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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
  • Cystic fibrosis
  • Synergistic use of gentamicin
  • Surgical prophylaxis

Refer to local guidelines for the management of these patients.

This guideline covers:

Contraindications and cautions 

See the BNF / Summary of Product Characteristics for full details

Contraindications
  • Hypersensitivity
  • Myasthenia gravis
  • Decompensated liver disease (jaundice, ascites, encephalopathy, variceal bleeding or hepatorenal syndrome).
  • Personal or family history of vestibular / auditory toxicity whilst on aminoglycosides.
  • Maternal relative with deafness due to mitochondrial mutation A1555G.
  • Patients with a known mitochondrial mutation.
Cautions
  • Creatinine Clearance (CrCl) <21ml/minute, ≥50% increase in serum creatinine from baseline or oliguria for >6 hours in the past 48 hours.
    • If gentamicin is clinically indicated, give the first dose as per guidance and check with microbiology, infectious diseases or pharmacy before giving a second dose.
  • Pre-existing auditory / vestibular impairment.
  • Co-administration with neurotoxic or nephrotoxic agents, e.g. neuromuscular blockers, nonsteroidal anti-inflammatory drugs, ACE Inhibitors; potent diuretics; other aminoglycosides (see Summary of Product Characteristics for further information).
  • Patients with known conditions characterised by muscular weakness.

Prescribing and documentation

  • Prescribe gentamicin on HEPMA (see FAQs) but do not add a dose / administration time as this causes confusion as times may vary.
  • Use the PAM chart to record all doses and concentration measurements.
  • Give the patient / carer the NHSGGC gentamicin patient information leaflet (PIL) which is also attached to the PAM chart as a 'tear off' document. If not issued, record the reason on the PAM chart.

Below is a 4-step guide on how to prescribe the first dose of gentamicin, assess and monitor gentamicin therapy.

Step 1: Calculate, prescribe and administer the first dose

Commence gentamicin within 1 hour of recognising sepsis. Prescribe gentamicin on HEPMA and on the gentamicin PAM chart (see FAQs and How to prescribe gentamicin documents for further information). 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. 

*For transgender patients (following ≥6 months of hormonal gender affirming therapy or at any time after completion of gender affirming surgery) calculate CrCl and ideal body weight (IBW) according to the patient's gender identity. Use the calculator within the GGC Medicines App or via the StaffNet Hub (link to page only active if connected to NHSGGC network). If gender affirming therapy does not meet the criteria above, use the patient's sex at birth according to their electronic records. The second last digit of a CHI number informs of a patient's assigned sex at birth - for those assigned male it is odd and for those assigned female it is even.

For all patients, if:

  • Creatinine is known - use the gentamicin calculator within the GGC Medicines App or via the StaffNet Hub (link to page only active if connected to NHSGGC network). Do not use another health board's calculator. Use table 1 below only if the gentamicin calculator is not available. Do not use eGFR. The CrCl equation can be found here.
  • Creatinine is unknown - give 5mg/kg actual body weight (maximum 400mg) or, if CKD 5, give 2.5mg/kg (maximum 180mg). Calculate the dose as above once creatinine result is available.
  • Weight >150kg may require higher doses (up to 600mg) of gentamicin. Please contact pharmacy or the local antimicrobial pharmacist (see Appendix 6 for contact details)

Prescribe doses individually on the gentamicin PAM chart (inform the nursing staff that the dose is due, to ensure prompt administration). 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.

Important note: Do not prescribe doses >24 hours in advance. There is a risk that gentamicin prescribed 48 hourly may be inadvertently prescribed and/or administered 24 hours earlier than intended, this can lead to toxicity (to avoid this, see How to prescribe gentamicin).

Table 1: Initial GENTAMICIN doses and dose intervals 

Only use if gentamicin calculator is unavailable.

  Actual body weight
CrCl (ml/minute) <40 kg 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 5mg/kg (max 180mg) 48 hourly 180mg 48 hourly 200mg 48 hourly 240mg 48 hourly 240mg 48 hourly 260mg 48 hourly
31-40 5mg/kg 48 hourly 200mg 48 hourly 240mg 48 hourly 280mg 48 hourly 300mg 48 hourly 320mg 48 hourly
41-50 5mg/kg 48 hourly 240mg 48 hourly 280mg 48 hourly 320mg 48 hourly 360mg 48 hourly 400mg 48 hourly
51-60 5mg/kg 24 hourly 200mg 24 hourly 240mg 24 hourly 280mg 24 hourly 300mg 24 hourly 320mg 24 hourly
>60 5mg/kg 24 hourly 240mg 24 hourly 280mg 24 hourly 320mg 24 hourly 360mg 24 hourly 400mg 24 hourly

CrCl = creatinine clearance, equation can be found here

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

Check creatinine daily, record the results on the gentamicin PAM chart to ensure consideration of renal function when prescribing. Review therapy and seek advice from pharmacy if renal function is unstable (e.g. change in creatinine of >15-20% or oliguric).  

Monitoring gentamicin concentrations

If CrCl is ≥21ml/minute:

Take the first blood sample 6-14 hours after the start of the first gentamicin infusion then at least 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). N.B. Concentrations are meaningless unless the dose and sample time are recorded accurately.

Record the serum concentration on the gentamicin PAM chart. Plot the concentration measurement on the graph at the back of the PAM chart. This will indicate one of 3 options:

  • Continue the present dosage regimen or
  • Adjust the dosage interval or
  • Withhold and resample after 24 hours.

Seek advice from pharmacy if rapidly changing renal function. Document the action taken in the medical notes and in the "Action / Comments" section of the gentamicin PAM chart, and if appropriate prescribe the next dose.

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 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). N.B. Concentrations are meaningless unless the dose and sample time are recorded accurately. 

Review the concentration measurement, this will indicate one of two options (see table 2 below).

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.

Document the action taken in the medical notes and in the "Action/Comments" section of the gentamicin PAM chart, and if appropriate prescribe the next dose. 

Table 2: Measured concentration and action advised for patients with CrCl <21ml/minute

Measured Concentration Action
<1 mg/L Give further dose
≥1 mg/L

Do not give further dose and take additional blood samples at least every 24 hours.

Only give a further dose once the measured concentration is <1mg/L.

Additional guidance

Before prescribing, assess the risk of renal toxicity and ototoxicity (see below).

Seek advice from pharmacy if you are unsure of how to interpret the result or if the concentrations are very low or very high (see below for potential causes).

In obesity, doses up to 600mg may be required for some patients, discuss with pharmacy.

If a gentamicin concentration was not taken, taken at the wrong time, is lost, or there is any concern about renal function, take a sample as soon as possible and wait for the result before giving the next dose.

Avoid taking drug samples from lines as results are often inaccurate - see the GGC Vascular Access Devices (VADS), Care and Maintenance guideline.

The mitochondrial DNA mutation m.1555A>G predisposes to severe hearing loss following aminoglycoside exposure. Consider the need for genetic testing, especially in patients requiring recurrent or long term treatment with aminoglycosides (e.g. enterococcal endocarditis, complex drug resistant infections including tuberculosis, cystic fibrosis or recurrent neutropenic sepsis). Do not delay urgent treatment in order to test.  Results can take up to 28 days to be reported. If required, please see the Genetic testing form available via TrakCare under "Germline molecular genetic analysis or storage" catalogue.

If in doubt, take another sample before re-prescribing and/or contact pharmacy for advice

If the measured concentration is unexpectedly HIGH or LOW, consider the following:

  • Were dose and sample times recorded accurately?
  • Was the correct and full 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?

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

Take further gentamicin samples at least every 2 days (see above).

If the gentamicin concentration is unexpectedly high or if renal function changes, sample daily. 

Maximum duration of treatment is normally limited to 4 days to prevent toxicity. Any continuation beyond 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. If patient is not suitable for IVOST, advice can be found here. 

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 may be 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 occur at any time). Stop gentamicin treatment if ototoxicity is suspected and refer to an infection specialist for advice on antibiotic therapy.
  • In prolonged courses of gentamicin (greater than 5 days of therapy), refer to audiology for ongoing assessment. Document daily discussion with patient regarding symptoms of cochlear / vestibular toxicity. Ensure Gentamicin Patient Information Leaflet (PIL) is issued to patient / carer.

Step 4: Issue Gentamicin PIL to patient / carer

The prescriber should issue the Gentamicin PIL to the patient / carer as soon as possible unless this is considered to be inappropriate. Complete Step 4 on the PAM chart to indicate whether the PIL has been given. 

Advice for nursing staff on using HEPMA and the GGC gentamicin PAM chart

Please note, gentamicin is prescribed "PRN" on HEPMA. To administer gentamicin on HEPMA, see FAQs.

Before administering gentamicin, check both HEPMA and the gentamicin PAM chart to ensure gentamicin has not been discontinued.

Check that creatinine and gentamicin levels are being monitored. These should be recorded on the gentamicin PAM chart. Discuss with the prescriber promptly if you are unsure if this monitoring is being done.

Alert a member of medical staff or pharmacy if you detect a change in renal function (e.g. reduced urine output).

Record the date and exact time that doses were given on both the gentamicin PAM chart and HEPMA.  

Useful links:

 

Guideline reviewed: August 2024

Page last updated: August 2024