Skip to content
Blood Test Tracker

Gentamicin (Peak)

Shows the highest level of the antibiotic gentamicin in your blood shortly after a dose, to check the dose is strong enough without being too strong.

AI-verified against cited sources. Awaiting expert reviewLOINC 3663-2

Check your result

Sex (optional)

Type your result to see where it falls.

Nothing you type here leaves your browser.

What it is

Gentamicin is an antibiotic given by injection or IV drip for serious bacterial infections. A peak level is a blood test drawn shortly after a dose finishes, when the drug is at its highest. Your care team uses it, often with the trough level drawn right before the next dose, to check that the dose is high enough to work but not so high that it harms your kidneys or ears.

Why it matters

Gentamicin works best when levels are high enough to kill bacteria, but too much can damage the kidneys and the inner ear. Hearing damage from this type of antibiotic is usually permanent. Peak levels help your team confirm the dose suits your infection, your size and your kidney function.

If it’s low

A peak below your team's target can mean the dose is not reaching the level needed to fight the infection. It can also be expected with some dosing plans, or come from the timing of the draw. Your care team reads it alongside your infection, kidney function and symptoms.

Common causes

  • Fever, anemia or severe burns, which the label says can shorten how long the drug stays in the body
  • A sample drawn long after the infusion ended
  • An infusion that ran slowly or was interrupted
  • A dosing plan that aims for lower peaks

What can help

  • Ask whether the draw time was recorded correctly
  • Ask how your result compares with the target for your type of infection

If it’s high

A peak above about 12 mcg/mL is higher than the FDA label recommends if it stays that high for long. It can mean the dose was larger than your body needed, or that the drug is clearing slowly, for example when the kidneys are not working well. A sample drawn too soon after the dose can also read high. Some dosing plans, such as high-dose once-daily dosing, are meant to give higher peaks. Your care team decides whether to change the dose or the timing.

Common causes

  • Reduced kidney function, which slows how fast gentamicin leaves the body
  • A dose that was larger than needed for your size
  • A sample drawn too soon after the infusion ended, or from the same line used for the infusion
  • Dehydration
  • Older age

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: 5–10 mcg/mL. Source: Journal of Pharmaceutical Policy and Practice, Hospira, via DailyMed (NIH National Library of Medicine), Australian Prescriber. Labs treat above 12 mcg/mL as urgent.

What can affect your result

  • How long after the infusion the sample was drawn
  • Kidney function
  • Body weight and fluid status
  • Fever, anemia or severe burns (the label notes shorter half-life in these states)
  • Dosing plan: several doses a day versus high-dose once-daily dosing
  • Other medicines that stress the kidneys or ears, including some strong water pills
  • The exact draw time matters. A sample taken too early or too late relative to the infusion changes the number.
  • 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.

Peak sample: about 30 minutes after an IV infusion ends, or 30 to 60 minutes after an injection into a muscle. The level reflects the dose just given and changes within hours.

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 peak above 12 mcg/mL is on ARUP's critical-values list, which means that lab calls the ordering clinician right away. A higher peak can be planned with once-daily dosing, so ask your team which target applies to you. 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?
  • Was the blood drawn at the planned time after the infusion?
  • What is my target peak, and is it based on my type of infection?

Questions people ask

What is the difference between a peak and a trough?

A peak is drawn shortly after a dose, when the drug is highest. A trough is drawn right before the next dose, when it is lowest. The peak shows whether the dose is strong enough, and the trough shows whether the drug is building up.

Why isn't there one normal range?

Gentamicin is a medicine, not something your body makes. The target depends on the kind of infection, your kidney function and the dosing plan your team uses. The numbers on this page are typical targets, and your team's goal comes first.

How do I convert mcg/mL to µmol/L?

Multiply mcg/mL (the same as mg/L) by about 2.09. For example, 10 mcg/mL is about 20.9 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 Peak unit converter

For clinicians and researchers

Gentamicin is an aminoglycoside with concentration-dependent killing and a post-antibiotic effect. It is eliminated unchanged by glomerular filtration (70% or more of a dose is recovered in urine within 24 hours) and has low protein binding (0 to 30%). The FDA label advises avoiding prolonged peaks above 12 mcg/mL and troughs above 2 mcg/mL, and expects 4 to 6 mcg/mL 30 to 60 minutes after an intramuscular injection. Traditional dosing targets depend on the infection site: one 2024 hospital protocol used 5 to 6, 6 to 8 and 8 to 10 mg/L. Many centers now use high-dose extended-interval dosing, where Australian Prescriber (2025) prefers area-under-the-curve monitoring, with a target of 80 to 100 mg·h/L for empirical Gram-negative therapy, and says peaks (aiming for 7 to 10 times the pathogen's MIC) are only a fallback when that is unavailable; such peaks can sit well above 12 mg/L. Gentamicin is a mixture of C1, C1a and C2 components; labs convert with about 2.09 µmol/L per mcg/mL.

LOINC 3663-2 (primary), 22746-2, 31091-2. 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.

Related markers