Intravenous gentamicin in adults (Antimicrobial)

Contra-indications:

• hypersensitivity
• myasthenia gravis

Cautions:

• Patients with decompensated liver disease - aminoglycosides are associated with an increased risk of renal failure.
• Concurrent administration of neurotoxic and/or nephrotoxic agents increases the risk of gentamicin toxicity. Review therapy and consider amending or withholding nephrotoxic drugs during gentamicin treatment. Avoid co-administration with the following:
  • neuromuscular blockers
  • other potentially nephrotoxic  (eg NSAIDs and ACE Inhibitors) or ototoxic drugs
  • potent diuretics
  • other aminoglycosides
    This list is not exhaustive – consult the Summary of Product Characteristics for a full list (www.medicines.org.uk)
• Chronic Kidney Disease (CKD) Stage 4 or more*
• Known or suspected acute kidney injury in the previous 48 hours* (>50% increase in baseline serum creatinine or oliguria >6 hours).
  *If gentamicin is clinically indicated, give one dose of 2·5mg/kg to a maximum of 180mg, check serum levels at 24 hours post dose and discuss with microbiology or an infection specialist before giving a second dose. 

To improve the prescribing of gentamicin, ensure consistency and reduce risk, standardised gentamicin prescribing charts should be used to document the prescription, administration and monitoring of gentamicin. These should be used in conjunction with the existing inpatient prescribing chart and medical/nursing documentation. 

Renal toxicity

Ototoxicity

• To reduce the risk of mortality, commence gentamicin administration within 1 hour of recognising sepsis.
• If serum creatinine is known – use the gentamicin calculator (preferred option).
  • If the online calculator is not available, the guidelines in Table 1 can be used. The dose and dosage interval are based on estimated creatinine clearance and actual body weight.
• If serum creatinine is not known – give an initial dose of 5mg/kg gentamicin (maximum 400mg) or, if Chronic Kidney Disease (CKD) 5, give 2·5mg/kg (maximum 180mg) on advice of senior staff. Calculate the dose using actual body weight.
• Give the recommended dose by infusion in 100mL sodium chloride 0·9% over 30 minutes and ensure the time of administration is noted on the medicine chart.

Estimation of creatinine clearance (CrCl)

The following 'Cockcroft Gault' equation can be used to estimate creatinine clearance (CrCl)

Cautions:

• Use actual body weight or maximum body weight whichever is lower. 
  For maximum body weight table see here
• In patients with low creatinine (< 60 micromol/L), use 60 micromol/L.
• Note: Use of estimated glomerular filtration rate (eGFR) is not recommended.

Table 1: Initial GENTAMICIN doses and dose intervals

Caution:
If the patient weighs les than 40kg and CrCl is 21 mL/min or greater, give a single dose of 5mg/kg then take a sample 6 to 14 hours after the dose and follow the instructions in Step 2.

Concentrations are meaningless unless the dose and sample time are recorded accurately

If creatinine clearance is 21mL/min or greater

• Take a blood sample 6 to 14 hours after the start of the first gentamicin infusion.
• Record the exact time of all gentamicin samples on the gentamicin prescribing chart AND on the sample request form.
• Record the serum concentration on the gentamicin prescribing chart. 
• Plot the concentration measurement on the graph and reassess the dose/dosing interval as indicated.
• This will indicate one of 3 options:  
  1. continue the present dosage regimen
  2. adjust the dosage interval
  3. withhold and resample after 24 hours

If creatinine clearance is less than 21mL/min

• Take a blood sample 24 hours after the start of the first gentamicin infusion.
• Record the exact time of all gentamicin samples using the gentamicin prescribing chart AND on the sample request form.
• If therapy is to continue, take additional blood samples at least every 24 hours and give a further dose once the measured concentration is below 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.
• 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. 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 20 to 24 hours after the start of the gentamicin infusion and wait for the result before giving the next dose. Otherwise, take a blood sample after the next dose.

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?

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

• Take a further blood sample 6 to 14 hours after the dose, at least every 2 days.
• If the gentamicin 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 72 hours. All gentamicin prescriptions that continue beyond 3 to 4 days of treatment must be discussed with microbiology or an infection specialist.

Renal toxicity

Ototoxicity