Summary

• Blood cultures are done to detect and identify bacteria and yeasts (fungus) in the blood. 
• They are usually ordered if you have symptoms of septicaemia or sepsis. 
• This is when an infection has spread from the original site such as your lungs, bladder or skin, and entered your bloodstream to be carried throughout your body. 

Septicaemia and sepsis

Septicaemia is when an infection enters your bloodstream. Sepsis is a life-threatening medical emergency when you have a severe response to an infection.

In sepsis, your immune system, which normally protects you from infection, turns on your own body and damages your tissues and organs. This can lead to permanent tissue damage, organ failure and death.

If your immune defences and white blood cells cannot contain a local infection – such as a urinary tract infection in your bladder, or pneumonia in your lungs – the infection may spread to your bloodstream and be carried throughout your body.  Sepsis needs immediate diagnosis and treatment. Bacterial infections cause most cases of sepsis. Fungus and virus infections can also cause bloodstream infections, but this is less common.  

What happens in a blood culture

To do a blood culture, samples of your blood will be put into a substance called a culture medium (high nutrition liquid or solid preparation) that helps them grow. Different types of media contain different nutrients and are designed to grow different types of bacteria or fungi. Some bacteria need oxygen (aerobes), while others live a low oxygen environment (anaerobes). Blood cultures are usually put into two types of media to detect both types of bacteria.

If your blood culture is positive and bacteria or a fungus has been grown, the specific type will be identified, and antibiotic susceptibility testing done. This shows which antibiotic or antifungal agent will be the most effective treatment. 

Why get tested?

Your doctor may order blood cultures if you have symptoms of septicaemia or sepsis such as chills, fever, nausea, rapid heartbeat, confusion, decreased urine output or changes in the way your body normally functions.

People who have artificial heart valves or artificial joints have a high risk of infection after their surgery. Although these infections are not common, there is a chance of getting an infection from the needles, catheters or surgical drains and this can lead to bloodstream infections or sepsis.  Similarly, anyone with a compromised immune system due to an underlying health disorder or drug therapy, has a higher risk of bloodstream infections.

Further tests

Gram stain

A gram stain test is often performed because it is relatively quick to do and can detect the general type of bacteria present in other parts of your body, such as testing urine or sputum. A direct gram stain of blood cannot detect specific bacteria in the bloodstream which is why a blood culture is essential.

Susceptibility testing

Susceptibility testing is used to help decide which treatment will stop the growth of the microorganism causing your infection. A bacterial infection will be treated with an antibiotic and susceptibly testing will show which antibiotic will work best.  If your infection is being caused by a yeast (fungus) it will require antifungal treatment.

Full Blood Count (FBC)

This is often ordered along with or before the blood culture is done to find out whether you have increased white blood cells, or in some cases, fewer white blood cells than normal. This can be a sign you have a potential infection.

Sometimes other testing is performed. This includes electrolytes and liver function tests to check your general health. A urine, sputum or cerebrospinal fluid (CSF) culture may also be done to help show the source of the original infection, especially if you have symptoms suggesting a urinary tract infection, pneumonia, or meningitis.

Having the test

Sample

Multiple samples of blood will be taken from different sites on your body. This is done because it increases the possibility of detecting an infection if it is present. 

Taking two or more samples also reduces the risk of contamination from bacteria on the skin getting into your blood when it is being collected. Contamination by other bacteria can lead to a false positive result.

If you have two blood culture collections that return positive results with the same bacteria, it is more likely that the bacteria found in the cultures are causing your infection. If one set is positive and one set is negative, it could be either an infection or sample contamination. 

Blood from each sample will be added to two different culture bottles - one containing nutrients that support the growth of aerobic bacteria and the other containing nutrients that support the growth of anaerobic bacteria. These two bottles make up one blood culture set. 

Further blood cultures can sometimes be collected from different sites at between five minutes and one hour later. Collecting blood cultures from different sites at different times helps ensure accuracy and reduces the risk of contamination. It also provides a better chance of detecting bacteria or yeast (fungi) that might be present in low numbers.

Any preparation?

Antiseptic is applied to your skin before blood is taken to minimise the possible contamination of the sample by normal skin bacteria.

Your results

Positive result

If your blood culture is positive, the specific bacteria or fungi causing the infection will be identified and susceptibility testing will be done to show which antibiotics, or antifungal will be most effective for treatment.

If your medical team suspects you have septicaemia, they may start you on broad antibiotic treatment while waiting for the culture results. Broad antibiotic treatment is antibiotics that are effective for most common types of bacteria and is sometimes given intravenously (into your veins) so that it reaches the bacteria or fungi quicker. When susceptibility test results are available, they may modify your treatment accordingly.

For some infections, several weeks of treatment are needed. A bloodstream infection can cause endocarditis, an inflammation and infection of the lining of the heart and/or of the heart valves. This requires long-term therapy to cure.

Negative result

If the blood culture sets are both negative, it is unlikely that you have sepsis caused by bacteria or fungi. However, some microorganisms are more difficult to grow in culture and more testing may be required.

Your symptoms may be due to a virus which will not grow in routine blood culture media and will need to be diagnosed by other laboratory tests. It is important to differentiate between bacterial and viral infections because although they both need to be treated promptly, the treatments are different.

Any more to know?

Blood cultures are taken more frequently in newborns who may have an infection but may not have the typical signs and symptoms of sepsis. 

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.