A blood culture is a test to find an infection in the blood. The blood does not normally have any bacteria or fungi in it. A blood culture can show what bacteria or fungi are in the blood.
A bacterial infection in the blood, called bacteremia, can be serious because the blood can spread the bacteria to any part of the body. A blood infection most often occurs with other serious infections, such as those affecting the lungs, kidneys, bowel, gallbladder camera.gif, or heart valves.
A blood infection may also develop when the immune system is weak. This can occur in infants and older adults, and from disease (such as cancer or AIDS) or from medicines (such as corticosteroids or chemotherapy) that change how well your body can fight infections (immunity).
To test for an infection in the blood, a sample of blood is collected and placed in a cup with special substances that allow the bacteria or fungus to grow. The type of bacteria or fungus that grows is checked with chemical tests and by looking at the culture under a microscope. Two or three blood samples from different veins are often taken to make sure a bacteria or fungus is not missed. If no bacteria or fungus grows, the blood culture is called negative. A blood culture is often done when a person has a fever because this is the time when the bacteria or fungus is most likely to have spread to the blood.
HOW TO TAKE BLOOD CULTURES?
Problems with blood cultures:
- Blood culture contamination is a common and preventable problem, especially in the emergency department.
- False positive blood cultures due to specimen contamination with skin bacteria are a common problem that can lead to unnecessary antibiotic use, additional laboratory tests and increased length of hospital stay, therefore leading to significant extra hospital costs.
- True and false positive rates for blood cultures vary but are commonly each in the range 5-10%.
- The use of phlebotomy teams has been shown to be effective for reducing blood culture contamination rates.
- Changing the method of blood culture collection to a more sterile process has been shown to result in significant reductions in blood culture contamination.
TYPES OF CULTURE BOTTLES
- There are a various different types of blood culture systems, both manual and automated.
- Other culture bottles are available and their use depends upon the clinical scenario – eg, culture bottles for tuberculosis or fungi.
- New technologies are being introduced which can lead to the rapid identification of pathogens in positive blood cultures.
- Several molecular platforms can identify bacteria associated with bloodstream infections much more quickly than standard methods.
BLOOD CULTURE PROCESSING
- Once blood cultures are taken they should be labelled and sent to the laboratory without delay.
- In the laboratory the bottles are agitated and incubated at body temperature. If it is out of hours then cultures are usually incubated overnight.
- Basic sets of cultures are incubated for 14 days and blind culture performed after 3, 7 and 14 days or as soon as there are signs of growth (eg, turbidity, haemolysis, colonies on agar slope in Castaneda’s bottles).
- Other bottles may be incubated for 7 days or up to 3 weeks if subacute bacterial endocarditis (SBE)/fastidious organisms are suspected.
- Observation of bottles, looking for a positive result, is performed at least twice a day when using manual systems.
- Automatic systems are available and are being increasingly used. Blood Culture System where a positive growth releases CO2 which is detected by a sensor that alerts the laboratory staff (bottles are placed in a special cabinet and linked to the patient by a barcode).
- The automatic blood culture system bottles containing antibiotic-binding polymeric beads have been shown to improve the diagnosis of bacteraemia.
- If growth is detected, the bottles are subcultured and a Gram stain performed. From this, relevant sensitivities are performed. In some cases the organism can be identified within hours of detecting a positive result, using Gram stain and further tests.
- Further tests that may be performed directly on the blood culture to hasten identification include streptococcal grouping, coagulase testing, antigen tests for pneumococcus and meningococcus, etc.
- The microbiologist will then review the results and inform the staff involved in the patient’s care. Thus, patients will begin provisional therapy and this will be confirmed once sensitivities are known.
- Usually, if there is to be a growth, there is generally only one organism; however, very rarely, there may be more than one organism and the laboratory will perform sensitivities and further tests on all.
WHY A BLOOD CULTURE IS DONE
Blood cultures are ordered when a doctor suspects you may have a blood infection. Some blood infections can be mild, but one such serious consequence of a blood infection is sepsis. In this condition, the organisms release toxins that trigger the immune system to attack the body.
Symptoms of sepsis include:
- decreased urine
- rapid breathing
- rapid heartbeat
The results of the test can help your doctor determine which specific organism or bacteria is causing the problem and how best to combat it.
THE RISKS OF A BLOOD CULTURE
The only risk of the test is when you give blood; however, blood draws are routine procedures and rarely cause any serious side effects.
The risks of giving a blood sample include:
- bleeding under the skin (hematoma)
- excessive bleeding