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
Please see Turnaround times
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:
microbiology.advice.ouh@nhs.net
Confidential patient information should only be sent from nhs.net accounts.
Microbiology Laboratory Users Manual (pdf, 684 KB)