What is a Culture and Sensitivity (C&S) Test?

A culture and sensitivity test, often abbreviated as C&S, is a laboratory test that performs two functions. First, it determines whether microorganisms are growing in a sample taken from your body. Microorganisms (also called microbes) are living things too small to see without a microscope, such as bacteria and fungi. Second, if a microorganism grows, the test checks which antibiotics are likely to stop it. An antibiotic is a medicine used to treat infections caused by bacteria, and a related group of medicines called antifungals is used for infections caused by fungi.

This article explains what a culture and sensitivity report shows, what the different results mean, and what to ask your doctor, so you can better understand a report you have received. In most cases, a result of “no growth” is reassuring, while a result that names a microorganism tells you and your doctor exactly what is causing the infection and which medicines should work against it.

What is a culture, and what is sensitivity testing?

The test has two parts, and your report may describe them separately.

A culture is a way of growing microorganisms in the laboratory. A sample from your body is placed in or on a special material that microorganisms like to grow on, called a culture medium. The sample is then kept warm for a period of time. If bacteria or fungi are present, they multiply until there are enough of them to see and identify. If nothing grows, the result is reported as “no growth.”

Sensitivity testing, also called antibiotic susceptibility testing, is only performed if something grows in the culture. It measures how well different antibiotics stop the microorganism that was found. The word “sensitive” means the microorganism is affected by the antibiotic, and “resistant” means it is able to survive it. The purpose of this part of the test is to help your doctor choose a medicine that is likely to work against your specific infection.

Why is a culture and sensitivity test done?

This test is ordered when your healthcare team suspects an infection and wants to know two things: whether a microorganism is causing it, and which medicine is most likely to treat it. The sample tested depends on where the infection is thought to be. Common samples include:

Urine — when a urinary tract (bladder or kidney) infection is suspected.
Blood — when there is concern that an infection has spread into the bloodstream.
A wound swab or fluid — when a cut, surgical site, or skin infection is not healing as expected.
Sputum — mucus coughed up from the lungs, when a chest or lung infection is suspected.
Stool — when an infection of the digestive tract is suspected.
Other fluids or tissue — such as fluid from around the lungs, joints, or spine.

Because the test grows living microorganisms, it can both confirm that an infection is present and identify the exact microorganism responsible. This is its main advantage over many other tests.

How the test is performed

Understanding the basic steps helps make sense of the timing and the wording of your report.

After the sample is collected, it is placed in the laboratory under conditions that promote microbial growth. For some samples, the laboratory first performs a Gram stain, a quick test in which the sample is treated with a dye and examined under a microscope. The Gram stain provides an early clue about the general type of bacteria present, based on their color and shape, before the culture is fully completed. Bacteria that hold the purple dye are called gram-positive, and bacteria that do not are called gram-negative. Round bacteria are called cocci, and rod-shaped bacteria are called bacilli.

If microorganisms grow, they form small visible clusters called colonies. The laboratory then identifies exactly which microorganism is present and, in most cases, performs sensitivity testing on it. Because growing and identifying microorganisms takes time, results usually arrive in stages. An early or preliminary report may describe what has been observed so far, such as a Gram stain result or early growth. A final report, usually available after one to several days, gives the full identification and sensitivity results. Some microorganisms grow slowly and take longer.

How to read your culture result

The first part of your report describes what grew, if anything. You may see any of the following.

No growth — No microorganisms grew in the sample. For many tests, this is the expected, reassuring result. The preliminary report may say “no growth after 24 hours,” and the final report may say “no growth after 48 hours” or longer.
The name of a microorganism — If something grew, the report names it, for example, Escherichia coli (often written E. coli) or Staphylococcus aureus. The name identifies the exact microorganism causing the infection.
A colony count — For urine samples, the report often includes the amount of bacteria found, measured in colony-forming units per milliliter (CFU/mL). A high count of a single bacterial type usually indicates a true infection, while a low count may be less significant.
Normal flora or mixed flora — The body is normally home to harmless microorganisms, especially on the skin and in the bowel. These are called normal flora. When several different microorganisms grow in small amounts, the report may say “mixed flora,” which often means the sample picked up these normal microorganisms rather than a single cause of infection.
Possible contaminant — Sometimes a microorganism from the skin gets into the sample during collection. When the laboratory suspects this has happened, it may describe the result as a possible contaminant, meaning it may not reflect a true infection.

Deciding whether a result represents a true infection or a contaminant is not always straightforward, and it is one reason your doctor reads the report alongside your symptoms rather than on its own.

Understanding the sensitivity (antibiotic susceptibility) results

If a microorganism grew, the report lists the antibiotics tested against it. Each antibiotic is given a result that predicts how likely it is to work. The three results you are most likely to see are:

Susceptible (S) — The antibiotic is expected to work against this microorganism at the usual dose. These are the medicines most likely to treat the infection.
Intermediate (I) — The result is in between. The antibiotic may still work, but often only at a higher dose or in parts of the body where the medicine becomes concentrated, such as the bladder for a urine infection. It may also mean the result is uncertain.
Resistant (R) — The microorganism can survive this antibiotic, so the medicine is not expected to work and is usually avoided.

Some reports also include a number called the minimum inhibitory concentration, or MIC. This is the smallest amount of an antibiotic required to prevent the microorganism from growing in the laboratory. The laboratory compares the MIC to a standard cutoff to decide whether the result is susceptible, intermediate, or resistant. It is important to know that a lower MIC for one antibiotic does not mean it is “stronger” or a better choice than another antibiotic with a higher MIC, because each antibiotic has its own cutoff. For this reason, MIC values cannot be directly compared between medicines.

You may notice that not every available antibiotic appears on the report. Laboratories usually test and report on a selected group of antibiotics appropriate for the type of microorganism and the site of infection. Some results may be omitted on purpose to encourage the use of the most appropriate medicine and to help limit the overuse of antibiotics, a practice known as antibiotic stewardship.

What “no growth” means, and what it does not mean

A lack of growth usually indicates that no bacteria or fungi were found in the sample. In many situations, this rules out the kind of infection the test was looking for. However, no growth does not always mean there is no infection at all. A few situations can explain a negative result even when an infection is present:

Antibiotics were already started — If you began taking an antibiotic before the sample was collected, it may have stopped the microorganism from growing in the laboratory.
The microorganism is hard to grow — Some bacteria and fungi grow slowly or need special laboratory conditions, so they may not appear on a standard culture.
The infection is caused by something a culture does not detect — Routine bacterial cultures do not detect viruses. An infection caused by a virus will show no growth on a bacterial culture, which is why other tests are used to look for viruses.

For these reasons, your doctor considers a no-growth result together with your symptoms and any other test results, rather than relying on it alone.

What happens after this test

A culture and sensitivity report describes what was found and which medicines are likely to work. It informs the decisions you and your healthcare team make together, but it does not, by itself, prescribe a treatment.

When an infection is suspected, doctors often start an antibiotic right away, before culture results are available, choosing one likely to work against the most common causes. This is called empiric treatment. Once the culture and sensitivity results return, the report helps the team decide whether to continue, change, or stop that antibiotic. If the microorganism is susceptible to a more targeted or narrower medicine, the team may switch to it, which can reduce side effects and the risk of resistance developing. If the report identifies the result as a contaminant or normal flora, the team may decide that no treatment is needed. In some cases, such as a bloodstream infection, repeat cultures may be ordered to confirm that the infection has cleared.

Questions to ask your doctor

What sample was tested, and what infection were you looking for?
Did anything grow in the culture, and what was it?
Does this result mean I have an infection, or could it be a contaminant or normal flora?
If my result was “no growth,” could I still have an infection that this test does not detect?
Which antibiotics did the microorganism turn out to be susceptible to?
Why was the antibiotic I am taking chosen, and is it one the report lists as susceptible?
Should my antibiotic be changed now that the sensitivity results are back?
What does it mean that some antibiotics were marked intermediate or resistant?
Could a recent antibiotic have affected my result?
Will I need a repeat test to confirm the infection has cleared?
Is this type of infection contagious, and do I need to take any precautions?
Who should I contact if my symptoms get worse while I wait for the final report?