20. Protocol for extended interval gentamicin dosing

Exclusions to this protocol
Until further evidence is available the following groups of patients should be excluded from this protocol:
Children, patients with ascites, endocarditis, major burns, pregnancy, cystic fibrosis and dialysis patients.

Renal impairment
In patients with renal impairment gentamicin needs to be used with extreme caution. If patients have a serum creatinine of >300micromol/l or CrCl <40ml/min consult microbiology for advice on dosing or possible alternative antibiotics.

Gentamicin dosing
1. First dose
Give 5mg/kg in 100ml 5% glucose or sodium chloride 0.9% over 60 minutes (round to the nearest 10mg) to a maximum of 400mg. Prescribing the dose at the 1pm time slot will make monitoring of levels easier.

2. Calculate creatinine clearance (CrCl ml/min)
The following formula should be used to calculate creatinine clearance in order to determine the dosing interval when prescribing gentamicin

3. Calculation of Creatinine Clearance
Clearance (ml/min ) = (140 - age (years)) x N x weight (kg)
Serum creatinine (micromol/l)
Where N = 1.04 for female patients
N = 1.23 for male patients

4. Subsequent doses

Creatinine clearance

Dosing Interval

> 60ml/min

24 hours

40 to 59ml/min

check level at 24 hours and await result - only give dose if level is below 1mg/l

< 40ml/min

seek advice from Microbiology or Pharmacy

Normal SERUM creatinine concentrations:
Female 40 - 90micromols/l
Male 60 - 120micromols/l

Gentamicin monitoring and subsequent doses

Levels should be taken 18-24 hours after the initial dose is given - please consult the table below:

If the first dose is given between:

Time of blood collection for gentamicin assay

Comments

8am to 2pm

Collect at 8am next day and
send to lab for same-day result
(available soon after 10am).

State clearly on the request form the time of collection of blood and the time gentamicin was given.

2pm to 10pm

After 18 hours

This means blood for assay will be collected during routine working hours, namely between 9am and 3pm. The blood must reach the biochemistry laboratory before 4pm in order to ensure a same-day result.
State clearly on the request form the time of collection of blood and the time gentamicin was given.

10pm to 8am

These would require pre-dose
levels being collected between
5pm and 7am - and the
laboratory does not offe
a 24h assay service

If renal function normal give second dose but aim to reschedule the timing of subsequent doses.
If renal function impaired the serum level must be checked and be < 1mg/l before subsequent doses given.
Ring Biochemistry department during working hours- tel 2136 - to discuss special arrangements if out-of-hours assay will be required.

For patients with normal renal function (CrCl >60ml/min) pre-dose levels and renal function should be checked every 3-5 days. Levels should be less than 1mg/l.
Check renal function (urea and creatinine) regularly
For patients with impaired renal function, pre-dose levels and renal function should be checked daily. The result must be received and be less than 1mg/l before the dose is given.
If levels are >1mg/l and taken at the correct time seek advice on adjustment of levels.

Additional Notes

For once-daily gentamicin dosing, estimates of peak levels are not required.
Please contact Microbiology (2885) or Pharmacy (3331) for advice if required.
There are few clinical conditions where an aminoglycoside needs to be continued for longer than 5-7 days.
Some patients, particularly the frail elderly may have a normal serum creatinine but still have moderately impaired renal function. For these patients always calculate (or measure) their creatinine clearance

Important
Please indicate clearly on the pathology request cards the time that the dose was given and the time the level was taken.

Creatinine clearance	Dosing Interval
> 60ml/min	24 hours
40 to 59ml/min	check level at 24 hours and await result - only give dose if level is below 1mg/l
< 40ml/min	seek advice from Microbiology or Pharmacy

If the first dose is given between:	Time of blood collection for gentamicin assay	Comments
8am to 2pm	Collect at 8am next day and send to lab for same-day result (available soon after 10am).	State clearly on the request form the time of collection of blood and the time gentamicin was given.
2pm to 10pm	After 18 hours	This means blood for assay will be collected during routine working hours, namely between 9am and 3pm. The blood must reach the biochemistry laboratory before 4pm in order to ensure a same-day result. State clearly on the request form the time of collection of blood and the time gentamicin was given.
10pm to 8am	These would require pre-dose levels being collected between 5pm and 7am - and the laboratory does not offe a 24h assay service	If renal function normal give second dose but aim to reschedule the timing of subsequent doses. If renal function impaired the serum level must be checked and be < 1mg/l before subsequent doses given. Ring Biochemistry department during working hours- tel 2136 - to discuss special arrangements if out-of-hours assay will be required.