The Test Sample
What is being tested?
Gentamicin, tobramycin and amikacin are aminoglycosides, a group of antimicrobials (antibiotics) that are used to treat serious bacterial infections. The level of the prescribed aminoglycoside in the blood is measured in order to adjust doses as necessary and ensure effective treatment while avoiding toxic side effects. (For more information about this, see the article on Therapeutic Drug Monitoring.)
Gentamicin, tobramycin, and amikacin are the most commonly prescribed aminoglycosides, and they are used to treat infections caused by certain types of 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 having an adequate level in the blood. Aminoglycosides are associated with serious toxic side effects, including damage to hearing and/or balance (ototoxicity) and acute kidney damage (nephrotoxicity). Though kidney damage caused by aminoglycosides is usually reversible, hearing and/or balance loss is frequently permanent. These side effects can occur at any time, but the risk is greater with elevated blood levels and when the drugs are given for an extended period of time.
Aminoglycosides are not well absorbed by the digestive system so they are typically be administered either through a needle into a vein (intravenously, IV) or by injection into a muscle (intramuscularly, IM). Aminoglycosides can be given using dosing intervals (such as every 8-12 hours) or given as a large 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 measured concentration, the next 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 an 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 and is unable to maintain a sufficient level in the blood, it is unlikely that treatment will be effective.
For extended-interval dosing, testing may be performed similarly to interval dosing, using a peak sample and a sample taken 6-12 hours later, or testing can be performed on a single sample taken 6-14 hours after the first dose of antibiotic.
Aminoglycosides are sometimes prescribed alone but are often combined with other antibiotics. Monitoring the antibiotic blood level is especially 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 blood sample is important. For interval dosing, trough levels are collected just prior to a person's next aminoglycoside dose. Peak levels are collected 30-45 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, 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 healthcare provider's directions for collection. It may be helpful to tell the laboratorian when the administration of the last dose was completed.