Gentamicin Level

Gentamicin is an antibiotic administered to treat severe blood infections caused by gram-negative bacilli.

Table 1. Optimal and Toxic Gentamicin levels ^[1] (Open Table in a new window)

Specimen: Blood

Container: Red-top tube

Collection method: Routine venipuncture

Panels: Gentamicin testing is not typically a part of a panel and should be obtained specifically. ^[1]

Description

Gentamicin is an antibiotic administered to treat severe blood infections caused by gram-negative bacilli. It may be used in combination with beta-lactam antibiotics.

Indications/Applications

Gentamicin is considered helpful in the treatment of bacterial septicemia, bacterial neonatal sepsis, and serious bacterial infections of the central nervous system, urinary tract, respiratory tract, gastrointestinal tract, skin, bone, and soft tissue.

Patients who require treatment with gentamicin often require monitoring, which includes peak and trough levels. Peak levels are typically evaluated 30 minutes after completion of the infusion. Trough levels should be drawn immediately before the dose is administered. Ideally, these levels should be evaluated by the third dose to allow the drug to reach a steady state and to provide adequate information for calculated adjustments in dosing regimens.

In underweight patients, the total body weight is used to calculate dose. In patients whose weight is 1-1.2 times their ideal body weight, calculate dosing using their ideal body weight. In patients who weight more than 1.2 times their ideal body weight, instead formulate the dosing with their adjusted body weight. The dosing of gentamicin may be 1, 2, or 3 times per day via intravenous or, less commonly, intramuscular injections to achieve peak blood concentrations between 5 and 12 µg/mL. Dosing should be adjusted to avoid prolonged levels in excess of 12 µg/mL. ^[2]

A study by O’Connor et al indicated that a regimen of 2.5 mg/kg of gentamicin administered every 36 hours in neonates of less than 30 weeks’ gestational age and every 24 hours in neonates of at least 30 weeks’ gestational age is suboptimal for reaching the drug’s therapeutic peak. However, at a dosage of 3.5 mg/kg, therapeutic peak concentrations were obtained in 98% of the study’s neonates. ^[3]

Considerations

Because gentamicin is excreted in the urine rather than metabolized, the administered dosing amount or dosing interval must be adjusted to compensate for reduced renal function. Therefore, any decrease in renal function without a concomitant decrease in the dose amount or interval of administration may increase the daily serum gentamicin concentration. An increase in the serum gentamicin level results in additional nephrotoxicity, beginning a cycle of damage.

Excessive concentrations may also cause damage to the eighth cranial nerve, resulting in temporary or permanent hearing loss and vestibular dysfunction due to ototoxicity. ^[4]

For long durations of drug use, baseline audiology/vestibular testing should be considered along with periodic testing during therapy. ^[5] The risk of toxicity is increased with coadministration of other ototoxic or nephrotoxic drugs, including, but not limited to, cisplatin, amikacin, colistin, cephaloridine, kanamycin, vancomycin, ethacrynic acid or furosemide, paromomycin, streptomycin, tobramycin, neomycin, polymyxin B, and viomycin. Other factors that may increase patient risk to toxicity include advanced age and dehydration. Thus, patients should be well hydrated during treatment.

The use of gentamicin in the neonatal intensive care unit using a weight-based gentamicin dosing protocol should also account for serum creatinine levels and urine output as indicators for identifying neonatal patients at risk for supratherapeutic trough levels. Physiological shock and nonstandard dosing also creates increased risk for supratherapeutic trough levels. ^[6]

Once-daily dosing of gentamicin in children is performed in certain circumstances; however, clinical data regarding toxicity and effectiveness are evolving. Some authors suggest nephrotoxicity to be uncommon and reversible. Ototoxicity occurred more frequently and was irreversible; however, those who experienced it were also receiving other nephrotoxic or ototoxic medicines. Usefulness of therapeutic drug monitoring to predict or prevent temporary or permanent toxicity in these situations is variable and unclear. ^[7]

Creatinine clearance of less than 60 mL/min is an exclusion criterion.

A literature review by Dillard et al found a high prevalence of self medication with aminoglycosides, with gentamicin being the most commonly overused of these drugs. ^[8]