Gentamicin - adult standard dosing

Antimicrobial prescribing: NHS Lothian
NHS Lothian

This guidance is for NHS Lothian staff wishing to use ONCE DAILY GENTAMICIN.

In some conditions (e.g. endocarditis) synergistic-dose gentamicin is used. 

Exclusions and contraindicationsDo not use this guidance in the following groups:

  • Hypersensitivity to aminoglycosides 
  • Myasthenia gravis
  • Dialysis patients
  • Surgical antimicrobial prophylaxis

Use in the these groups ONLY on advice from infection specialists & antimicrobial pharmacist

Cautions:

  • Acute or chronic renal impairment or unstable creatinine
  • Patients on medicines that increase risk of toxicity (consider stopping):
    • Nephrotoxicity: NSAID, ACE inhibitor
    • Ototoxicity: diuretics (particularly loop diuretics like furosemide)
    • Neuromuscular blockers
  • Decompensated liver disease (increased risk of renal failure)

Avoid co-administration of gentamicin with other aminoglycosides

Required monitoring

Check U+E + trough level daily; if renal function changes significantly (by >15%), recalculate creatinine clearance and adjust dosing.

Audiometry: If therapy continues for >7days, obtain baseline audiometry then repeat monthly

Gentamicin toxicity

Nephrotoxicity:

  • Dose-dependent: further rise in creatinine/oliguria will lead to gentamicin accumulation
  • Ensure good hydration, avoid co-prescription nephrotoxic medication (eg. furosemide)

Ototoxicity

  • Dose-independent; related to drug accumulation in the inner ear
  • Associated with prolonged use (usually >10 days but may be >72 hours).
  • Symptoms: new tinnitus, dizziness, poor balance, hearing loss or oscillating vision.

If ototoxicity/vestibulotoxicity is suspected, discontinue aminoglycoside therapy, refer to ENT for assessment and call microbiology to discuss alternative antimicrobials.

If therapy continues for >7days, obtain baseline audiometry then repeat monthly.

Calculate once daily gentamicin dose

Only do this if the gentamicin calculator is unavailable

Document your calculations in TRAK (Obtain a second check for all data entry/calculations if possible). Ensure clinical pharmacist review at earliest opportunity.

1. Calculate the patient's creatinine clearance

  • Calculate using patient's age, height, weight & serum creatinine.
  • Calculator available here (NB: Intranet only)
  • Other online calculators can be used as long as they use the Cockcroft-Gault formula
  • As a last resort calculate manually. 

2. Calculate dosage

Dose depends on the patient’s creatinine clearance and actual body weight

Creat Cl (ml/min) 40 - 49 kg 50 - 59 kg 60 - 69 kg 70 - 80 kg > 80 kg
< 21 2.5 mg/kg (max 180 mg) then take a sample after 24 hours
21 - 30

180 mg

48 hourly

200 mg

48 hourly

240 mg

48 hourly

240 mg

48 hourly

260 mg

48 hourly
31 - 40

200 mg

48 hourly

240 mg

48 hourly

280 mg

48 hourly

300 mg

48 hourly

320 mg

48 hourly
41 - 50

240 mg

48 hourly

280 mg

48 hourly

320 mg

48 hourly

360 mg

48 hourly

400 mg

48 hourly
51 - 60

200 mg

24 hourly

240 mg

24 hourly

280 mg

24 hourly

300 mg

24 hourly

320 mg

24 hourly
> 60

240 mg

24 hourly

280 mg

24 hourly

320 mg

24 hourly

360 mg

24 hourly

400 mg

24 hourly

3. Prescribing gentamicin

  • Prescribe on the drug chart or HEPMA (electronically) the gentamicin dose/interval as calculated above
  • Gentamicin is administered in 100ml sodium chloride 0.9% or dextrose 5% as a 30 minute intravenous infusion

Initial dosing if creatinine unknown

In an emergency if creatinine is not known, use last known eGFR and actual body weight: 

eGFR (ml/min/1.73m2) Dose
>15 Give 5mg/kg (max 400mg)
<15
 Give 2.5mg/kg (max 180mg)

Round doses to the nearest multiple of 20mg

Obtain a serum creatinine level ASAP and then use calculator to determine dosing regimen

Check level as recommended (either at 6-14 hours or 24 hours post dose) and plot results on the chart to determine dose interval.

Monitoring levels and adjusting dosing interval

Check U+E daily; if renal function changes significantly, recalculate creatinine clearance and adjust dosing.

Check trough level after each dose: 

Creatinine clearance (ml/min) When to check trough
>21 6-14h post dose
<21 24h post dose

Then check recommended dosing interval as per the chart below:

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If the measured concentration is unexpectedly HIGH or LOW consider the following;

  • Were dose and sample times recorded accurately?  
  • Was the correct dose administered?  
  • Was the sample taken during drug administration?  
  • Was the sample taken from the line used to administer the drug?  
  • Has renal function declined or improved?  
  • Does the patient have oedema or ascites?

If in doubt take another sample 24 hours after the dose was given before re-prescribing and/or contact pharmacy for advice.