Antimicrobial Dosing Guidelines (Adults) – Gentamicin/Tobramycin and Vancomycin

Gentamicin/Tobramycin Dosing Guidelines (Adults)

Most patients receive once daily dosing as outlined in the guideline below. Exceptions include patients with:

• tetraplegia – see aminoglycoside guideline for patients with spinal injuries, or
• bacterial endocarditis – see aminoglycoside guideline for patients with bacterial endocarditis.

Calculate the first dose of gentamicin/tobramycin

• For patients with severe sepsis, give IV 7 mg/kg ideal body weight unless renal function is poor, i.e. creatinine clearance (CrCl) < 20 mL/min (consult Infectious Diseases for alternative antimicrobial agent).

  Ideal body weight

  Height	Ideal body weight (M/F)
  150 cm	50 kg / 46 kg
  152 cm	52 kg / 47 kg
  155 cm	55 kg / 50 kg
  157 cm	56 kg / 52 kg
  160 cm	59 kg / 55 kg
  163 cm	62 kg / 57 kg
  165 cm	64 kg / 59 kg
  168 cm	66 kg / 62 kg
  170 cm	68 kg / 64 kg
  173 cm	71 kg / 66 kg
  175 cm	73 kg / 68 kg
  178 cm	75 kg / 71 kg
  180 cm	77 kg / 73 kg
  183 cm	80 kg / 75 kg
  185 cm	82 kg / 77 kg
  188 cm	84 kg / 80 kg
  191 cm	87 kg / 83 kg

• For patients requiring gentamicin/tobramycin for other indications, e.g. UTIs:
  1. Calculate the patient’s CrCl based on the adapted Cockcroft and Gault equation. Alternatively the estimated glomerular filtration rate (eGFR) supplied by the laboratories can be used for patients whose body surface area is close to 1.73m². See Prescribing in the Larger Adult.
  2. Calculate the first dose of gentamicin/tobramycin using ideal body weight:

     Ideal body weight

     Height	Ideal body weight (M/F)
     150 cm	50 kg / 46 kg
     152 cm	52 kg / 47 kg
     155 cm	55 kg / 50 kg
     157 cm	56 kg / 52 kg
     160 cm	59 kg / 55 kg
     163 cm	62 kg / 57 kg
     165 cm	64 kg / 59 kg
     168 cm	66 kg / 62 kg
     170 cm	68 kg / 64 kg
     173 cm	71 kg / 66 kg
     175 cm	73 kg / 68 kg
     178 cm	75 kg / 71 kg
     180 cm	77 kg / 73 kg
     183 cm	80 kg / 75 kg
     185 cm	82 kg / 77 kg
     188 cm	84 kg / 80 kg
     191 cm	87 kg / 83 kg

     CrCl (mL/min)	Dose in mg/kg (ideal body weight)	Time of second blood sample (hours)
     Severe sepsis
     ≥ 20	7	12 – 18
     < 20	Consult Infectious Diseases
     Most indications other than severe sepsis
     > 40	5	12 – 18
     20 – 40	3	18 – 24
     < 20	Not recommended

Prescribe and administer gentamicin/tobramycin

1. Round dose, where able, to the nearest 40 mg (half of an 80 mg/2 mL vial) for ease of administration. Prescribe in MedChart with frequency set as "variable dose".
2. Give gentamicin/tobramycin once daily by IV infusion over 30 minutes in 100 mL of sodium chloride 0.9%.
3. Document times of starting and finishing infusion to enable accurate dose prediction.

Blood sampling

Patients expected to receive only one dose (e.g. for UTIs) do not require blood sampling for gentamicin/tobramycin concentrations. For all other patients, take two blood samples for gentamicin/tobramycin concentrations at:

• 30 minutes after the end of infusion (i.e. 1 hour post-infusion start), and
• 12 to 24 hours after the end of the infusion depending on renal function (see table above).

Document accurate times of blood samples to enable accurate dose prediction. Consult readily with Infectious Diseases/Clinical Microbiology for patients requiring more than one dose.

Follow-up doses

1. Contact the ward pharmacist during normal working hours with the drug concentration results to get a prediction for the next dose. Alternatively, contact the pharmacist on call after hours (call before 10.30 pm unless the dose must be given before 8 am). Target AUC (24 hours) is 70 – 100 mg/L.hr for most indications.
2. Repeat 30 minute and 6 to 20 hour samples every 3 days. This may need to be more or less frequent depending on the clinical situation.
3. Monitor renal function. If it is changing, dose alteration will be required – contact the ward pharmacist. Stop therapy if hearing or vestibular function appears to be affected.
4. The Department of Infectious Diseases (Respiratory Service, where relevant) must be consulted for any patient being sent home on aminoglycosides for ongoing monitoring. Contact Medicines Information (03 364 0900) for dose predictions for these patients.

Topic Code: 200409

must be prescribed or recommended by a consultant.