Protocol for Antibiotic Assays

Written by Clare Overton Lewis Created on Updated on .

Samples For all antibiotic assays, 5-10mls blood into red-topped tube (no anticoagulant) DO NOT sample from the line through which the antibiotic has been given.


Antibiotic Sampling Time First Sample Expected Levels Usual interval to re-assay




Pre-dose only (i.e. within 15 mins before dose)


Around 2nd – 4th dose

10 – 20mg/L

3 days


GENTAMICIN / TOBRAMYCIN (b.d or t.d.s regime)

Pre-dose (within 15 mins before dose)


Post-dose (1hr after dose)

Around 2nd – 4th dose

Pre less than 2mg/L

Post 5 – 10 mg/L

3 days

GENTAMICIN / TOBRAMYCIN (once – daily regime) Post-dose only taken between 6 and 14 hours after dose.

N.B. Times of dosing and sampling must be given on request form.

1st to 3rd dose

Interpretation by Microbiologist

3 days

Important Notes

  1. After taking pre-dose levels, give the prescribed dose as normal – DO NOT wait for the results of the assay.
  2. “Random” levels should seldom be needed and should only be taken by prior arrangement with the Duty Microbiologist.
  3. For patients with stable renal function, whose results are within the expected range, assays do not need to be performed more than twice a week. Where renal function is unstable or poor, assays should be discussed with the Duty Microbiologist.