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Blood culture

Blood culture test

A blood culture detects the presence of bacteria or fungi in the blood. It is done when you suspect that an infection may be present in the blood, such as sepsis.

A blood culture is a laboratory test used to detect any foreign bodies in the blood, such as bacteria, yeast, and other pathogens. The presence of these invaders in your bloodstream may indicate an infection, which may be a serious case.


What is the blood culture test?

Normally, the blood doesn’t have any pathogens. The blood culture is a test used to detect the presence of bacteria or fungi that may cause infection of the blood. The bacterial infection of the blood is called bacteremia. It may be a serious case because the blood may transfer the infection into other body parts. The blood infection may affect infants and adults; it may be a result of several factors, including:

  • Serious infections in other parts of the body, such as lungs, kidneys, heart valves, and gallbladder.
  • Certain diseases weaken the immune system, such as AIDS and cancer.
  • Some medications that affect the ability of your body to fight infections, such as chemotherapy and corticosteroids.
  • Bacteria and yeasts may also enter the bloodstream directly through intravenous drugs, catheters, or surgical drains.

This test uses a substance that allows the growth of germs. It will be added to a blood sample, then the certain type of germ will be detected using a microscope or specific chemical tests. The healthcare provider will often take two or three blood samples from different veins to avoid missing a bacteria or fungus and accurate results.

Other tests may be used in addition to this test to detect the exact right medicines to treat the infection; these tests are known as sensitivity testing.

When is it required?

Blood infection complications
Purpose of the test

blood cultures procedures are done to identify the infection and the cause. The bloodstream infections are usually caused by bacteria (bacteremia), but they also can result from viruses (viremia) and fungi (fungemia).

Septicemia is the term that refers to the infection in the blood, while sepsis is the severe response of the body that sometimes threatens life.

A healthcare provider may order the blood culture test if the patient has blood infection signs which refer to an infection of the blood with bacteria, fungi, or their toxic byproducts. These infection signs usually include:

  • Fever and chills.
  • Rapid breathing.
  • Palpitations and increased heart rate.
  • Headache.
  • Muscle pain.
  • Fatigue.

Without treatment, this infection may progress into the most serious stage that is known as sepsis or blood poisoning. Sepsis symptoms include the blood infection symptoms, damaged organs symptoms, and other signs, such as

  • Confusion and dizziness.
  • High decrease in body temperature.
  • Nausea and vomiting.
  • Less frequent urination.
  • Skin discoloration.

The complications of this infection also may cause:

  • Inflammation of different body parts.
  • Serious blood pressure drop.
  • Failure of organs.
  • Tiny blood clots form in the smallest blood vessels.

Symptoms of infections in newborns include:

  • Abnormal body temperature.
  • Breathing difficulty.
  • Abnormal heartbeat.
  • Diarrhea or low bowel movement.
  • Low levels of blood sugar.
  • Irritability.
  • Feeding problems.
  • Fever (body temperature more than 38.1°c).
  • Being very sleepy.
  • Pale skin.

Blood infection complications

this test is also used to diagnose some conditions, such as endocarditis, that occur when the bacteria in your blood affect the heart valves. It may be life-threatening.

If you have a fever lasting for more than 3 days, you must get your healthcare provider’s advice. If infants younger than three months have a fever, they also must be seen by a doctor.

Blood infections risk factors.

Some people are at higher risk for septicemia, especially those who have:

  • certain diseases, such as 
    • Diabetes.
    • HIV and AIDS.
    • An autoimmune disorder.
    • Cancer.
  • Recent surgeries.
  • Immunosuppressive therapy.
  • A prosthetic heart valve replacement or prosthetic joints.
  • Recent serious infection.

A blood cultures test is also used in newborns and infants who have a fever for no reason, even in absence of other typical symptoms of infections in the blood. However, adults are at higher risk for this infection than infants.

You have to ask your healthcare provider for advice if you are at high risk for blood infections or any symptoms. 

How to prepare for the test?

Before the blood cultures procedures, tell your doctor about any medications you use, including prescription drugs or nutritional supplements. Your healthcare provider may tell you to stop some medications that may interfere with the test results.

If you are wary of the needle insertion, talk to your doctor about how to decrease your anxiety. There is no special preparation required for the cultures test.

Tell your doctor if you took any antibiotics recently.

How is a blood culture test performed?

Blood culture procedure
How the test is performed?

A blood cultures test is used to detect the type of infection that affects the blood and that may lead to complications. It also detects the correct treatment. Patients with a suspected blood infection often need to be treated in the hospital, so the test is usually performed in the hospital or emergency department.

The test procedures

  • The healthcare specialist will clean your skin with an antiseptic, such as chlorhexidine, to avoid sample contamination with microorganisms on the skin.
  • An elastic band will be tied around your arm, to make it more visible and increase the blood in it.
  • needles are used to draw several blood samples.
  • Samples are often taken from different body sites, they may also be collected at two different times, a few hours apart.
  • In the laboratory, your sample will be mixed with a special material called culture.
  • It will allow the growth of bacteria or fungi if they are present in the blood sample.
  • Early results may be available within 24 hours of your test.
  • You have to wait from 48 to 72 hours to detect the type of bacteria or fungi that cause the blood infection.
  • Other tests may be needed.
Some people need chemotherapy or nutrition supplements for weeks or months, so they have long-term catheters inserted in a major vein. For those people, the samples will be taken from the catheters.

Other related tests

Other related tests that may be performed along with this test include:

Gram stain

A relatively quick test that is used to detect the general type of bacteria in other body sites, such as sputum or urine. A direct gram stain of the blood is insensitive and can’t be used to detect bacteria in the bloodstream.

Susceptibility testing (sensitivity tests)

These types of tests help to detect the most effective drug (antimicrobial) for infection treatment.

Complete blood picture (CBC)

This test is usually done along with or before the blood cultures to detect the white blood cell count. Abnormal white blood cells count may refer to a potential infection.

Other tests may include

  • Cerebrospinal fluid culture.
  • Urine and sputum cultures.
  • A chemistry panel to check the health of the organ.

These tests will help to detect the original site of the infection. They are very helpful if the patient has symptoms related to urinary tract infections, meningitis, or pneumonia.

“Read more: Bilirubin Test


What does the result mean?

Sepsis
The test results

Most types of bacteria can appear in the culture within 2 to 3 days. However, some species may take up to 10 days or more to appear. The fungus may need 30 days to appear in the culture plate. Results of the test may include:

Two blood cultures positive or more

Two blood cultures positive or more for the same bacteria or fungi refer to an infection with this pathogen. The results usually indicate the type of bacteria or fungi causing the infection.

Blood infections are serious conditions, so they require immediate treatment. Most patients need to receive treatment in the hospital. Sepsis is a complication that may be life-threatening especially in people with immune disorders.

If your healthcare provider suspects a blood infection, he will not wait for the sensitivity test results. Instead, he will give you an intravenous broad-spectrum antibiotic that is effective against a wide range of bacteria while waiting for the blood cultures or the susceptibility test results.

After that the treatment may be changed to a more specific antimicrobial agent for the bacteria or fungi that are detected in the blood, depending on the test results when they become available. 

One sample is positive and one set is negative

This result means that there is an infection or skin contaminant in the samples. Diagnosis will depend on the patient’s clinical state and the type of germs that are found in the sample. Additional tests also will be ordered.

All sets are negative after several days

No germs growth in the culture means that the probability of infection is low. However, if the symptoms persist, such as fever for no reason, and don’t go away, additional tests will be required.

Reasons for a negative result, in presence of infection symptoms:

  • Certain microbes are more difficult to be detected in the culture, additional blood cultures may be done to allow its growth.
  • Routine blood culture media can not allow viruses growth, so viruses will not be detected in cultures designed to grow bacteria.
If your doctor suspects a viral infection (viremia), other tests will be ordered. These tests depend on the patient’s symptoms and the type of virus suspected to cause the infection.

Other tests results

Some tests are usually performed along with blood cultures. The results of these tests may refer to sepsis even if the culture result is negative.

  • Complete blood count (CBC)

An increase in white blood cells (WBCs) number (or in some cases low WBCs count) may indicate an infection.

  • A sputum or urine culture

Positive results may help to identify the possible source of infection that has spread into the blood.

  • A Cerebrospinal fluid culture

This test also may indicate the source of infection.

  • Procalcitonin

This test may be an indication of sepsis.

  • Complement blood test

Measuring the amount and the activity of complement proteins in the blood may help in blood infection diagnosis. Elevated levels of C3 may indicate infection.


Blood cultures risks

This test has a few risks related to drawing the blood sample. Blood vessels vary in size from one to another and according to their site in the body, so taking a blood sample may be more difficult in some people rather than others.

Other risks may include:

  • Accumulation of blood under the skin (hematoma).
  • Infections.
  • Excessive bleeding in certain cases.
  • Formation of a small bruise at the site of needle entry.
  • Dizziness and lightheadedness.
  • Multiple punctures to get the vein.

Is there anything else I should know about the test?

Blood infections or sepsis mean that the bacteria or fungi spread throughout the whole body, so this will cause many different symptoms. Your immune system will work hard to fight the foreign bodies. It will produce many factors to get rid of these bacteria or fungi. These factors will also affect your health and cause illness.

Septicemia may decrease the blood flow to the heart, kidneys, and brain. It also may cause a shock (blood pressure drop) and rapid heart rate, in addition to affecting the clotting factors Causing disseminated intravascular coagulation (DIC) that may cause bleeding. Bacteria in the blood may also transfer into the joints resulting in septic arthritis.

Recent innovations in blood cultures test interest in the development of new testing methods that can quickly detect the type of microbes found in the blood when the culture result is positive.

Some rapid tests are available to detect certain types of bacteria that usually cause infections in the blood. These tests may detect methicillin-resistant Staphylococcus aureus (MRSA), which can not be treated easily, and gram-negative rods such as E.coli that live in the gastrointestinal tract. Rapid identification of the microbe facilitates the treatment using the proper antibiotics.

Medical researchers are also trying to make progress and develop tests to speed up the diagnosis, by:

  • Common pathogens detection directly from the blood Instead of or in addition to culture test.
  • Perform susceptibility tests rapidly to determine the appropriate antibiotic for treatment.
  • Distinguish between infection-positive inflammation and infection-negative inflammation to rule out sepsis.

When will the test results be available?

The blood cultures results may take a few days because the bacteria or fungi must grow in the blood culture media for sufficient numbers to be detected. Mostly, this will take two days but some microbes will need a longer time to grow.

In addition to that, some microbes may be present in the blood in very small numbers, so they will need more time to grow and reproduce quantities to be detected.


Why additional blood cultures may be needed?

Your healthcare provider may order more cultures after drawing the first cultures to determine if the pathogens that grow in the culture are persistent in the bloodstream and really cause the infection or the blood sample is contaminated from the skin pathogens.

If the germs grow in the first cultures and don’t arise in the follow-up cultures, this means that the first sample was only contaminated with the skin bacteria.

Additional blood cultures are also required if the first culture results are negative, but the patient still has symptoms of infections.

Sepsis treatment

Sepsis treatment starts immediately without waiting for the culture’s results or the sensitivity tests because it is a very serious condition. Treatment mostly begins in the emergency room (ER) and then it may be continued and monitored in an intensive care unit (ICU).

Intravenous broad-spectrum antibiotics will be used first then the treatment may be changed into more targeted antibiotics after the susceptibility test results are available (that detect the exact type of microbes causing the infection).

IV fluids are also used to improve blood pressure and maintain it within the normal levels. In some cases, medications that constrict the blood vessels may be given.


What infections can be detected by the blood cultures?

The test is performed to detect the bacterial infections that have spread into the blood, including meningitis, pneumonia, kidney infections, osteomyelitis, or sepsis. The culture can also show the type of bacteria that causes the infection and detect fungal infections such as yeasts. Viruses can not be detected by blood cultures bottles designed for bacterial growth.


Which is the best method for taking the sample of the test?

Samples for this test are typically taken through venipuncture. Taking the sample using an intravenous line is not preferred, because this is associated with a higher risk of contamination. However, samples may be collected from either venipuncture or through an intravenous line to detect catheter-associated infections.


Types of blood culture media

There are different media, classified into 6 types, including:

  • Basal media.
  • Enriched media.
  • Indicator media.
  • Selective media.
  • Transport media.
  • Storage media.

Generally, to detect the pathogen causing the infection, the blood sample will be subcultured on a solid medium. McConkey’s and blood agar are the most common media that can be used.


Blood cultures are the procedures used to diagnose blood infection and detect the pathogens that cause septicemia. Multiple blood samples are usually collected from different body sites to have accurate results. After that, a sensitivity test will help your healthcare provider identify the most targeted antibiotic that can kill the germs and treat the infection.

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