Blood cultures are used to detect the presence of bacteria or fungi in the blood, to identify the type present, and to guide treatment. Testing is used to identify a blood infection (septicemia) that can lead to sepsis, a serious and life-threatening complication. Individuals with a suspected blood infection are often treated in intensive care units, so testing is often done in a hospital setting.

Although blood samples may be used to detect viruses, this article focuses on the use of blood cultures to detect and identify bacteria and fungi.

Other related tests that may be performed include:

Gram stain—a relatively quick test used to detect and identify the general type of bacteria Susceptibility testing—determines the drug (antimicrobial) that may be most effective in treating the infection Often, a complete blood count (CBC) is ordered along with or prior to the blood culture to determine whether the person has an increased number of white blood cells, indicating a potential infection. Sometimes other testing is also performed, such as a chemistry panel to evaluate the health status of a person's organs, or a urine, sputum, or cerebrospinal fluid (CSF) culture to help identify the source of the original infection. This is especially true when a person has symptoms associated with a urinary tract infection, pneumonia, or meningitis.

When is it ordered? A health practitioner may order blood cultures when a person has signs and symptoms of sepsis, which indicates that bacteria, fungi, or their toxic by-products are causing harm in the body. A person with sepsis may have:

Chills, fever Nausea Rapid breathing, rapid heartbeat Confusion Less frequent urination As the infection progresses, more severe symptoms may develop, such as:

Inflammation throughout the body The formation of many tiny blood clots in the smallest blood vessels A dangerous drop in blood pressure The failure of one or more organs When a person has had a recent infection, surgical procedure, prosthetic heart valve replacement, or immunosuppressive therapy, the person is at a higher risk of a systemic infection and drawing blood cultures would be appropriate when symptoms are present. Blood cultures are drawn more frequently in newborns and young children, who may have an infection but may not have the typical signs and symptoms of sepsis.

What does the test result mean? Two or more blood cultures that are positive for the same bacteria or fungi means that the person tested likely has a blood infection with that microorganism. The results typically identify the specific bacteria or fungi causing the infection.

Blood infections are serious and need to be treated immediately, usually in a hospital. Sepsis is a complication that can be life-threatening, especially in people with weakened immune systems. Health practitioners who suspect sepsis may begin patients on intravenous broad spectrum antibiotics that are effective against a wide range of bacteria while waiting for the blood culture or susceptibility testing results. When results become available, the treatment may be changed to an antimicrobial agent that is more specific for the bacteria or fungi detected in the blood cultures.

If one blood culture set is positive and one set is negative, it may mean that an infection or skin contaminant is present. The health practitioner will consider the person's clinical status and the type of bacteria or fungi found before making a diagnosis. Also, additional testing may be warranted in this case.

Blood culture sets that are negative after several days (often reported as “no growth”) mean that the probability that a person has a blood infection caused by bacteria or fungi is low. If symptoms persist, however, such as a fever that does not go away, additional testing may be required. A few reasons that symptoms may not resolve even though blood culture results are negative may include:

Some microorganisms are more difficult to grow in culture, and additional blood cultures using special nutrient media may be done to try to grow and identify the pathogen. Viruses cannot be detected using blood culture bottles designed to grow bacteria. If the health practitioner suspects that a viral infection may be the cause of the person's symptoms, then other laboratory tests would need to be performed. The tests that would be ordered depend upon the person's clinical signs and the type of virus the health practitioner suspects is causing the infection. Results from other tests that may be done in conjunction with blood cultures may indicate sepsis even though blood cultures may be negative. Some of these may include:

Complete blood count (CBC). An increased white blood cell (WBC) count may indicate infection. Complement. Levels of C3 may be increased. A urine or sputum culture may be positive, indicating a possible source of infection that may have spread to the blood. A CSF analysis may reveal a possible source of infection.

Is there anything else I should know? Because sepsis means that the bacteria or fungi have spread throughout the body, an affected person may experience many different symptoms of illness. As the immune system works to fight the infection, it produces many factors to kill the bacteria or fungi that can also make a person feel sick. Septicemia can cause a fall in blood pressure (shock), a rapid heart rate, and a decrease in blood flow to the brain, heart, and kidneys. It can also affect blood clotting factors, leading to disseminated intravascular coagulation (DIC), which can cause generalized bleeding. Bacteria in the blood may also spread to the joints and cause septic arthritis.

Rapid tests are available that can detect several different types of bacteria that are commonly known to cause infections of the blood. These tests are used in follow up to positive blood cultures to quickly identify the bacteria that are present. They can identify types such as methicillin-resistant Staphylcoccus aureus (MRSA), which is typically difficult to treat, and gram negative rods such as E. coli that live in the gastrointestinal tract. Rapid identification can facilitate treatment with appropriate antibiotics.