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Gentamicin

Chemical Pathology

Notes

  • All patients receiving gentamicin must have levels monitored, primarily to avoid renal toxicity
  • Renal function should also be regularly monitored
  • For most patients requiring a gentamicin course a once daily dosing regime can be used
  • Instructions for prescribing gentamicin and monitoring levels can be found in the Gentamicin Policy
  • Gentamicin levels can be taken at any time of day but the collection time should be convenient for the patient and for the testing laboratory – sampling and testing between 12 midnight and 6 am should be avoided –see "when to test"
  • Most gentamicin levels are a planned non-urgent investigation
  • Gentamicin levels are performed both at CGH and GRH Chemical Pathology – please send requests using green Chemistry request forms to avoid delays in sample receipt and analysis
  • For requests out of hours please contact the on call biomedical scientist in Chemical Pathology to inform them that a sample is being sent

When to test

  • For a once daily dosing regime samples should be taken 12 to 18 hours after the dose. For ease of interpretation it is recommended to take the levels at exactly 12 or 18 hours after the dose. If the levels are within the recommended range with normal renal function then the levels should be repeated twice weekly
  • For patients on bd or tds gentamicin regimes paired pre and 1 hour post-dose levels should be taken – the first levels should be taken when the patient has been on gentamicin for 48 hours. Repeat levels should be taken every 5-7 days if the first levels were satisfactory and the renal function is satisfactory and stable. If the first levels were unsatisfactory and the regime was altered, the levels should be repeated 48 hours after the regime change
  • If the renal function is abnormal or changing the levels should be monitored more frequently (every 2 days)

Sample requirements

Samples must NOT be taken via lines

For adults

5ml gold tube





For children

3.5ml rust top tube





For neonates

0.8ml minicollect tube











Storage/transport

Do not store. Send at ambient temperature to the laboratory as soon as possible.

Clinical Information required

  • Gentamicin dose and frequency
  • Time of last dose
  • Date and time sample collected

Turnaround time

  • The assays are run throughout the day and night. The in-lab turnaround time is less than 6 hours. Results are made available on the computer as soon as they are technically approved. Additional comments may be added later by the Consultant Microbiologist when results are clinically approved.
  • The test can be ordered as an urgent investigation

Result interpretation

If there is any doubt regarding interpretation, or a dose change needs to be discussed, then contact the Consultant Microbiologist

Normal levels

  • Target levels for once daily dosing in adults:

12 hours post-dose <2 mg/L

18 hours post-dose <1 mg/L

To interpret levels from between these times see graph (nomogram) in the gentamicin policy.

  • Target levels for once daily dosing in children:

Trough level (taken up to 4 hours before the next dose) <1 mg/L

  • Target levels for extended daily dosing in neonates:

Trough level (taken up to 4 hours before the next dose) <2 mg/L

See neonatal policy via link below:

Neonatal Gentamicin - Extended Dosing Regime

  • Target levels for bd or tds dosing in adults:

Pre-dose <2 mg/L

Post-dose (1 hour) 5-10 mg/L

  • Target levels for bd or tds dosing in infective endocarditis:

Pre-dose <1 mg/L

Post-dose (1 hour) 3-5 mg/L

If you need advice please contact the duty consultant microbiologist or ward pharmacist.

Page last updated 11/05/2015

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