ANTIBIOTIC ASSAY

ANTIBIOTIC ASSAY

Specimen: At least 1 mL blood in plain tube.

Method: EIA, HPLC.

Reference Interval: Depends on antibiotic and dose regimen.

Application: Monitoring plasma levels of potentially toxic drugs such as aminoglycosides, vancomycin, flucytosine and, in neonates, chloramphenicol.

Therapeutic Intervals: For 8-12 hourly dosage:

	Peak	Trough
Gentamicin	5-10 mg/L	<2 mg/L
Tobramycin	5-10 mg/L	<2 mg/L
Amikacin	25-30 mg/L	<10 mg/L

Interpretation: For once-daily dosage of gentamicin, tobramycin or amikacin, a single measurement of plasma concentration should be made 6-14 hours after a dose.

Interpretation of aminoglycoside levels for monitoring once-daily dosage

If the measured concentration of gentamicin or tobramycin, or one quarter the measured concentration of amikacin, does not lie between the curves on the graph, then dose adjustment is required. For 8- and 12-hourly dosage of aminoglycosides, a peak level is measured to assess adequacy of the dose in serious infections. The peak level should be measured 30 minutes to 1 hour after a dose, given either as an IV infusion over 20-30 minutes or as an IM injection.
A trough level (immediately prior to the next dose) is measured to assess whether accumulation is occurring. For vancomycin, peak levels do not correlate with either efficacy or toxicity; trough levels should be maintained at 5-10 mg/L. Flucytosine toxicity occurs at levels >100 mg/L; trough levels should be maintained at >25 mg/L for efficacy. Chloramphenicol toxicity is associated with peak levels of >20 mg/L.

Reference: Victorian Drug Usage Advisory Committee Antibiotic Guidelines. 10th ed. 1998.