Dr Leigh Martens

General Practitioner

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Release Version: Beta

Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Sensitivity Testing


At a Glance

Why Get Tested?

To determine the likelihood that a particular antibiotic or antifungal drug will be effective in stopping the growth of the bacteria or fungi causing your infection

When to Get Tested?

As follow up to a positive bacterial or fungal culture; 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

Sample Required?

A sample of a pure culture of bacteria or fungi grown and isolated from an infected body site

Test Preparation Needed?

None

The Test Sample

What is being tested?

Susceptibility is a term used when microorganisms such as bacteria and fungi are unable to grow in the presence of one or more antimicrobial drugs. Susceptibility testing is performed primarily on bacteria but also on fungi that have been identified with a culture as causing an individual's infection. Testing is used to determine the potential effectiveness of specific antibiotics on the bacteria and/or to determine if the bacteria have developed resistance to certain antibiotics. The results of this test can be used to help select the drug(s) that will likely be most effective in treating an infection.

Although viruses are microorganisms, testing for their resistance to antiviral drugs is performed differently, so this article is limited to the discussion of bacterial and fungal susceptibility testing.

Bacteria and fungi have the potential to develop resistance to antibiotics and antifungal drugs 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 a culture of a sample collected from the site of a suspected infection is positive for the presence of one or more pathogens. (For more about cultures, see specific articles: Blood Culture, Urine Culture, Wound Culture, AFB Smear and Culture, Fungal Tests).

During the culture process, pathogens – if present – are isolated (separated out from any other microorganisms present) and each is identified using biochemical, enzymatic, or molecular tests. Once the pathogens 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) that can be treated with penicillin.

Susceptibility testing is performed 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 the antibiotic together in a growing environment, such as nutrient media 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 culture of bacteria or fungi suspected of causing an infection.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

Susceptibility testing is used to determine which antimicrobials will inhibit the growth of the bacteria or fungi causing a specific infection. The results from this test will help a health practitioner determine which drugs are likely to be most effective in treating a person's infection.

Like bacteria, viruses can develop resistance to the drugs used to treat them, but that type of testing is performed differently, so this article is limited to the discussion of bacterial and fungal susceptibility testing.

Some types of bacteria and fungi are known to be susceptible to certain antimicrobials, so routine testing may not always be necessary. For example, it is well known that most streptococci, including the type that causes strep throat, can be treated with penicillin, so these types of infections may be treated without susceptibility testing. However, if the usual drug of choice fails to treat the infection, then susceptibility testing may be necessary to determine a more effective drug.

Other types of infections may require testing because the bacteria or fungi isolated from an infection site are known to have unpredictable susceptibility to the drugs usually used to treat them. Some examples include staphylococci ("staph") and Pseudomonas aeruginosa. Sometimes there may be more than one type of pathogen isolated from an infected site. Susceptibility testing may be used to determine which antibiotic or antibiotic combinations will be most effective in treating all of the different types of bacteria causing the infection. This may be true, for example, with wound infections.

When is it ordered?

Susceptibility testing is often ordered at the same time as a culture of a potentially infected site, such as a wound, urine, or blood culture. However, the test will usually only be performed when the culture is positive for one or more pathogens. The test may also be ordered when an infection does not respond to treatment to see if the pathogen has developed resistance and to determine which antimicrobial agent would be more effective in treating the infection.

What does the test result mean?

Results of the testing are usually reported as:

  • Susceptible — likely, but not guaranteed to inhibit the pathogenic microorganism; may be an appropriate choice for treatment
  • Intermediate — may be effective at a higher dosage, or more frequent dosage, or effective only in specific body sites where the antibiotic penetrates to provide adequate concentrations
  • Resistant — not effective at inhibiting the growth of the organism; may not be an appropriate choice for treatment

Sometimes results are reported as minimum inhibitory concentration (MIC), in units such as milligrams per microliter (mg/mcL). This is the highest dilution (lowest concentration) of an antibiotic that will still be effective in inhibiting growth of the bacteria. Though results may be expressed this way, the laboratory will often include in the report an interpretation of what the results mean (e.g., susceptible, intermediate or resistant).

If there is more than one pathogen identified in a culture, the laboratory will report results for each one.

A health practitioner will choose an appropriate drug from those on the report that were categorized as "Susceptible." If there are no "Susceptible" choices, then the practitioner may select one categorized as "Intermediate." This may require a higher dosage and may involve a longer duration of therapy as well as a higher risk for medication side effects.

A pathogen may be "Resistant" to all of the drugs that are usually used to treat that type of infection. If this is the case, then the practitioner may prescribe a combination of antibiotics that work together to inhibit the bacteria when neither one alone will be effective. These drug therapies may be more expensive and have to be given intravenously, sometimes for extended periods of time. Some infections caused by resistant bacteria have proven very difficult to treat.

Is there anything else I should know?

A sample for culture and susceptibility testing should be collected before the start of any treatment with an antimicrobial unless the test is used to monitor the effectiveness of treatment.