Microbiological agar diffusion assay for metronidazole concentrations in serum

Area of Science:

• Microbiology
• Pharmacology
• Clinical Chemistry

Background:

• Accurate measurement of serum metronidazole is crucial for therapeutic drug monitoring.
• Existing methods may be affected by the presence of other antibiotics.
• Clostridium perfringens is a suitable organism for metronidazole susceptibility testing.

Purpose of the Study:

• To describe a novel agar diffusion method for quantifying serum metronidazole concentrations.
• To evaluate the sensitivity and accuracy of the assay.
• To assess the assay's performance in the presence of common antibiotics.

Main Methods:

• An agar diffusion assay was developed using paper disks on plates seeded with Clostridium perfringens.
• Plates were incubated anaerobically to allow for bacterial growth.
• Serum samples were applied to the disks, and zones of inhibition were measured.

Main Results:

• The assay could measure metronidazole concentrations as low as 0.7 mug/ml.
• An average error of 4% was observed for measurements at 5 mug/ml.
• The assay demonstrated reliability in the presence of gentamicin, penicillin (with penicillinase), and a combination of penicillin, methicillin, and cephalothin (with prior beta-lactamase exposure).

Conclusions:

• The described agar diffusion method provides a sensitive and accurate means to determine serum metronidazole levels.
• The assay is robust and can be employed in the presence of several commonly co-administered antibiotics.
• This method offers a valuable tool for therapeutic drug monitoring of metronidazole.