These guidelines aim to produce a trough of < 1 mg/L. Doses of up to 300mg can be given as a bolus injection over 3 to 5 minutes; higher doses should be administered as a short infusion over 30 minutes. This will not alter interpretation of trough levels.

Weight banded dosing is given in the table below. Doses must be calculated using maximum body weight if patients are obese (actual weight more than 20% of Ideal Body Weight) (see maximum body weight table).

Maximum body weight table

Use to determine dosing weight for obese patients (defined as actual weight more than 20% of Ideal Body Weight)

1. Take a blood sample for gentamicin analysis at the end of the first dosage interval (trough concentration) then give the next dose. Do not delay giving the second gentamicin dose while waiting for trough concentration.
2. If the gentamicin trough is <1mg/L, continue the present dosage regimen.
3.  Record the exact time of gentamicin samples on the sample request form.
4.  Seek advice from pharmacy if you are unsure how to interpret the result or if the concentrations are not within range.

• Monitor renal function and gentamicin trough levels daily for the first three days.
• If renal function is stable, check the trough concentration every 3 days.
• If renal function deteriorates, check the trough daily.

If the gentamicin trough concentration is >1mg/L and a further dose has been administered, re-analyse the trough after the next dose. Do not give a further dose until the gentamicin concentration is <1mg/L.

Suggested dose adjustment for serum levels out with range

High Trough: 

• Increase dosage interval. Extension of the dose interval would normally be from 24 hourly to 48 hourly.
• Discuss with pharmacy if subsequent levels remain out with range

Gentamicin therapy should continue for 2 weeks except in the case of enterococcal infective endocarditis (IE) when it may be given for 2 to 6 weeks on Infection Specialist advice.

Renal toxicity

• Monitor creatinine daily. Seek advice if renal function unstable (eg change in creatinine of >15%).
• Signs of renal toxicity include increase in creatinine.
• Falling urine output or oliguria will lead to accumulation of gentamicin and increase the risk of toxicity.
• Consider an alternative agent if creatinine is rising or the patient becomes oliguric.

Ototoxicity

• Ototoxicity secondary to gentamicin is independent of drug concentration. It is suggested by any of the following: new tinnitus, dizziness, poor balance, hearing loss or oscillating vision.
• Toxicity is associated with prolonged aminoglycoside use (usually >10 days but may be >72 hours) and is secondary to drug accumulation within the inner ear.
• Stop treatment if ototoxicity is suspected and refer to microbiology or an infection specialist for advice on future therapy.
• If gentamicin is to continue for more than 7 days, consider referring to audiology for assessment.