{"id":19729,"date":"2026-08-22T15:47:46","date_gmt":"2026-08-22T08:47:46","guid":{"rendered":"https:\/\/www.vietstock.org\/?p=19729"},"modified":"2026-08-21T15:52:26","modified_gmt":"2026-08-21T08:52:26","slug":"antimicrobial-susceptibility-testing","status":"publish","type":"post","link":"https:\/\/www.vietstock.org\/en\/industry-news\/antimicrobial-susceptibility-testing\/","title":{"rendered":"Antimicrobial Susceptibility Testing in Veterinary Medicine: When It Is Needed and How to Interpret the Results"},"content":{"rendered":"

Antimicrobial susceptibility testing (AST) in veterinary medicine is a microbiological test used to determine how susceptible an isolated bacterium is to specific antimicrobial agents. The results provide additional evidence that can help veterinarians select a more appropriate antibiotic rather than relying solely on empirical treatment, particularly when antimicrobial resistance is suspected or initial treatment does not produce the expected response.<\/span><\/p>\n

AST is an important tool as antimicrobial resistance becomes an increasing concern in livestock production. However, susceptibility results should not be interpreted in isolation. They need to be considered alongside the clinical presentation, bacterial species, site of infection, animal health status, approved medicines, and current regulations.<\/span><\/p>\n

This article explains what antimicrobial susceptibility testing is, when it may be indicated, how the process works from a livestock producer\u2019s perspective, how to interpret S\/I\/R categories and MIC values, and what factors should be considered when selecting antibiotics based on test results.<\/span><\/p>\n

What Is Antimicrobial Susceptibility Testing in Veterinary Medicine?<\/b><\/h2>\n
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Bacterial culture and isolation from clinical specimens are important steps before antimicrobial susceptibility can be assessed.<\/figcaption><\/figure>\n

Definition and Purpose of Veterinary Antimicrobial Susceptibility Testing<\/b><\/h3>\n

Antimicrobial susceptibility testing (AST) is a microbiological test in which bacteria isolated from an animal clinical specimen are tested for their susceptibility to specific antimicrobial agents.<\/span><\/p>\n

Under laboratory conditions, the results indicate the susceptibility of the bacterial isolate to each antimicrobial according to the testing method and breakpoints being applied. This information can help veterinarians assess which medicines may remain suitable and which may have a higher risk of failing to achieve the desired treatment outcome.<\/span><\/p>\n

The main purpose of AST is to provide additional evidence for antimicrobial treatment decisions, particularly when disease does not respond to initial treatment, recurs after treatment, or may involve antimicrobial-resistant bacteria.<\/span><\/p>\n

AST alone cannot determine an entire treatment protocol. Results still need to be interpreted in the context of case history, clinical signs, site of infection, the isolated organism, the pharmacokinetic and pharmacodynamic characteristics of the medicine, and current veterinary medicine regulations.<\/span><\/p>\n

How Is AST Different From Empirical Antibiotic Treatment?<\/b><\/h3>\n

Empirical antibiotic treatment refers to the initial selection of an antibiotic by a veterinarian based on the clinical presentation, suspected pathogen, epidemiological information, previous treatment history, and antimicrobial use within the herd or flock before complete microbiological results are available.<\/span><\/p>\n

This approach can save valuable time when early treatment is needed. However, there is a risk of selecting an inappropriate antibiotic if the underlying cause has not been accurately identified or if the bacteria have reduced susceptibility to commonly used medicines.<\/span><\/p>\n

AST differs because it provides experimental data from a bacterial isolate obtained from a clinical specimen. When the specimen has been collected correctly and the isolated bacterium is considered clinically relevant, the results can help veterinarians assess its susceptibility to each antimicrobial tested.<\/span><\/p>\n

This may help reduce inappropriate antimicrobial use, reduce repeated trial-and-error changes in antimicrobial treatment, and support more responsible antimicrobial stewardship on farms.<\/span><\/p>\n

Role in Disease Management and Reducing Antimicrobial Resistance<\/b><\/h3>\n

AST can provide value beyond an individual disease case. When data are collected consistently from a sufficient number of representative samples, susceptibility results from multiple disease episodes may help farms monitor changes in susceptibility patterns among commonly identified bacteria.<\/span><\/p>\n

These data can be considered alongside treatment records, epidemiological information, and production data to help veterinarians develop an antimicrobial management program that better reflects the farm\u2019s actual situation.<\/span><\/p>\n

However, a small number of individual AST results cannot represent the farm\u2019s overall antimicrobial resistance situation. The value of the data depends substantially on sampling quality, the number of bacterial isolates tested, sample representativeness, laboratory methods, and appropriate interpretation of the results.<\/span><\/p>\n

When Should Antimicrobial Susceptibility Testing Be Performed in Livestock?<\/b><\/h2>\n

Clinical Situations Where AST May Be Valuable<\/b><\/h3>\n

Not every disease case requires antimicrobial susceptibility testing. The following are situations in which AST may provide significant value:<\/span><\/p>\n