?
Usually no. For tobramycin, a lower trough is the goal, because it means the drug clears between doses. The concern is a trough that rises above about 2, which FDA labeling says may mean the drug is building up. If your trough is low and your peak is also low, ask your team whether the dose is large enough.
Why does tobramycin need blood tests and kidney tests?
Tobramycin leaves the body through the kidneys, and too much for too long can harm the kidneys and hearing. Levels and creatinine tests let your team keep the dose in a useful and safe range as your kidney function changes.
Show the science and sources
Units
- 1 µg/mL = 1 µg/mL
- 1 µg/mL = 1 mg/L
- 1 µg/mL = 2.139 µmol/L
Open the Tobramycin trough unit converter
For clinicians and researchers
Tobramycin is an aminoglycoside eliminated almost entirely by glomerular filtration (serum half-life about 2 hours with normal renal function, longer in renal impairment and in premature or neonatal infants). FDA labeling states that nephrotoxicity is more likely if trough concentrations fail to fall below 2 mcg/mL, that rising troughs above 2 mcg/mL may indicate tissue accumulation, and that nephrotoxicity is most closely related to the AUC, so a trough is an incomplete exposure measure. Quest reports a 1.0–2.0 mg/L trough interval by immunoassay (mg/L equals µg/mL); ARUP asks for collection 30 minutes before the next dose and lists no interval for its tobramycin tests. Extended-interval regimens aim for a very low or undetectable trough, and protocols may use a random level instead. Hospital critical limits vary widely for the trough (median 2, range 1–12 µg/mL).
LOINC 4059-2 (primary), 22752-0. Specimen: serum-or-plasma.
Sources
- Tobramycin, Trough (test code 8577), Quest Diagnostics
- Tobramycin injection prescribing information, DailyMed, National Library of Medicine
- Drug Critical Limits for Urgent Physician Notification, Clinical Pharmacology & Therapeutics (Kuang, Ibrahim, Cheng, Kost) (2026)
- Tobramycin (PubChem compound 36294), PubChem, National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.