Testing activity of antibiotics by agar diffusion

When the quality of antibiotics and antibiotic dosage forms cannot be assessed with the help of chemical methods, one can use two variants of the agar diffusion test described in the State Pharmacopoeia XII, part 1 (General pharmacopoeial monograph 42-006807): the three-doses variant of the agar diffusion test and determination with the help of a standard curve. The method is based on directly proportional relationship between the size of test microorganisms inhibition zones and log concentration (dose) of the antibiotic in the solution placed onto agar bacterial medium. The size of the zone depends on a number of factors: sensitivity of the test microorganism to the antibiotic, homogeneity and density of the bacterial lawn, composition of the growth medium, amount of the growth medium placed on Petri dishes, chemical nature of the antibiotic, the dose of antibiotic used in the experiment. Laboratory employees who use this method to test the quality of antibiotics should have appropriate skills. The activity of the drug under testing is determined by comparing it to the established activity of the reference standard which should have compendial quality. Statistical methods are used to assess obtained results and confirm validity of test conditions. The precision of test results is confirmed by computing the 95% confidence interval, and the repeatability is supported by calculation of the relative standard deviation.

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