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Oxford University Hospitals NHS Foundation Trust
Microbiology

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Antibiotic assays

Assays of serum drug concentrations are indicated in the following situations:

  • drugs with a known or suspected relationship between concentrations in blood and toxicity
  • drugs with a known or suspected relationship between concentrations in blood and efficacy
  • where there is pharmacokinetic variation such that concentrations in blood cannot be predicted
  • to confirm oral absorption
  • to test compliance.

Specimen requirements

Samples should be transported to the laboratory as soon as possible. If specimen transport will be delayed, e.g. from primary care, specimens should be stored in a refrigerator until transported to the laboratory.

Blood in yellow top SST preferred or paediatric tube.

It is essential to give details of times and dosage of antibiotic to be assayed, the date and time specimens collected and whether peak or trough.

Laboratory turnaround time

Vancomycin, Gentamicin, Amikacin and Tobramycin assays are processed on the day of receipt (24/7).

Other antibiotic assays (e.g. teicoplanin) have to be sent away, so the turnaround time is two to three days.

Laboratory method

Antibiotic assays are run on immunoanalysers in the core automated laboratory

When to assay

Aminoglycosides

Gentamiciin

Tobramycin

Amikacin

 

Streptomycin

These are best given once daily except in renal failure and endocarditis.

Assay after 48 hours therapy (unless being discontinued).

Single assay in renal failure and for once daily dosing.

For ds or tds dosing, assay paired and trough samples (immediately pre-dose and 60 minutes post completion iv dose).

Glycopeptides

Vancomycin


Teicoplanin

 

Vancomycin: through before third dose or random assay if renal impairment or using continuous infusion. See Dosing of Gentamicin and Vancomycin in the adult Microguide OUH application available via App store.

Teicoplanin: after 3-5 doses, trough only to ensure therapeutic levels.

Reference range and units

Assay type Regime Trough target level (mg/L) Peak target level (mg/L)

Gentamicin

(Once daily 7mg/Kg or 5 mg/Kg) Use nomogram

Tobramycin

(Once daily 7mg/Kg or 5mg/Kg)

See Dosing of Gentamicin and Vancomycin in the adult Microguide

OUH application available via App store

Gentamicin or Tobramycin

bd or tds < 2 > 5

Gentamicin or Tobramycin

Infective endocarditis < 1 3-5

Amikacin

bd or tds < 10 > 20

Amikacin

Once daily < 5 N/A

Streptomycin

  < 5 15-40
Vancomycin

od or bd

See Dosing of Gentamicin and Vancomycin in the adult Microguide

OUH application available via App store

10-20 N/A

Vancomycin infusion

ITU only 15-25 N/A

Teicoplanin

Severe Staph. aureus infections including endocarditis > or = 20 but < or = 60 N/A

Teicoplanin

Other severe infections > or = 10 but < or = 60 N/A

Antifungal

Pre-dose

Post-dose

Target Levels (mg/L)

Flucytosine

Just before dose*

Oral: 2 hours*

IV: 30 minutes*

Adult:
Pre-dose 30-40
Post-dose 70-80

Neonatal:
Pre-dose 20-40
Post-dose 50-80

Levels > 100 are potentially toxic.

Itraconazole

Oral: not needed

IV: just before dose

Oral: random*

IV: 1 hour and 4-6 hours

Target 5-17.

Steady state reached after 1-2 weeks on oral therapy, with little variation throughout the day.

Voriconazole

Just before dose*

Oral: not needed

IV: not needed

< 1.3: low pre-dose level

1.3 - 5.7: satisfactory level

> 5.7: high level

Posaconazole

   

Levels not recommended.

Where to find results of these tests

  • Vancomycin and Gentamicin results are not usually phoned.
  • All results are returned electronically where possible (EPR, SunquestICE).
  • Some results may be phoned to clinicians and GPs.
  • Results are never given directly to patients by the laboratory staff.

Further information and contact details

For further information please email:

Confidential patient information should only be sent from nhs.net accounts.