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
At regular intervals during treatment with an aminoglycoside
A blood sample drawn from a vein
No test preparation is needed, but the timing of the blood sample is important.
- For interval dosing, trough levels are collected just prior to your next aminoglycoside dose. Peak levels are collected 30-45 minutes after the completion of an intravenous (IV) dose or 60 minutes after a dose injected into a muscle.
- 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 your healthcare provider's directions for collection. It may be helpful to tell the laboratory professional drawing your blood the time your last dose was completed.
Aminoglycosides are a group of 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. The risk of side effects is lower with some of the aminoglycosides that have been developed recently.
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
- 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 you may be taking, your age and weight.
When a dose of aminoglycosides is given, the level typically rises in the blood to a peak concentration and then falls over time to a lower (trough) concentration. Sometimes these drugs are prescribed using interval dosing, in which the subsequent dose is timed to be given in anticipation of the falling level. The goal is to dose a sufficient amount of drug to maintain a therapeutic level that will kill the bacteria causing the infection. The dose and the dosing interval are optimized to give the body enough time to clear most of the drug from the previous dose before the next dose is given. This minimizes the risk of complications and helps ensure that an adequate drug level is always maintained in the blood.
- 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, you may not be able to clear the drug out of your system efficiently if you have kidney disease, 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 you are given too little drug and have an insufficient 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 test used?
This test is used to monitor the level of the prescribed aminoglycoside antibiotic in the blood. Testing is used to ensure that the level of the drug in the blood is sufficient to treat the infection but not so high as to increase the risk of side effects.
In some cases, a dose of the drug is given only once every 24-48 hours (called extended-interval or pulse dosing). A test of the drug level on a sample taken 6-14 hours after the dose is used to ensure adequate dosing.
Blood drug levels are used by clinical pharmacists and healthcare providers to calculate the rate at which your body clears the drug from your blood. These results are then used to determine the appropriate amount of drug and the appropriate timing between doses to assure that the blood concentration is adequate for treating the infection but is not so high as to increase the risk of toxic side effects. For additional information on how the test is used, see Therapeutic Drug Monitoring.
When is it ordered?
Blood levels of gentamicin, tobramycin or amikacin may be monitored under a variety of conditions. For example, your age, kidney function, overall health, and presence of underlying conditions or symptoms of toxicity may be considered in the decision to perform the testing. The length of treatment and type of protocol used for dosing can also be factors.
Monitoring of aminoglycosides may be recommended when you will be receiving the drug for more than 3 days.
- For interval dosing, testing is usually ordered after 2 to 4 doses of the aminoglycoside have been given and when the drug is expected to have reached a relatively stable level in the blood (steady state). Drug levels then may be measured again every few days or once a week and with any change in the amount or timing of the dose or with change in kidney function.
- With individuals receiving extended interval dosing, no steady state of the drug will be achieved. Typically, a timed random sample is drawn 6 to 14 hours after the dose for testing.
More frequent aminoglycoside monitoring may be performed for people with impaired kidney function (renal insufficiency) and for people who have an increased risk of toxic side effects, such as those taking other drugs known to adversely affect hearing and the kidneys (ototoxic or nephrotoxic).
What does the test result mean?
- A trough level of the aminoglycoside below the target level indicates that you are clearing the drug at an adequate rate.
- A peak level within the therapeutic range means there should be sufficient drug in the blood to be effective in treating your infection. The target level typically depends on the type of infection and the organ infected.
- A peak level below the maximum level indicates that you are at less risk of developing toxic side effects, though you may still experience a complication.
- If the trough and/or peak level is above the maximum level, then you are at an increased risk of toxicity and your healthcare provider may either alter the dose or alter the dosing schedule.
For an extended-dose regimen, the results can help your healthcare provider decide when to give the next dose. In general, if the blood level is at the low end of the range, the healthcare practitioner may decide to dose every 24 hours. If the level is at the higher end (indicating that the drug is being cleared more slowly), the healthcare practitioner may wait 48 hours before giving the next dose.
If your infection is not responding to the treatment, then your healthcare provider may either continue the drug for a longer period of time or consider other treatment options.
Is there anything else I should know?
Intravenous doses of aminoglycosides are given slowly over about 30 minutes.
Other forms of aminoglycosides, such as eye drops, ear drops, and inhaled drugs, may be used to treat specific types of infections. Monitoring is not used in these cases.
The first aminoglycoside, streptomycin, was developed in the 1940s and used successfully to treat tuberculosis. Its use declined with the introduction of other aminoglycosides.
Aminoglycosides are cleared from the body by the kidneys, thus dosages are modified based upon kidney function. Tests that reflect the health of the kidneys, such as a creatinine and estimated glomerular filtration rate (eGFR), are often ordered prior to the initiation of aminoglycoside therapy and then at intervals to monitor kidney function.
Risk of toxicity is increased in people who are taking other drugs that affect hearing and the kidneys such as certain diuretics, particularly furosemide, or NSAIDS (nonsteroidal anti-inflammatory drugs) such as ibuprofen or naproxen, or other antibiotics such as vancomycin.
Because of the potential for complications, extended-interval dosing is not recommended in people who:
Can I have my level tested at home?
Although you may receive intravenous aminoglycoside therapy at home, usually administered by a home health professional, blood levels cannot be monitored at home. The test requires specialized equipment and must be performed in a laboratory. The home health professional may draw a blood sample prior to administering the next dose of drug. This sample will be sent to a laboratory for analysis.
Why are aminoglycosides used at all if they can cause permanent hearing loss?
Should all antibiotic therapy be monitored like aminoglycosides?
No, not all antibiotics require monitoring. Unlike aminoglycosides, most antibiotics are not associated with significant side effects that are predictable with drug levels. They have a larger therapeutic range in which they are effective. Because of this, they can be prescribed based upon pre-established dosing schedules.