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Blood Culture
A blood culture is a test that your doctor will order if you're showing signs of a blood infection. Learn how to prepare for this test and what...

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Blood culture

A blood culture is a test that checks for foreign invaders like bacteria, yeast, and other microorganisms in your blood. Having these pathogens in your bloodstream can be a sign of a blood infection, a condition known as bacteremia. A positive blood culture means that you have bacteria in your blood.

This type of infection involves the blood that circulates within your entire body. Bacteria that start on your skin or in your lungs, urine, or gastrointestinal tract are common sources of blood infections.

An infection can spread to your blood and become systemic if it’s severe or if your immune system isn’t able to keep it contained. A systemic infection is known as sepsis.

The test for a blood culture involves a simple blood draw. A laboratory tests the blood sample and forwards the results to your doctor, who will use the findings to help determine what’s needed to treat any infection.

Purpose of a blood culture

Blood cultures are ordered when your doctor suspects you may have a blood infection. It’s important to test for blood infections because they can lead to serious complications. One such complication of a blood infection is sepsis.

In sepsis, the pathogens that are causing the infection in your bloodstream interfere with your body’s normal defenses and prevent your immune system from working properly. The pathogens also produce toxins that can damage your organs.

The results of the test can help your doctor determine which specific organism or bacteria is causing the blood infection and how best to combat it.

Symptoms of blood infection and sepsis

You should call 911 or visit a doctor immediately if you’re experiencing any symptoms of a blood infection. These include:

  • shaking chills
  • moderate or high fever
  • rapid breathing
  • increased heart rate or palpitations
  • excessive fatigue
  • muscle aches
  • headache

Without treatment, a blood infection can progress to its most severe stage, sepsis. The symptoms of sepsis include those listed above, as well as signs of damaged organs. The following are additional symptoms of sepsis:

  • confusion
  • decreased urine
  • dizziness
  • nausea
  • mottled skin

As the infection progresses, more serious complications of sepsis may develop. These can include:

  • inflammation throughout your body
  • formation of many tiny blood clots in your smallest blood vessels
  • a dangerous drop in blood pressure
  • failure of one of more organs

Blood infection risk factors

Blood cultures are done more frequently for those who are at a higher risk of developing a blood infection. You’re at a higher risk if you’ve been diagnosed with:

The following situations also put you at risk for blood infection:

  • You’ve recently had an infection.
  • You’ve recently had a surgical procedure.
  • You’ve had a prosthetic heart valve replacement.
  • You’re undergoing immunosuppressive therapy.

Blood cultures are also drawn more frequently in newborns and children with fever who may have an infection but don’t have the typical signs and symptoms of sepsis. Older adults are also at higher risk for blood infections.

Blood culture for other conditions

A blood culture can also be used to detect conditions such as endocarditis. Endocarditis is a condition that occurs when bacteria in your bloodstream sticks to your heart valves. It can be life-threatening.

Potential risks of a blood culture

Complications you may experience from this test only occur 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 your skin, or hematoma
  • excessive bleeding
  • fainting
  • infection

How to prepare for a blood culture

Tell your doctor what kinds of medication you’re taking, including prescriptions and nutritional supplements. They may ask you to stop taking certain medications that may affect the blood culture results.

If you’re wary of needles, talk to your doctor or your nurse to discuss ways to ease your anxiety.

How a blood culture is performed

The blood draw may be performed in a hospital, emergency department, or specialized testing facility. Blood cultures are rarely done in an outpatient setting.

To start, your skin is cleaned to prevent any microorganisms on your skin from contaminating the test. Your nurse or technician then usually wraps a cuff or an elastic band around your arm to allow your veins to fill with blood and become more visible. They next use one needle to draw several samples of blood from your arm.

Multiple blood samples are generally collected from different veins to help increase the chance of detecting the bacteria or fungi in your bloodstream. If you’re an adult, your doctor or healthcare team usually collects two to three blood samples, often drawn on different visits.

After the draw, your nurse or technician covers the puncture site with some gauze and a bandage. The blood sample is then submitted to a laboratory where it’s cultured: Each blood sample is added to a bottle containing a liquid known as broth. The broth encourages any microorganisms present in the blood sample to grow.

Interpreting the results

If the blood culture is positive, this means you have a bacterial or yeast infection in your blood. The results usually help your doctor identify the specific bacteria or fungi that’s causing the infection.

Depending on the type of organism discovered in your blood, your doctor will perform another test called a sensitivity or susceptibility test. This helps determine which specific medication will work best against that organism. It’s standard practice to run a sensitivity test as a follow-up to a positive blood culture test. It can also be done when an infection isn’t responding to treatment.

After a blood culture

If your doctor suspects that you have a blood infection, they may begin treatment right away via intravenous broad-spectrum antibiotics. This medication can start fighting a wide range of bacteria while you’re waiting for the blood culture or susceptibility testing results.

Blood infections require immediate treatment, usually in a hospital. If sepsis develops, it can be life threatening, especially if you have a weakened immune system. If you have sepsis, you’ll be hospitalized so you can be completely treated.

Blood infections can lead to serious complications, so talk to your doctor if you’re at risk or if you’re showing any symptoms. Any fever lasting longer than three days should always be evaluated by a doctor or other healthcare provider. If an infant younger than 3 months has a fever, they should be seen by a doctor immediately.

Written by: Brian Krans and Brian Wu
Edited by:
Medically Reviewed by:
Published: Aug 7, 2012
Published By: Healthline Networks, Inc.
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