At a glance
Also known as
Sensitivity testing; Antimicrobial susceptibility testing; AST; Drug resistance testing; Culture and sensitivity; C&S
Why get tested?
To determine the likelihood that a particular antimicrobial agent will be effective in stopping the growth of the or causing your infection
When to get tested?
As follow-up to a positive bacterial or fungal ; when you have an infection and one or more types of bacteria or fungi have been grown and isolated in a culture from a sample obtained from the site of suspected infection; when your infection is not responding to treatment
A sample of a pure culture of bacteria or fungi grown and isolated from an infected body site
Test preparation needed?
What is being tested?
Susceptibility is a term used to describe the condition in which are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing determines the potential effectiveness of an antimicrobial agent on the organism causing an infection and/or determines if the organism has developed resistance to certain antibiotics. The results of this test can be used to predict the potential effect in the patient. Although are microorganisms, testing for their resistance to antiviral drugs is performed differently, so this article is limited to the discussion of and susceptibility testing.
Bacteria and fungi have the potential to develop resistance to antimicrobial agents at any time. This means that antibiotics once used to kill or inhibit their growth may no longer be effective. Susceptibility testing is a way to determine if this is the case when your is positive for the presence of pathogens. A culture of the infected area must first be performed on a sample from the site of suspected infection to see if any bacteria or fungi are present that may be causing your infection. (For more about cultures, see the specific articles: Blood culture, Urine culture, Wound culture, AFB smear and culture, Fungal Tests).
During the culture process, (if present) are isolated (separated out from any other microorganisms present) and identified using biochemical, enzymatic, mass spectrometry or molecular tests. Once they have been identified, a determination can be made as to whether susceptibility testing is required. Susceptibility testing is not performed on every pathogen; there are some that respond to established standard treatments. An example of this is strep throat, an infection caused by Streptococcus pyogenes (also known as group A streptococcus).
Susceptibility testing is performed separately on each type of bacteria or fungi that may be clinically significant in the specimen and whose susceptibility to treatment may not be known. Each pathogen is tested individually to determine the ability of antimicrobials to inhibit its growth. This is can be measured directly by bringing the pathogen and a specific antibiotic together in a growing environment, such as in a test tube or agar plate, to observe the effect of the antibiotic on the growth of the bacteria.
How is the sample collected for testing?
A sample is obtained from a pure of or suspected of causing your infection.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.