Blood cultures are done to detect the presence of bacteria or yeasts, which may have spread from a specific site in the body into the bloodstream. For example, if a person has bacterial pneumonia or bacterial meningitis, the causative organism may be recovered from the blood and the culture results will help your physician understand why you are not feeling well and how to treat you.

Your doctor may order blood cultures when you are having symptoms of septicemia or sepsis, which indicates that bacteria or their products are causing harm in your body. You may have chills, fever, nausea, rapid heartbeat, confusion, decreased urine output, and changes in other organ systems. If you have had a recent infection, surgical procedure, prosthetic heart valve replacement, or immunosuppressive therapy, you are at higher risk of a systemic infection and drawing blood cultures would be appropriate if you have these symptoms. Blood cultures are drawn more frequently in newborns who may have an infection but may not have the typical signs and symptoms of sepsis. Similarly, blood cultures are collected in young children to detect serious infections.

If your blood culture is positive, it may mean that you have a bacterial or fungal infection in your bloodstream that needs to be treated immediately. Septicemia can be life- threatening, especially in immunocompromised patients. Your doctor may have started you on a broad spectrum antibiotic, often intravenous, that would be effective against a wide range of bacteria while waiting for the test results. Your doctor may change your antibiotic to be specific for the bacteria grown in your blood culture once that information is available from the laboratory.

A positive result could also be a false positive caused by skin contamination. If you have two blood culture sets positive with the same bacteria, it is more likely that the bacteria found in the culture are causing your infection. If one set is positive and one set is negative, it could be an infection or contamination. Your doctor will need to evaluate your clinical status and the type of bacteria found.

If the blood culture sets are both negative, the probability that you have sepsis caused by bacteria or yeasts is low; however, some microorganisms are more difficult to grow in culture and more testing may be required. Your symptoms may be due to a virus that would not grow in routine blood culture media designed to grow bacteria. Other laboratory tests would be required to diagnose a viral infection. Your doctor will have to evaluate your individual case.

Because septicemia means the bacteria or yeasts have spread throughout the body, you may experience many different symptoms of illness. The immune system is struggling to overcome the infection and produces many factors to kill the bacteria that also make you feel sick. Septicemia can cause shock, a rapid heart rate, and can decrease the blood flow to your brain, heart, and kidneys as well as alter your blood clotting components, leading to disseminated intravascular coagulation (which can cause generalized bleeding). Bacteria in your blood may attach to your heart valves and cause damage and heart murmurs (endocarditis) or spread to your joints and cause septic arthritis.

Symptoms such as fever, chills, muscle pains, and exhaustion may also be seen with influenza (the flu). If you are ill during the flu season, your doctor may do an influenza test to rule out this viral respiratory infection. Both the flu and septicemia can be especially serious in the very young, elderly, and immunocompromised patient. It is important to differentiate between them because while they both need to be treated promptly, the treatments are different (antibacterial versus antiviral).