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Free Gentamicin in Serum/Plasma (Mass Concentration)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of free (unbound) gentamicin, a medication used to treat serious bacterial infections, in your blood.

Quick summary

["Free gentamicin levels help doctors monitor how much of the drug is actively working in your bloodstream."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
80971-5
Short code
Gentamicin free SerPl-mCnc
Synonyms
Gentamicin free SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Matters

Gentamicin is a broad-spectrum antibiotic, but high levels can damage your kidneys or nerves. Free gentamicin is the portion that actually reaches these organs. Measuring it helps prevent toxic side effects while ensuring the drug remains effective against infections.

What to Expect Before the Test

No special preparation is needed. You’ll provide a blood sample drawn from a vein, usually in your arm. The test is quick and typically done in a lab setting.

What the Results Mean

Results are used to guide your antibiotic dosing. Your doctor will compare your free gentamicin level to established therapeutic ranges to adjust your medication safely.

Possible Causes for Testing

Your doctor may order this test if you’re on gentamicin therapy to check for toxicity, optimize dosing, or monitor kidney function during treatment.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Free Gentamicin Level

Frequently asked questions

How often should I get this test?
Your doctor will determine the frequency based on your treatment plan, infection severity, and kidney function. Common schedules include daily or every few days during active therapy.
What if my results are outside the normal range?
If your free gentamicin level is too high, your doctor may reduce the dose or switch medications to protect your kidneys. If too low, they may increase your dose to ensure the infection is fully treated.
Does this test interact with other medications?
Yes, some medications (like certain diuretics or other antibiotics) can affect gentamicin levels. Always inform your healthcare provider about all medications, including over-the-counter drugs and supplements, before starting gentamicin.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy