INTRODUCTION

Microbiological assay methods have traditionally been used to quantify the potency, or antimicrobial activity, of antibiotics. These microbiological procedures were historically used to certify antibiotics on a lot-by-lot basis to ensure sufficient activity. Before 1998, monographs for several approved antibiotics were published in FDA’s Code of Federal Regulations, 21 CFR. These regulatory antibiotic assay procedures were later published in USP as the official referee methods to determine the potency of antibiotics. The details of the microbial assay procedures for individual antibiotics, including the challenge organisms and test parameters, are described in general chapter Antibiotics—Microbial Assays 〈81〉. Microbial assays provide a direct measure of the effectiveness of the antibiotic against a reference microorganism. Although these microbiological methods have continued to serve as the official compendial referee methods since their publication in USP, many manufacturers have replaced these microbiological analyses with high-performance liquid chromatography (HPLC) methods.