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Blood Test Tracker

Gentamicin (Trough)

Shows how much gentamicin is still in your blood just before your next dose, a check that the antibiotic is clearing and not building up.

AI-verified against cited sources. Awaiting expert reviewLOINC 3665-7

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What it is

Gentamicin is an antibiotic given by injection or IV drip for serious bacterial infections. A trough level is drawn right before your next dose, when the drug is at its lowest. Your care team uses it to check that gentamicin is clearing from your body and not building up, often together with the peak level drawn after a dose.

Why it matters

If gentamicin clears slowly, levels stay high between doses. Long exposure raises the chance of kidney and inner-ear damage, and hearing damage from this type of antibiotic is usually permanent. The trough is one of the main safety checks during treatment.

If it’s low

A very low or undetectable trough is common and often expected, especially with once-daily dosing, because the drug clears out between doses. It is usually not a safety concern. Whether the dose is working is judged from your infection and from other measures such as the peak.

Common causes

  • Once-daily or less frequent dosing, which is designed so the drug clears between doses
  • Good kidney function, which clears the drug quickly
  • Fever, anemia or severe burns, which the label says can shorten how long the drug stays in the body
  • A sample drawn after the drug had already cleared

What can help

  • Ask how your result fits your dosing plan
  • Ask whether a peak or other level is also being checked

If it’s high

A trough above 2 mcg/mL is higher than the FDA label recommends. It can mean the drug is not clearing as fast as expected, often because the kidneys are working less well. It can also mean the sample was drawn too soon after a dose. Your care team may adjust the dose or the time between doses.

Common causes

  • Reduced kidney function, including kidney strain from the infection itself
  • Dehydration
  • Older age
  • A sample drawn too early, before the lowest point, or from the line used for the infusion
  • Other medicines that stress the kidneys or ears, such as other aminoglycosides, vancomycin, cisplatin or strong water pills

What can help

  • Tell your care team right away about new hearing loss, ringing in the ears, dizziness, or much less urine than usual
  • Ask whether your kidney function is being checked during treatment
  • Ask whether staying well hydrated is right for you while you are on this medicine

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 mcg/mL. Source: Hospira, via DailyMed (NIH National Library of Medicine), Journal of Pharmaceutical Policy and Practice, Clinical Pharmacology & Therapeutics. Labs treat above 2 mcg/mL as urgent.

Wellness targets

Narrower ranges some clinicians aim for. Being outside one doesn’t mean something is wrong.

up to 1 mcg/mL (Adults). Evidence: Limited. Australian Prescriber (2025) aims for a trough under 1 mg/L, and a 2020 international position paper (cited in a 2026 survey) treats troughs above 1 mg/L as a toxicity threshold. The exact cutoff has limited outcome evidence, and US labeling uses under 2.

What can affect your result

  • Kidney function
  • Dosing plan and the time between doses
  • How close to the next dose the sample was drawn
  • Age and hydration
  • Fever, anemia or severe burns (the label notes shorter half-life in these states)
  • Other medicines that stress the kidneys or ears
  • A sample drawn after the dose is not a trough and will read higher than a true trough.
  • Drawing from the same line or arm used for the infusion can contaminate the sample and give a falsely high result.
  • In a tube that sits for a long time, high doses of some penicillin-type antibiotics can inactivate gentamicin and lower the measured result.

Trough sample: just before the next dose (ARUP's critical-values list says to draw 30 minutes before the infusion). Troughs reflect how quickly the kidneys clear the drug between doses. They can rise within days if kidney function drops.

When to talk to a clinician

Tell your care team right away if you notice hearing loss, ringing in your ears, dizziness or balance trouble, numbness or tingling, muscle twitching, or much less urine while on gentamicin. A trough above 2 mcg/mL is on ARUP's critical-values list, which means labs call the ordering clinician right away. If you feel faint or confused, or have trouble breathing, call 911 or your local emergency number.

Questions you could ask

  • Should I have a hearing check during or after treatment?
  • How long will I be on this medicine?
  • Is my dose or the time between doses being adjusted?
  • How is my kidney function being checked while I take this?
  • Does my low trough match my dosing plan?
  • Is a peak or other level also being checked?

Questions people ask

Why is my trough often undetectable?

With once-daily or less frequent dosing, the dose is meant to clear before the next one, so the trough is expected to be very low. That is a design feature, not a problem.

Why do some sources say under 1 and the FDA label says under 2?

US labeling uses under 2 mcg/mL. Australian guidance and a 2020 international position paper use under 1 mg/L as a stricter safety goal. Your team follows the target set by its own protocol.

How do I convert mcg/mL to µmol/L?

Multiply mcg/mL (the same as mg/L) by about 2.09. For example, 2 mcg/mL is about 4.2 µmol/L.

Show the science and sources

Units

  • 1 mcg/mL = 1 mcg/mL
  • 1 mcg/mL = 1 mg/L
  • 1 mcg/mL = 2.094 µmol/L

Open the Gentamicin Trough unit converter

For clinicians and researchers

Gentamicin is an aminoglycoside eliminated unchanged by glomerular filtration, so trough concentrations track renal clearance and accumulation. The FDA label says to adjust dosing so troughs above 2 mcg/mL are avoided, and ARUP and the median of US hospital lists use the same critical limit (above 2). Stricter goals exist: Australian Prescriber (2025) aims for under 1 mg/L for gentamicin or tobramycin on multiple-daily or synergy regimens, notes that troughs correlate poorly with total exposure and efficacy, and prefers area-under-the-curve monitoring (target 80 to 100 mg·h/L) with once-daily dosing in adults. A 2020 international position paper, as cited in a 2026 survey, treats troughs above 1 mg/L as a toxicity threshold. A trough is meaningful only if it is drawn immediately before the next dose. Gentamicin is a mixture of C1, C1a and C2 components; labs convert with about 2.09 µmol/L per mcg/mL.

LOINC 3665-7 (primary), 22747-0, 31092-0. Specimen: serum-or-plasma.

Sources

  1. Gentamicin Sulfate Injection, USP: prescribing information, Hospira, via DailyMed (NIH National Library of Medicine) (2026)
  2. ARUP Laboratories Critical Values List, ARUP Laboratories (2026)
  3. Drug Critical Limits for Urgent Physician Notification, Clinical Pharmacology & Therapeutics (2026)
  4. Appropriateness of gentamicin therapeutic drug monitoring at a Middle Eastern tertiary hospital setting: a retrospective evaluation and quality audit, Journal of Pharmaceutical Policy and Practice (2024)
  5. Aminoglycosides: an update on indications, dosing and monitoring, Australian Prescriber (2025)
  6. Gentamicin C1 (compound summary), PubChem (NIH National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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