About the Test
Purpose of the test
The purpose of a blood culture test is to see if there is an infection in the bloodstream. In testing for infections, a blood culture test has several roles:
- Diagnosis: Testing for bacteria and fungi in the blood can confirm that an infection is the cause of a person’s symptoms. If a virus is suspected, special viral tests can be performed.
- Germ identification: By growing the germs from a patient’s blood in the lab, a blood culture test makes it possible to determine the specific type or strain of germ causing an infection.
- Assessing infection severity: Along with identifying the type of germ, a blood culture test can show how extensive an infection is in the body, including whether it has entered the bloodstream. This is important because an infection in the blood can spread to other parts of the body. Blood infections can also trigger a potentially life-threatening inflammatory reaction called sepsis.
- Treatment planning: Once a germ has been grown in the laboratory during a blood culture test, the lab can perform susceptibility testing, which checks to see what type of antibiotic works best against that germ.
- Monitoring: Repeat blood culture tests can be used to see how well an infection is responding to treatment and to determine when the blood is free of the germ.
What does the test measure?
A blood culture test measures and analyzes the growth of germs that can cause infections, such as bacteria or fungi. Special tests can be used to see if the infection is caused by a virus.
To perform a blood culture test, a laboratory technician places part of a blood sample onto a dish containing material that encourages germs to grow. This material is known as the culture. The dish is then observed over days or weeks to see if any germs become detectable. The laboratory can also observe how quickly and how many germs developed.
If germs are found in the culture, further analysis is done to identify them. This may involve looking at the germs under a microscope while applying chemical dyes, such as a Gram stain, that help identify bacteria or fungi. Other laboratory methods, including molecular tests using a polymerase chain reaction (PCR) technique, may be able to identify certain types of germs.
Blood cultures are frequently used for drug susceptibility testing. In drug susceptibility testing, one or more antibiotics are added to portions of the culture to see how well they kill the germs. This type of testing can help doctors choose the most appropriate treatment for an infection.
When should I get a blood culture test?
Blood culture testing is generally ordered if your doctor believes that you may have an infection in your blood.
Your doctor may suspect that you have an infection in your blood if you have symptoms of sepsis. Sepsis is an inflammatory reaction that can occur if dangerous bacteria or other germs are in your bloodstream. Sepsis can become severe and life-threatening. Symptoms of sepsis that may lead your doctor to suggest a blood culture test include:
- Fast heartbeat
- Rapid breathing
- Low blood pressure
- Gastrointestinal problems
A blood culture test may also be recommended if you have an infection that is often caused by germs in the blood. Examples of these infections include:
- Infections of the heart valves
- Infections affecting the bones
- Meningitis, which is an infection in the tissue that surrounds the brain and spinal cord
- Infections around implantable medical devices like catheters, artificial joints, or pacemakers
Blood culture testing may also be recommended if you have an infection and are at a higher risk of developing more serious complications. In addition, with some types of infectious conditions, such as severe pneumonia, blood culture tests may be done alongside culture tests that use other body fluid or tissue.
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.