Gentamicin

• Baseline renal function should be measured, and creatinine clearance calculated. Monitor daily. Re-calculate creatinine clearance if there is a greater than 15% change in serum creatinine and adjust dose. 
• Take a baseline weight, calculate ideal and adjusted body weight (see below), and then monitor weekly. Adjust dose if weight changes. 
• Take daily U+Es.
• Monitor serum gentamicin levels (see below). 
• Auditory function may be affected. Please refer to Audiometry and intravenous aminoglycosides. 

• Nephrotoxicity - dose dependent. Monitor creatinine clearance and urine output. 
• Ototoxicity - dose dependent. Associated with prolonged use (usually greater than 7 days). Monitor hearing function as directed. 

• Hypersensitivity to gentamicin.
• Myasthenia gravis. 
• Familial susceptibility to aminoglycoside toxicity.

• Concurrent use of nephrotoxic drugs:
  • Examples include ACE-inhibitor, diuretics (especially IV), NSAIDs, ciclosporin, colistimethate sodium, radio-contrast media, high-dose methotrexate or amphotericin B liposomal (Tillomed/Gilead/AmBisome).
  • This is not a complete list. Refer to the BNF or speak to your ward pharmacist for advice. 
• Impaired renal function - CKD4 or AKI (increase in creatinine greater than 50% in the last 48 hours). 
• Mitochodrial mutations (MHRA safety update January 2021)
  • Evidence suggests an increased risk of aminoglycoside-associated ototoxicity in patients with certain mitochondrial mutations (particularly the m.1555A>G mutation), including cases in which the patient's aminoglycoside serum levels were within the recommended range.
  • These mitochondrial mutations are rare, and frequency of complication is uncertain. 
  • Genetic testing should not delay urgently needed aminoglycoside treatment, but consider need for genetic testing, particularly where treatment is expected to last more than 7 days. 
  • Where the patient has a susceptible mutation, consider the need for aminoglycoside treatment versus alternative options. 
• Concurrent use of ototoxic drugs - loop diuretics, capreomycin, platinum products, vincristine, vinblastine.
• Concurrent use of neurotoxic drugs - anaesthetic muscle relaxants, botulinum toxin. 
• Muscle weakness. 

Where possible, creatinine clearance should be calculated before deciding the dosing of gentamicin. 

The EPR gentamicin calculator will launch automatically when gentamicin is prescribed. This calculator will calculate the dosing weight. In order to do this, an accurate weight and height will need to be entered on EPR. This calculator will calculate a dose and rounds down to the nearest 20mg. This calculator is capped with a maximum dose of 480mg.

gentamicin 5mg/kg: dose should be capped at maximum dose of 480mg. 

gentamicin 3mg/kg (for both surgical prophylaxis and patients with reduced creatinine clearance): dose should be capped at a maximum dose of 240mg

For Adult Critical Care areas that use CareVue, or in the case of EPR downtime, use the following guideline (On OUH Intranet): EPR Downtime Guide 

STEP 1. Determine Creatinine Clearance (CrCl)

Determine Creatinine Clearance (CrCl) using the Cockcroft-Gault equation (below).

CrCl (mL/min) = F x (140-age) x weight (kg)   

                             Serum Cr (micromol/L)

where F=1.04 (females) or F=1.23 (males)

• Use Ideal Body Weight (IBW) if Actual Body Weight (ABW) is over 20% above IBW.
• In patients with low serum creatinine (less than 60 micromol/L), use 60 micromol/L to calculate creatinine clearance). 

STEP 2. Calculate initial gentamicin dose

STEP 3. Prescribe on EPR. The Gentamicin Calculator will launch automatically. 

Prescribers are required to ensure: 

1. Dosing weight and height is accurate or best estimate. 
2. Dose provided by calculator is in the expected range. 

Ensure that "Apply dose" is clicked to prescribe the dose on drug chart. For further information on how to use the calculator see: Gentamicin calculator - how to guide (v1.1).pdf 

NOTE: If actual body weight is less than ideal body weight, the EPR calculator will not calculate the dose. Prescribers need to:

1) Manually enter the patient's actual body weight into the box for 'Adjusted weight'. 

2) Change 'Adjustment' box to Actual (no adjustment) as shown in the example image below:

The calculator will now calculate the correct dose. 

STEP 4. Monitoring 

Review need for gentamicin daily. Gentamicin courses should not exceed 3 days unless advised by Micro/ID.

Creatinine clearance 31ml/min or greater (Dosing gentamicin 5mg/kg) 

• Take daily U+E.
• Take serum gentamicin level (yellow blood tube) 6-14 hours after administration of first dose of gentamicin. Once level available, plot on Urban and Craig nomogram below and adjust dosing interval as required. 
• At 24 hours post first dose, if gentamicin level is not available use the dosing interval suggested in table below.
• If the normogram advises q12hr then the patient should be discussed with Micro/ID.
• If renal function is stable, repeat gentamicin levels should be taken twice a week for prolonged courses.
• If renal function is deteriorating, this is a nephrotoxic medicine therefore stop gentamicin.

   

• For gentamicin 3 day courses:
  • • After calculating dosing interval from Urban and Craig nomogram below then:
      • Interval is 24 hours - give 2 further doses at the appropriate time.
      • Interval is 36 or 48 hours -  give 1 further dose at the appropriate time.

Creatinine Clearance 30 ml/min or less (Dosing gentamicin 3mg/kg)

• Take daily U+E. 
• Take gentamicin level 24 hours after dosing. Only re-dose once level is less than 1mg/L. 

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Suggested dosing intervals for 5mg/kg dose if gentamicin level is not available: (Only applicable for once daily dosing regimens)