Blood cultures are done to detect the presence of bacteria or yeasts, which may have spread from another site in the body. 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 doctor understand how to treat you.

Your doctor may order blood cultures if you have symptoms of septicaemia or sepsis, which indicate 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 experience these symptoms following a recent infection, surgical procedure, artificial heart valve replacement, or immunosuppressive therapy, you are more likely to have a systemic infection and taking blood cultures would be appropriate. Blood cultures are taken 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. Septicaemia can be life- threatening, especially in immunocompromised patients. Your doctor may have started you on a broad spectrum antibiotic, often intravenous, while waiting for the test results and will adjust the medication depending on the antibiotic susceptibility results.

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 either 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 and would require other laboratory tests to diagnose. Your doctor will have to evaluate your individual case.

Because septicaemia 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. Septicaemia 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 generalised bleeding). Bacteria in your blood may attach to your heart valves and cause damage and heart murmurs (endocarditis).

Septicaemia 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 septicaemia can be especially serious in the very young, elderly, and immunocompromised patients. It is important to differentiate between septicaemia and viraemia because while they both need to be treated promptly, the treatments are different (antibacterial versus antiviral).