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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.