What is being tested?

Blood cultures are done to detect and identify bacteria and yeasts (a type of fungus) in the blood. Some bacteria prefer oxygen (aerobes), while others thrive in a reduced oxygen environment (anaerobes). Blood cultures are usually drawn into two types of media to detect both types of bacteria. If your blood culture is positive, the specific bacteria causing the infection will be identified and antibiotic susceptibility testing will be done to tell your doctor which antibiotics will be effective for treatment. If yeasts are causing the infection, treatment will be given that is appropriate for fungal infections.

Infections of the bloodstream are caused most commonly by bacteria (bacteraemia), but can also be caused by a fungus (fungaemia) or a virus (viraemia). If your immune defences and white blood cells cannot contain an infection at its source, for example, in the bladder for a urinary tract infection, the infection may spread to your bloodstream and be carried throughout your body. Endocarditis, an inflammation and infection of the lining of the heart and/or of the heart valves, can result from a bloodstream infection.

People who have artificial heart valves or artificial joints have a higher risk of infection following their surgery, although these infections are not common. The direct contamination of the blood from ‘dirty needles’ (with intravenous drug use) or potentially from intravenous catheters or surgical drains can lead to bloodstream infections. Similarly, anyone with a compromised immune system due to underlying disease (for example, leukaemia) or drug therapy (for example, immunosuppressive agents) has a higher incidence of bloodstream infections.

How is it used?

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 choose the best treatment for you.

Other related tests that may be performed include:

• Gram stain- a relatively quick test used to detect and identify the general type of bacteria present in other body sites, such as urine or sputum. A direct gram stain of blood is too insensitive to detect bacteria in the bloodstream.
• Susceptibility testing- determines the drug (antimicrobial) that may be most effective in treating the infection.

Often, a full blood count (FBC) is ordered along with or prior to the blood culture to determine whether the person has an increased number of white blood cells (or in some cases, a decreased number of white blood cells), indicating a potential infection. Sometimes other testing is also performed, such as electrolytes and liver function tests 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 requested?

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.

What does the result mean?

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.

Is there anything else I should know?

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).

Common questions

• Why do I have to take antibiotics for so long if I feel better already?

It is important to eliminate all of the bacteria that are causing the problem. For some infections, several weeks of treatment are necessary. This is especially important if you have endocarditis, which requires long-term therapy to cure.

More information

RCPA Manual: Blood culture