Antibiotic assay

Keywords: Gentamicin, Tobramycin, Vancomycin

Specimen:

5 mL blood in plain tube.

Method:

HPLC, EIA.

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.

Interpretation:

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

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 min to 1h after a dose, given either as an IV infusion over 20-30 min or as an IMI.

A trough level (immediately prior to the next dose) is measured to assess whether accumulation is occurring.

The following are recommended levels for 8-12 hourly Aminoglycoside Therapy:

Gentamicin: >8 mg/L (peak), <2 mg/L (trough)

Tobramycin: >8 mg/L (peak), <2 mg/L (trough)

Amikacin: >32 mg/L (peak), <8 mg/L (trough) 

For vancomycin, peak levels do not correlate with either efficacy or toxicity. Trough levels should be maintained at 10-20mg/L.

Flucytosine toxicity occurs at levels >100 mg/L. Trough levels should be maintained at >25 mg/L for efficacy.

Reference:

Victorian Drug Usage Advisory Committee. Antibiotic Guidelines. 13th ed. Melbourne: Therapeutic Guidelines Ltd, 2006