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
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
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
The results of the test can help your doctor determine which
specific organism or bacteria is causing the blood infection and how best to
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
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
- decreased urine
- 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
- 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
Blood culture for other conditions
A blood culture can also be used to detect conditions such
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
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.