Blood Culture

Sh Nilica Devi *

Workshop on Voluntary Blood Donation and Blood Donation Camp on 31 October 2012 :: Pix - Banti Phurailatpam

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. Testing is done to identify a blood infection (septicaemia) that can lead to sepsis, a serious life-threatening complication. Individuals with a suspected blood infection are often treated in intensive care units.

Often, a complete blood count (CBC) is done along with or prior to the blood culture to determine whether the person has an increased number of white cells, indicating a potential infection. Sometimes other tests are also performed to evaluate the health status of a person’s organs or to help identify the source of original infection.

WHEN IS IT DONE?

A doctor may ask for blood culture when a person has signs and symptoms of septicaemia, which indicates that bacteria, fungi, or their toxic by-products are causing harm in the body. A person with septicaemia may have:
o Chills, fever.
o Nausea.
o Rapid breathing, rapid heartbeat.
o Less frequent urination.
o Confusion.

As infection progresses, more severe symptoms may develop, such as:
o Inflammation throughout the body.
o The formation of many tiny blood clots in the smallest blood vessels.
o A dangerous drop in blood pressure.
o The failure of one or more organs.

The result of the test can enable the doctor determine which specific organism or bacteria is causing the problem and how best to combat it.

WHY IS IT DONE?

A blood culture is done to:
o Find a bacterial infection that has spread into the blood, such as meningitis, osteomyelitis, pneumonia, a kidney infection, or sepsis.
o Confirm a fungal infection, such as yeast in the blood.
o Check for endocarditis, which is an infection of the valves of the heart.
o Find the best antibiotics to kill the bacteria or fungi (sensitivity testing).
o Find the cause of an unexplained fever (pyrexia of unknown origin) or shock or a person becoming extremely ill.

HOW TO PREPARE FOR THE TEST?

There are no special preparations needed for this test. However, it is important to tell the health professional if one has taken antibiotics recently.

HOW IS THE SAMPLE COLLECTED FOR TESTING?

The site from where blood will be drawn is first cleaned thoroughly with an antiseptic. This is done to reduce the chance of an organism from the skin getting into (contaminating) the blood sample and causing a false-positive result.

Usually, two to three blood samples are collected over a period of time and from different veins to increase the likelihood of detecting bacteria or fungi if they are present in the blood. Multiple blood samples helps to differentiate true pathogens from skin bacteria.

Blood is obtained by inserting a needle into a vein in the arm. The phlebotomist will put the blood into a blood culture bottle containing broth to grow microorganisms. For infants and young children, the quantity of blood sample will be smaller and appropriate for their body size.

HOW IS THE TEST DONE?

Blood cultures are incubated for several days before being reported as negative. This is because some types of bacteria or fungi grow more slowly than others and may take longer to detect if initially present in low numbers.

When a blood culture is positive, the specific microorganism causing the infection is identified and susceptibility testing is performed to inform the health practitioner which antibiotics are most likely to be effective for the treatment.

In many laboratories, the blood culture testing process is automated with instruments continuously monitoring the samples for growth of bacteria or fungi. This allows for timely reporting of results and for the health practitioner to direct antimicrobial therapy to the specific microorganism present in the blood.

Because treatment must be given as soon as possible in cases of sepsis, broad-spectrum antimicrobials that are effective against several types of bacteria are usually given intravenously while waiting for blood culture results. A more targeted antibiotic therapy is undertaken once the microorganism causing the infection is identified.

INTERPRETING THE RESULTS

If the blood culture is positive, this means the person has a bacterial or yeast infection in the blood. An infection of this kind can be life-threatening if left untreated. Depending on the type of bacteria discovered in the blood, sensitivity or susceptibility testing is performed to determine which type of antibiotic will kill the bacteria.

Sometimes, an abnormal result can come up which may be due to contamination. This means bacteria may be found, but it came from the skin or from lab equipment, instead of the blood. This gives a false-positive result.