Gentamicin Guideline

• Use the gentamicin calculator (also via intranet quicklinks) or use the table below to calculate an initial dose. (Calculate creatinine clearance (CrCl) using the Cockcroft–Gault equation below.  Do not use eGFR).
• Prescribe the first gentamicin dose on the "GENTAMICIN PRESCRIBING, ADMINISTRATION AND MONITORING CHART". Also, prescribe gentamicin 'as per chart' on the regular section of the NHS Fife drug chart.  Do not prescribe further doses until a level has been obtained.
• In patients with Acute Kidney Injury (≥50% increase in baseline serum creatinine or oliguria >6 hrs) give a single dose according to the table below and seek senior medical advice before giving a second dose.
• If the patient weighs less than 40kg and CrCl is >21ml/min, give a single dose of 5mg/kg, then monitor as below.
• If the creatinine is unknown give a single dose of 5mg/kg (maximum 400mg), then monitor as below.
• If known CKD 5, give 2.5mg/kg (maximum 180mg) on the advice of senior medical staff, then monitor as below.
• Obese patients or patients >90 years may require dose adjustments and require close monitoring. Contact Pharmacy for advice.

Table of initial gentamicin doses:

• If CrCl is ≥21ml/min take a blood sample 6-14 hours after the start of the gentamicin infusion. Record the time the sample was taken and the serum concentration on the gentamicin prescribing chart. 
• Plot the result on the graph below. This shows the time interval at which the dose should be repeated.  If the point falls on a line, take the longer time interval.  Document the action taken on the gentamicin prescribing chart.
• If CrCl is <21ml/min take a blood sample 24 hours after the start of the gentamicin infusion and ONLY give a further dose once the measured concentration is <1mg/L, the dosing graph does not apply.
• Concentrations are meaningless unless the dose given, the time it was given and blood sample time are recorded accurately. Record the exact time taken of all gentamicin samples in the clinical details section of the electronic request and on the gentamicin prescription chart.
• Subsequent levels should be taken every 2 days, 6 –14 hours after the dose. If CrCl is <21ml/min or if concentration is unexpectedly high or if renal function is unstable or patient is on concomitant nephrotoxic drugs monitor levels daily. 

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

Monitor creatinine daily.  If renal function declines or signs of ototoxicity withhold further doses and seek senior medical advice. 

Dose requirements may change if renal function alters.

Vestibular and ototoxicity due to gentamicin is independent of drug concentration and is suggested by any of the following: new tinnitus, dizziness, poor balance, oscillating vision, hearing loss, nausea or vomiting. 

Toxicity is associated with prolonged aminoglycoside and is secondary to drug accumulation within the inner ear.  To minimise the risk of toxicity duration should usually be limited to 72 hours and avoid multiple courses. 

All patients receiving gentamicin therapy should be warned of these potential side-effects and the drug should be discontinued at the earliest sign of toxicity.

If gentamicin continues for >7days consider referring to audiology for assessment.

Consider IV to Oral switch daily.

Contact the Microbiologist on-call to discuss treatment if gentamicin is to continue beyond 3-4 days.

Calculating Creatinine Clearance (CrCl)             

CrCl  =  (140 – age)  x  Weight (kg)*    x  1.23 (male) or 1.04 (female)

             Serum Creatinine (µmol/L)**

To avoid overestimating CrCl:

*Use actual or maximum body weight (MBW), whichever is lower

**Use 60µmol/L if the creatinine concentration is <60µmol/L