At a Glance
Why Get Tested?
To monitor the level of an aminoglycoside antibiotic such as gentamicin, tobramycin, or amikacin in the blood to ensure adequate dosing and help avoid toxic side effects
When to Get Tested?
At regular intervals during treatment with an aminoglycoside
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None, but timing of the sample for testing is important; follow your doctor's directions.
The Test Sample
What is being tested?
Aminoglycosides are a group of antimicrobials (antibiotics) that are used to treat serious bacterial infections. These tests measure the level of the prescribed aminoglycoside in the blood in order to adjust doses as necessary and ensure effective treatment while avoiding toxic side effects.
Gentamicin, tobramycin, and amikacin are the most commonly prescribed aminoglycosides, and they are used to treat infections cause by certain types of bacteria called Gram-negative bacteria as well as a few Gram-positive bacteria. (For more on these, see the article on the Gram stain).
It is important to monitor the concentration of aminoglycosides because their effectiveness depends on an adequate level in the blood. Furthermore, aminoglycosides are associated with serious toxic side effects, including damage to hearing and/or balance (ototoxicity) and acute kidney damage (nephrotoxicity). The kidney damage is usually reversible, but the hearing and/or balance loss is frequently permanent. These complications can occur at any time, but the risk is greater with elevated blood levels, indicating that the drug is not being cleared from the blood at a typical rate, and when it is given for an extended period of time.
Aminoglycosides are not well absorbed by the digestive system so they must typically be administered either through a needle into a vein (intravenously, IV) or by injection into a muscle (intramuscularly, IM). They are given using dosing intervals (such as every 8-12 hours) or given as a larger single dose once every 24 to 48 hours (also called extended-interval or pulse dosing). The amount of an aminoglycoside given per dose depends on a variety of factors, including kidney function, other drugs the person may be taking, age, and weight.
For interval dosing, drug monitoring typically involves assessing the maximum concentration soon after a dose is given (called a peak level) and the minimum concentration just before the next dose is given (called a trough level). Depending on the results, the dose of drug may be adjusted up or down. For example, a person with decreased kidney function may not be able to clear the drug out of his system efficiently, resulting in increased concentration in the blood, so the dose may be adjusted lower or the drug may be given less frequently. On the other hand, if a person is given too little drug to maintain a sufficient minimum level in the blood, then it is unlikely that treatment will be effective.
For extended-interval dosing, testing may be performed on a timed sample taken 8-12 hours after dosing.
Aminoglycosides are sometimes used alone but are often combined with other antibiotics. Monitoring the antibiotic blood level is important in the presence of other medications as they can affect the ability of the body to process (metabolize) and clear the drug.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed, but the timing of the sample for testing is important. For interval dosing, trough levels are collected just prior to a person's next aminoglycoside dose. Peak levels are collected about 30 minutes after the completion of an intravenous dose or 60 minutes after an intramuscular dose. For extended-interval dosing, the recommended collection time may vary (8-12 hours after dose), but the time of the completion of the last dose and the time of the blood sample collection will be recorded and compared. Follow the doctor's directions for collection. It may be helpful to tell the laboratorian when the administration of the last dose was completed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
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