What is a Culture and Sensitivity test?
A Culture and Sensitivity (C&S) test is a laboratory test used to determine whether bacteria or other microorganisms are present in a clinical specimen and, when a bacterial pathogen is identified, which antibiotics are most likely to be effective against it.
During the culture process, the laboratory places the specimen onto appropriate culture media and incubates it under conditions that allow microorganisms to grow.
If a clinically significant microorganism grows, the laboratory identifies the organism and may perform antimicrobial susceptibility testing (AST) to determine whether it is susceptible or resistant to specific antibiotics.
Culture and sensitivity testing can help healthcare providers select the most appropriate antimicrobial treatment and avoid unnecessary use of antibiotics.
Other names: C&S, culture and susceptibility, bacterial culture and sensitivity, culture and antimicrobial susceptibility testing (AST), culture and antibiotic sensitivity.
What is it used for?
A Culture and Sensitivity test may be used to help:
Detect an infection
Culture can identify microorganisms responsible for infections in different parts of the body.
Depending on the specimen, cultures may be performed on:
Urine
Blood
Sputum or respiratory specimens
Wound or pus specimens
Throat swabs
Stool
Genital specimens
Other body fluids or clinical specimens
Identify the microorganism causing the infection
If microorganisms grow in culture, the laboratory can identify the organism, such as a particular species of bacteria or yeast, depending on the specimen and testing method.
Determine antibiotic susceptibility
The sensitivity portion of the test determines how the isolated microorganism responds to selected antimicrobial agents.
Results may be reported using categories such as:
S — Susceptible
I — Intermediate / Susceptible, increased exposure, depending on the reporting standard used
R — Resistant
The exact terminology depends on the laboratory's susceptibility testing system and the interpretive standard being followed, such as CLSI or EUCAST.
Help guide antibiotic treatment
Culture and susceptibility results can help healthcare providers choose an antibiotic that is appropriate for the identified organism and its susceptibility pattern.
This can help reduce unnecessary or ineffective antibiotic treatment and support antimicrobial stewardship.
Why do I need a Culture and Sensitivity test?
You may need this test if your healthcare provider suspects a bacterial infection, especially when identifying the causative organism or selecting an effective antibiotic is important.
It may be requested if you have symptoms such as:
Fever
Pain or swelling
Redness or inflammation
Pus or discharge
Burning or frequent urination
Persistent cough or respiratory symptoms
Diarrhea or gastrointestinal symptoms
Wound infection
Other symptoms suggesting an infection
The type of specimen collected depends on the suspected site of infection.
What happens during a Culture and Sensitivity test?
The procedure depends on the type of specimen being tested.
Urine culture
You may be asked to provide a clean-catch midstream urine sample.
Wound culture
A healthcare professional may collect a specimen from the wound using a sterile swab or aspirate material from the infected area.
Throat or respiratory culture
A swab may be used to collect a specimen from the throat or respiratory tract.
Blood culture
Blood is collected from a vein using sterile techniques and placed into special blood culture bottles.
Other specimens
Depending on the suspected infection, samples may be collected from stool, sputum, genital sites, or normally sterile body fluids.
The laboratory then processes the specimen and incubates it under appropriate conditions.
If significant growth occurs, the microorganism is identified and antimicrobial susceptibility testing may be performed.
Will I need to do anything to prepare for the test?
Preparation depends on the type of specimen.
Generally, it is important to collect the specimen before starting antibiotic treatment, whenever clinically appropriate and without delaying urgent treatment.
Tell your healthcare provider if you are currently taking or have recently taken antibiotics, because antimicrobial treatment may reduce the chance of recovering the causative organism from a culture.
Follow the specific collection instructions provided by your healthcare provider or laboratory.
Are there any risks to the test?
The risks depend on the type of specimen collected.
For urine, stool, sputum, or swab specimens, there are generally no significant risks associated with collection.
Blood culture collection carries the usual minor risks of blood sampling, such as brief pain, bruising, or slight bleeding.
What do the results mean?
Culture and sensitivity results may include several components:
No growth / Negative culture
This means that no clinically significant microorganism was recovered from the specimen under the culture conditions used.
A negative culture does not always completely exclude infection. Previous antibiotic treatment, inadequate specimen collection, low numbers of microorganisms, or organisms that are difficult to grow can affect the result.
Positive culture
A positive culture means that a microorganism was recovered from the specimen.
The laboratory will identify the organism and determine whether the finding is clinically significant.
A positive culture does not always mean that a person has an active infection. Colonization or contamination may sometimes produce a positive result, depending on the specimen and organism.
Susceptible (S)
The microorganism is considered susceptible to the antimicrobial agent when the drug is used according to the relevant dosing and exposure conditions.
Intermediate / Susceptible, increased exposure (I)
The interpretation of I depends on the standard used by the laboratory.
Under current EUCAST terminology, I means "Susceptible, Increased exposure". This indicates that the microorganism may be successfully treated when exposure to the antimicrobial is increased through an appropriate dosing regimen or drug concentration at the site of infection.
Resistant (R)
The microorganism is considered resistant to the antimicrobial agent under the conditions defined by the applicable susceptibility standard.
Important note about Culture and Sensitivity results
A Culture and Sensitivity test should always be interpreted together with the patient's symptoms, clinical findings, specimen type, specimen quality, and medical history.
A microorganism growing from a specimen does not automatically prove that it is causing disease. For example, some organisms may represent normal flora, colonization, or contamination.
The quality of specimen collection is therefore extremely important.
Antibiotic susceptibility results should also be interpreted according to an established standard, such as CLSI or EUCAST, and the final choice of antimicrobial treatment should be made by the healthcare provider.
Do not start, stop, or change antibiotics based solely on a culture report without medical advice.
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