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Gentamicin dosing guidelines (for patients aged ≥16 years)

This guideline does not apply to:

  • Patients treated in renal units or receiving haemodialysis or haemofiltration
  • Major burns
  • Ascites
  • Cystic fibrosis
  • Synergistic use of gentamicin

Refer to local guidelines for managing these patients.

Contraindications and cautions

Contraindications include hypersensitivity, myasthenia gravis

Cautions:

  • Chronic Kidney Disease Stage 4/5, ≥50% increase in serum creatinine or oliguria for >6 hours in the past 48 hours.
    • If gentamicin is clinically indicated, give one dose as per guidance and check with microbiology, infectious diseases or pharmacy before giving a second dose.
  • Patients with hearing loss.
  • Avoid in decompensated liver disease (jaundice, ascites, encephalopathy, variceal bleeding or hepatorenal syndrome).
  • Avoid co-administration with neurotoxic or nephrotoxic agents, e.g. neuromuscular blockers, nonsteroidal anti-inflammatory drugs, ACE Inhibitors; potent diuretics; other aminoglycosides (see www.medicines.org.uk).

Prescribing and documentation

  • Outlined below are 3 steps, from calculating and prescribing the first dose to monitoring and assessing gentamicin therapy.
  • Prescribe gentamicin "as charted" on the kardex. Do not add a dose / administration time as this causes confusion as times may vary.
  • Use the gentamicin prescribing, administration and monitoring (PAM) chart to prescribe and record all doses and concentration measurements (see How to prescribe gentamicin and How to administer gentamicin for further information).
  • An NHSGGC gentamicin patient information leaflet (PIL) should be given to patients commenced on it. It is also available on NHSGGC StaffNet.

Step 1: Calculate, prescribe and administer the first dose

  • To reduce the risk of mortality, commence gentamicin administration within 1 hour of recognising sepsis.
  • Prescribe gentamicin on the inpatient kardex and on the gentamicin PAM chart.
  • On the gentamicin PAM chart, document the patient's sex, age, height, weight and creatinine (if known). Use these to calculate the gentamicin dose and frequency. 
  • If creatinine is known - use the gentamicin calculator on the NHSGGC StaffNet / Clinical Info page (access to GGC Sharepoint needed) or table 1 below. Use table 1 only if the gentamicin calculator is not available. The dose amount and dosage interval are based on estimated CrCl and actual body weight. Do not use eGFR.
  • If creatinine is not known - give 5mg/kg actual body weight (maximum 400mg) or, if CKD 5, give 2.5mg/kg (maximum 180mg).
  • If the patient weighs <40 kg and CrCl is ≥21 ml/minute, give a single dose of 5mg/kg then take a sample 6-14 hours after the dose. Higher doses (up to 600mg) may be necessary if the patient weighs >150kg. Please contact pharmacy or the local antimicrobial pharmacist (see Appendix 6 for contact details).
  • Prescribe each dose individually on the gentamicin PAM chart (inform the nursing staff that the dose is due, to ensure prompt administration). Do not prescribe >24 hours in advance. There have been multiple clinical incidents where the patient has been prescribed 48 hourly gentamicin, which has subsequently been administered 24 hourly due to prescribing a day in advance. For an example of this see How to prescribe gentamicin. 
  • Record gentamicin dose and predicted frequency in the initial gentamicin dose box on the gentamicin PAM chart. 
  • Give the recommended dose by infusion in 100ml sodium chloride 0.9% over 30 minutes.

Step 2: Monitor creatinine and gentamicin concentrations and reassess the dosage regimen

  • Check the patient's creatinine daily, record the results in the gentamicin PAM chart to ensure any changes impacting on dosing are easily recognised. 
  • Review therapy and seek advice if renal function is unstable (e.g. change in creatinine of >15-20%).  

Table 1: Initial GENTAMICIN doses and dose intervals

  Actual body weight
CrCl (ml/minute) 40-49 kg 50-59 kg 60-69 kg 70-80 kg >80 kg
<21 2.5mg/kg (max 180mg) then take a sample after 24 hours
21-30 180mg 48 hourly 200mg 48 hourly 240mg 48 hourly 240mg 48 hourly 260mg 48 hourly
31-40 200mg 48 hourly 240mg 48 hourly 280mg 48 hourly 300mg 48 hourly 320mg 48 hourly
41-50 240mg 48 hourly 280mg 48 hourly 320mg 48 hourly 360mg 48 hourly 400mg 48 hourly
51-60 200mg 24 hourly 240mg 24 hourly 280mg 24 hourly 300mg 24 hourly 320mg 24 hourly
>60 240mg 24 hourly 280mg 24 hourly 320mg 24 hourly 360mg 24 hourly 400mg 24 hourly
CrCl = creatinine clearance, equation can be found here

Concentrations are meaningless unless the dose and sample time are recorded accurately
If CrCl is ≥21ml/minute
  • Take a blood sample 6-14 hours after the start of the first gentamicin infusion and then every 2 days.
  • Record the exact time of all gentamicin samples on the gentamicin PAM chart and ensure that the TrakCare sample request form is printed at the time of sample collection (to ensure that the sample time on TrakCare is accurate).
  • Record the serum concentration on the gentamicin PAM chart.
  • Plot the concentration measurement on the graph below. This will indicate one of 3 options:
    • Continue the present dosage regimen or
    • Adjust the dosage interval or
    • Withhold and resample after 24 hours.
  • Document the action taken in the medical notes and in the action / comment section of the gentamicin PAM chart and if appropriate prescribe the next dose. 
  • Graph Gentamicin concentration plot graph
If CrCl is <21ml/minute
  • Take a blood sample 24 hours after the start of the first gentamicin infusion.
  • Record the exact time of all gentamicin samples on the gentamicin prescribing chart and ensure that the TrakCare sample request form is printed at the time of sample collection (to ensure that the sample time on TrakCare is accurate).
  • If therapy is to continue, take additional blood samples at least every 24 hours and give a further dose once the measured concentration is <1mg/L.
General points
  • Document the action taken in the medical notes and on the gentamicin prescribing chart.
  • Undertake pre-prescribing checks to assess the risk of renal toxicity and ototoxicity (see below). Prescribe the next dose as appropriate.
  • Seek advice from pharmacy or microbiology if you are unsure how to interpret the result or if the concentrations are very low or very high. Doses up to 600mg may be required for some patients.
  • If a blood sample is not taken, is lost or is taken at wrong time and if there is any concern about the patient's renal function, take a sample at 20-24 hours and wait for the result before giving the next dose. Otherwise, take a blood sample after the next dose. Nurses, if you are unsure how to assess renal function, contact a member of medical staff or pharmacy.

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 from the line used to administer the drug?
  • Was the sample taken during drug administration?
  • Has renal function declined or improved?
  • Does the patient have oedema or ascites?
  • Is the patient severely underweight or overweight?
If in doubt, take another sample before re-prescribing and/or contact pharmacy for advice

Step 3: Assess daily: the ongoing need for gentamicin; if signs of toxicity

  • Take further samples 6-14 hours after the previous dose and then at least every 2 days.
  • If the concentration is unexpectedly high or if renal function alters, daily sampling may be necessary.
  • To minimise the risk of toxicity, duration of treatment should normally be limited to 3 days. All gentamicin prescriptions that continue beyond 3–4 days of treatment must be discussed with an infection specialist. Consider changing to an oral alternative - refer to the IV to Oral switch policy.

Renal Toxicity

  • Monitor creatinine daily. Seek advice if renal function is unstable (e.g. a change in creatinine of >15-20%).
  • Signs of renal toxicity include an increase in creatinine or decrease in urine output / oliguria.
  • 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 >3 days) and is secondary to drug accumulation within the inner ear.
  • Stop treatment if ototoxicity is suspected and refer to a microbiology / infection specialist for advice on future therapy
  • If gentamicin continues for >7 days, suggest referring to audiology for assessment.

Advice for nursing staff on using the GGC gentamicin PAM chart

  • Before administering the dose, check the kardex to ensure gentamicin has not been discontinued there.
  • Check that creatinine and gentamicin levels are being monitored (these are recorded on the gentamicin PAM chart; discuss with the prescriber promptly if you are unsure if this monitoring is being done).
  • Record the date and exact time of administration on both the gentamicin PAM chart and the kardex with 2 nurses' signatures.  

 


Guideline reviewed: December 2023

Page updated: December 2023