Blood Culture Bacteria are one-celled organisms. There are many different kinds of bacteria. They live just about everywhere in your body and on your skin. Some types of bacteria are harmless or even helpful. Others can cause infections and disease.
A bacteria culture test can help find harmful bacteria in or on your body that may be making you sick. To do the test, you will need to give a sample of your blood, urine, skin, or other tissue. The type of sample depends on where the infection seems to be located.
To find out what type of bacteria you may have, a health care professional will need to examine a large number of bacteria cells. So, your sample will be sent to a lab where the bacteria cells will be grown until there are enough for the test. Test results are often ready within a few days. But some types of bacteria grow slowly, so sometimes your results may take several days or longer.
Does this test have other names?
Blood culture and sensitivity test, blood C&S
Why do I need this test?
You may need this test if you have symptoms of a systemic infection. These include:
- Fever or low temperature
- Severe tiredness (fatigue)
- Fast breathing or heart rate
- Passing urine less often
A blood culture and sensitivity test can be done to confirm an infection, such as pneumonia, and figure out the best way to treat it.
What is it used for?
Bacteria culture tests are used to help diagnose certain types of infections. The most common types of bacteria tests and their uses are:
- Used to diagnose or rule out strep throat
- Test procedure:
- A health care professional uses a special swab to take a sample from the back of your throat and tonsils.
- Used to diagnose a urinary tract infection and identify the bacteria causing the infection
- Test procedure:
- You provide a sterile sample of urine in a container, following special instructions.
Sputum is a thick mucus that is coughed up from the lungs. It is different from spit or saliva.
- Used to help diagnose bacterial or fungal infections in your respiratory tract, such as bacterial pneumonia, tuberculosis, bronchiectasis, and histoplasmosis.
- Test procedure:
- You provide a sputum sample, usually by coughing it up into a special cup. In certain cases, you may need to have a bronchoscopy to get a sample. A bronchoscopy involves inserting a flexible tube through your nose or mouth and into your lungs.
- Used to look for bacterial or fungal infections in your blood
- Test procedure:
- A health care professional takes a blood sample, usually from a vein in your arm.
Another name for stool (poop) is feces.
- Used to detect bacterial infections in your digestive system, including food poisoning. Because many things can cause digestive illness, this test is often done with other tests to look for viruses and parasites that may be causing symptoms.
- Test procedure:
- You provide a sample of your feces in a clean container.
- Used to find infections in open wounds or on burn injuries
- Test procedure:
- A health care professional uses a special swab to collect a sample of cells or pus from your wound. For deeper wounds, a syringe may be used to draw out fluid, or you may have a biopsy to remove a piece of tissue from the wound.
Purpose of a blood culture
Blood cultures are ordered when your doctor suspects you may have a blood infection. It’s important to test for blood infections because they can lead to serious complications. One such complication of a blood infection is sepsis.
In sepsis, the pathogens that are causing the infection in your bloodstream interfere with your body’s normal defenses and prevent your immune system from working properly. The pathogens also produce toxins that can damage your organs.
The results of the test can help your doctor determine which specific organism or bacteria is causing the blood infection and how best to combat it.
Symptoms of blood infection and sepsis
You should call 911 or visit a doctor immediately if you’re experiencing any symptoms of a blood infection. These include:
- shaking chills
- moderate or high fever
- rapid breathing
- increased heart rate or palpitations
- excessive fatigue
- muscle aches
Without treatment, a blood infection can progress to its most severe stage, sepsis. The symptoms of sepsis include those listed above, as well as signs of damaged organs. The following are additional symptoms of sepsis:
- decreased urine
- mottled skin
As the infection progresses, more serious complications of sepsis may develop. These can include:
- inflammation throughout your body
- formation of many tiny blood clots in your smallest blood vessels
- a dangerous drop in blood pressure
- failure of one of more organs
Blood infection risk factors
Blood cultures are done more frequently for those who are at a higher risk of developing a blood infection. You’re at a higher risk if you’ve been diagnosed with:
- HIV or AIDS
- an autoimmune disease
The following situations also put you at risk for blood infection:
- You’ve recently had an infection.
- You’ve recently had a surgical procedure.
- You’ve had a prosthetic heart valve replacement.
- You’re undergoing immunosuppressive therapy.
Blood cultures are also drawn more frequently in newborns and children with fever who may have an infection but don’t have the typical signs and symptoms of sepsis. Older adults are also at higher risk for blood infections.
Potential risks of a blood culture
Complications you may experience from this test only occur when you give blood. However, blood draws are routine procedures and rarely cause any serious side effects.
The risks of giving a blood sample include:
- bleeding under your skin, or hematoma
- excessive bleeding
Why do I need a bacteria culture test?
Your provider may order a bacteria culture test if you have symptoms of a bacterial infection. The symptoms vary, depending on the type of infection.
Why do I have to wait so long for my results?
Bacteria culture tests require a large number of cells to accurately identify what type of bacteria may be causing an infection. Most test samples don’t include enough cells for that. So your sample is sent to a lab to allow the cells to grow until there are enough to test. Most disease-causing bacteria will be ready for testing within one to two days, but some types of bacteria take five days or longer to grow enough cells.
Finding a Blood Culture Test:
How to get tested
A blood culture test is ordered by a health care professional and requires multiple blood samples taken from different veins. These blood samples are typically taken in a doctor’s office, hospital, laboratory, or health clinic.
Can I take the test at home?
Blood culture testing is not available at home. Special procedures must be followed when collecting a blood sample for a culture test, so it is important that the test be performed by a health care provider in a medical setting.
How much does the test cost?
There is no standard cost for a blood culture test. The cost of testing can depend on factors like:
- Where the blood samples are taken
- How many blood samples are taken
- The type of culture testing being performed
- If any additional tests are done along with the initial culture
- Whether you have health insurance
If you have insurance, many fees may be covered if the blood culture test is recommended by your doctor. However, you may still have to pay out-of-pocket costs toward a deductible or for a copay.
Check with your doctor, a hospital administrator, and/or your health insurance provider for more details about your expected costs from blood culture testing.
Taking a Blood Culture Test
Blood culture testing requires at least two blood samples that are taken from different veins. Multiple samples are used because it provides a more reliable test result. Often, three or four blood samples will be taken.
The blood draws can occur in various medical settings like a doctor’s office, laboratory, or hospital. Patients with severe symptoms from infections may have their blood drawn in an emergency room.
Before the test
There is usually no preparation required to take a blood culture test. However, testing is normally performed before starting antibiotic treatment because these drugs can affect test results. For that reason, you should tell your doctor if you have taken any antibiotics prior to the test.
During the test
It is normal to have between two and four separate blood draws for blood culture testing. Samples may be obtained at the same time, or two initial samples may be drawn with additional blood samples taken several hours later.
Blood samples are usually taken from veins in the arms. For each blood draw, a health care professional will tie an elastic band around your arm to increase blood flow in the vein. The skin will then be thoroughly cleaned with an antiseptic, which is an important step in making sure that bacteria from the skin do not contaminate your blood sample.
A needle will be inserted through the skin and into the vein, which may cause a brief stinging sensation. Blood will be withdrawn into a vial, and once enough blood is collected, the needle will be removed. The same process will be repeated for each blood sample that is needed. Each blood draw usually takes less than a few minutes.
After the test
A bandage or gauze will typically be applied to each area where your blood was drawn. You may notice some bruising or soreness around the puncture sites, but these effects usually go away quickly.
Serious complications from a blood draw are rare, but make sure to tell your doctor if you have any persistent or severe side effects after your blood culture test.
Blood Culture Test Results
Receiving test results
It may take a few days to a few weeks for results from your blood culture test to be available.
Germs must be given time to grow in the laboratory, and the amount of time required can depend on the specific type of germs that are believed to be causing an infection. In most cases, an initial culture test can be completed within five days, but tests for some conditions, such as tuberculosis, can take up to a month.
Once testing is complete, results are usually provided by your doctor at an in-person appointment or over the phone. A test report may also be available to view through an online health portal.
Interpreting test results
The results of a blood culture test are based on whether any germs were detected. The test report may list the blood culture test as either negative or positive.
- A negative test result means that no germs grew in the culture.
- A positive test result means that one or more germs developed in the dish. A positive result indicates the presence of germs in your blood.
Because a blood culture involves multiple test samples, it is possible for some samples to be positive and others to be negative.
Normally the blood is sterile, which means there should not be bacteria, fungi, or viruses present. For this reason, a positive test result can be a sign of an infection. If the test result is positive, other information may be listed on the test report including:
- How many and which of the test samples were positive
- The specific type of germ or germs that were found
- How quickly the germ or germs developed
- How much the germ or germs grew over a set period of time
Each of these factors can be important for interpreting your test result. Certain types of germs are almost always associated with a potentially serious infection. Other germs may grow because of contamination of your sample or the lab equipment and not because of an infection in your blood.
For this reason, the details of a positive result on a blood culture test must be reviewed by your doctor who can consider how the test result relates to your symptoms, overall health, and results from other tests.
If the blood culture test found germs that are likely to be harmful, the test report may also include details about drug susceptibility testing. This information can help your doctor choose the most appropriate treatment for the infection.
Are test results accurate?
Culture testing is very accurate and widely considered to be the best method for identifying the germs that cause an infection. However, no test is perfect, and the results of a blood culture test can be affected by certain factors:
- Germs that do not grow in lab cultures: Certain types of bacteria, fungi, and viruses are difficult or impossible to grow in the laboratory. A person infected with one of these germs can have a negative test result even when they have an infection.
- Sample contamination: Sometimes germs from the skin enter the blood during the blood draw, or germs from lab equipment get into the culture dish. In these cases, the test result may be positive even though the germs are not actually in the blood. This is called a false positive result. Health professionals take steps to reduce the risk of contamination, and doctors carefully interpret a positive test result to determine when contamination could have affected the results.
- Use of antibiotics: If you have started taking any antibiotic or other antimicrobial drug, it can limit the accuracy of a blood culture test. For this reason, samples for a blood culture test are typically taken before beginning treatment for an infection.
- Conflicting results: Sometimes the laboratory equipment detects germs on the blood culture test, but follow-up tests using a microscope or other methods do not identify any germs present. This type of conflicting result is rare, but when it occurs culture testing may need to be repeated.
The results from blood culture tests are generally dependable and widely used in the care of patients with infections. If you have any questions about the accuracy of your test, you can talk with the doctor for more information.
Do I need follow-up tests?
Depending on your symptoms and the results of your blood culture test, you may need follow-up testing.
If blood culture tests are negative but you have symptoms of an infection, your doctor may recommend more testing. Since a blood culture test can’t detect all germs, other types of tests may be used to look for an infection. The doctor may also suggest repeating blood culture testing.
If your blood culture test is positive, the doctor may order tests to try to determine where the infection started or to see if the infection has spread to other parts of the body. After treatment has started, you may need to have follow-up blood culture tests to see if the infection is getting better.
Because blood infections can cause serious complications, the doctor may also perform tests to check your vital signs, assess organ function, and monitor your overall health.
Questions for your doctor about test results
When you discuss the results of your blood culture test with your doctor, some of the following questions may be helpful:
- Were any of my blood culture test samples positive?
- If my test was positive, what type of germs were found?
- What does my blood culture test mean about my infection?
- Do I need any follow-up testing?
- What type of treatment do you recommend?
Blood culture testing in children vs. adults
Blood culture testing can be performed in both adults and children. Testing is generally similar, although the number of blood draws, the amount of blood in each sample, and the preparation of the samples for testing may be different for children.
Blood culture testing may also be performed in children up to age three who have a fever with no apparent cause and no other symptoms of an infection. In some very young children, this type of fever can develop when they have a blood infection. While vaccinations reduce the likelihood of these infections, the doctor may recommend blood culture testing to determine if there are germs in the blood.
What do the results mean?
If harmful bacteria are found in your sample, it usually means you have a bacterial infection. Your provider may order more tests to confirm the diagnosis or to see how serious the infection is.
Your provider may also order a test to find out which medicine will work best to treat the type of bacteria you have. This test is called an antibiotic sensitivity test or a susceptibility test. It checks to see how sensitive the bacteria are to different antibiotic medicines. If you have questions about your results, talk with your provider.
Learn more about laboratory tests, reference ranges, and understanding results.
Is there anything else I should know about a bacteria culture?
If your results show you don’t have a bacterial infection, you should not take antibiotic medicines. Antibiotics only treat bacterial infections. Taking antibiotics when you don’t need them won’t help you feel better and may lead a serious problem known as antibiotic resistance.
Antibiotic resistance allows harmful bacteria to change in a way that makes antibiotics less effective or not effective at all. This can be dangerous to you and your community, because antibiotic-resistant bacteria can spread to others.