Gentamicin

C. diff Risk

Low

Oral Bioavailability

NA

Approximate Cost

$10-30/d

Dosing

Renal

Renally cleared, requires dosage adjustment with changes in renal function. Consult a pharmacist for renal dosing.

General Information

Common Usage

  • Empiric (in combination) or targeted therapy for suspected or confirmed gram negative infections.

  • Empiric therapy for pyelonephritis.

  • Used synergistically in enterococcal endocarditis.

Drug Monitoring

Monitor creatinine at least weekly and more often if levels are elevated or other signs of renal dysfunction arise. Discontinue if any signs of ototoxicity (tinnitus, fullness in ears, dizziness).

Serum Level Monitoring for EXTENDED Interval Dosing, Pediatrics:
Obtain trough concentrations when treatment is anticipated to be longer than 48 hours.
Measure trough level 1 hour prior to second dose.Target trough: less than 1 microgram/mL.If trough is less than 1 microgram/mL, continue current dose and repeat trough every 7 days.

Serum Level Monitoring for EXTENDED Interval Dosing, Neonates:
Check gentamicin level at 22 hours post start of infusion, regardless of the dosing interval that the patient is started on for all patients with an anticipated gentamicin treatment greater than 48 hours.
For patients less than or equal to 7 days of age in whom anticipated gentamicin treatment duration is less than or equal to 48 hours (i.e. clinical suspicion for early-onset sepsis is low), only order 22 hour gentamicin level if any or the following criteria apply:

  • urine output less than 1 mL/kg/h or a serum creatinine greater than or equal to 120 umol/L

  • hypoxic ischemic encephalopathy

  • concurrent ibuprofen/indomethacin administration

  • concurrent furosemide, vancomycin, vasopressor, or inotrope administration

  • severe cardiac anomalies

  • less than or equal to 29 weeks gestation or less than 1500 grams birth weight

Adjust dosing interval based on the 22-hour level drawn with the first dose by referring to the information below or in discussion with the pharmacist.

  • If 22 h level is: 1.2 mcg/mL or less- give dose every 24h

  • 1.3 to 2.6 mcg/mL- give dose every 36h

  • 2.7 to 3.5 mcg/mL- give dose every 48h

  • 3.6 mcg/mL or more- Hold next dose, repeat level in 24 hours
    If levels are within target range continue current dosing regimen. Repeat 22 hour level every 7 days or sooner if clinically necessary. Peak levels are not routinely measured but may be ordered if severe gram negative infection or concern about clinical progress.

Serum Level Monitoring for EXTENDED Interval Dosing, Women's Health:
For anticipated duration of therapy more than 2 days monitor serum gentamicin levels as follows:
Measure level 18 hours after start of infusion (based on 30 minute infusion) to assess for adequate clearance. Target 18 hour level: 1 microgram/mL or less.
If 18 hour level is within desired range, repeat every 4 days or sooner if clinically necessary.

Serum Level Monitoring for CONVENTIONAL Dosing, Pediatrics :

Serum concentrations of aminoglycosides should be monitored to ensure therapeutic levels have been achieved and to avoid potentially toxic levels. Obtain peak and trough concentrations when treatment is anticipated to be longer than 48 hours.
Obtain drug levels with the third or fourth dose. Peak serum levels are drawn 30 minutes after the end of a 30 minute IV infusion or 1 hour following IM injection and should be 4-12 mcg/mL depending on infection . Trough levels are drawn just prior to the next dose (0-30 minutes) and should be less than 2 mcg/mL.

Serum Level Monitoring for SYNERGISTIC Dosing, Pediatrics :
Targets are: Peak 3 to 5 mcg/mL and trough less than 2 mcg/mL.

Adverse Effects

  • Nephrotoxicity (non-oliguric): less common with once daily dosing; greater toxicity with longer duration and supratherapeutic trough levels; avoid concomitant nephrotoxins

  • Vestibulocochlear toxicity (irreversible): suggest audiology testing if prolonged use

  • Can exacerbate neuromuscular blockade- e.g. contraindicated in patients with myasthenia gravis.

Major Interactions

  • Enhanced nephrotoxic effect with concomitant use of other nephrotoxins

  • Enhanced ototoxicity with loop diuretics (e.g. furosemide)

  • Non-depolarizing muscle relaxants may be potentiated

Pharmacology

Antimicrobial class: Aminoglycoside

Route of Elimination: Almost completely by glomerular filtration of unchanged drug with excretion into urine.

Average serum half life: - Neonates: 3-11.5 hours
- Infants: 4 ± 1 hour
- Children: 2 ± 1 hour
- Adolescents:1.5 ± 1 hour
- Adolescents: 1.5 ± 1 hour
- Adults: 1.5- 3 hours; End stage renal disease: 30-70 hours