Blood Culture and Sensitivity

Test Description

A blood culture is performed to detect infection in the blood. Such infections may be
bacterial in origin (as with meningitis, osteomyelitis, or sepsis) or fungal (yeast).
Bacteria may enter the bloodstream in a variety of ways, including invasion by bacteria
through the lymphatic system from infections of the kidneys, bowel, or gallbladder;
through indwelling venous or arterial catheters; or from bacterial endocarditis
associated with prosthetic heart valves. When bacteria enter the bloodstream, chills
and fever may occur. It is usually at this point that a blood culture is performed to confirm
the presence of bacteria in the blood, a condition called bacteremia. Bacteria
tested may include aerobic organisms such as Staphylococcus pneumoniae, or anaerobic,
such as Clostridium difficile.
Once the blood sample is obtained, the blood is cultured, during which time the
organisms are allowed to grow in special culture media. In 48 to 72 hours, the organism
is usually identified, although fungal cultures can take up to 30 days for final
results to be reported. If bacteria are found in the culture, additional sensitivity testing
is done to determine the antibiotic that is most effective at killing the bacteria.
The culture is considered negative for bacteria if no growth is observed after one week

THE EVIDENCE FOR PRACTICE

Initial empirical anti-infective therapy should include one or more drugs that have activity
against the likely pathogens (bacterial or fungal). Other factors to consider are the location
of the presumed source of the infection and the patterns of antibiotic resistance in the institution
and community. Once a causative pathogen is identified, there is no evidence that
combination therapy is more effective than monotherapy. However, treatment should be
guided by clinical response.
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Normal Values

Negative culture (no bacteria or fungus are found)
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Possible Meanings of Abnormal Values

Positive:
Bacteremia
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Contributing Factors to Abnormal Values

• False negatives or delayed growth may occur if antimicrobials are given prior to the
 specimen collection.
• Contamination of the specimen, whether from inadequate skin preparation or from
drawing the blood from an intravenous line.
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Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be
 drawn.
• Some institutions require two samples to be drawn, each from a different site. This provides
a verification of whether positive cultures are the result of contamination during
sample collection or the actual presence of bacteria. Contamination will generally be cultured
in one bottle, whereas pathogens will be found in both bottles.
• Sometimes two sets of blood cultures will be drawn 45 to 60 minutes apart. Additional
sets may be drawn in 24 to 48 hours.
• No fasting is required for the test.
• Whenever possible, blood cultures should be obtained prior to initiating antimicrobial therapy.

Procedure

• The venipuncture site is cleansed, starting at the center and moving outward in a circular
pattern. Most institutions cleanse the site with povidone-iodine (Betadine) if the
patient has no allergy to this. The skin is allowed to air dry.
• The stoppers of the culture bottles are cleansed with Betadine, allowed to air dry, and
then cleansed with 70% alcohol.
• A 10- to 20-mL blood sample is collected into a 20-mL syringe from each site. Institution
policy should be followed for transfer of the blood from the syringe to the culture bottle.
(Some institutions require a change of needle before the transfer; others use the original
needle for the transfer to prevent an accidental needle stick.)
• If two culture bottles are to be inoculated, one for anaerobic and one for aerobic organisms,
the anaerobic bottle is inoculated first.
• Mix the bottle(s) gently.
• Gloves are worn throughout the procedure.

Posttest

• Apply pressure at venipuncture site. Cleanse the site with alcohol to remove Betadine.
Apply dressing, periodically assessing for continued bleeding.
• Label the specimens, noting whether the patient is currently taking an antimicrobial, and
transport to the laboratory.

Clinical alert

• Once the specimen is obtained, the patient may be given a broad-spectrum
antibiotic known to be effective against the suspected bacteria.
• The final culture and sensitivity report will note the type of organism and provide
a list of antibiotics to which the organism is susceptible or resistant.
• The antibiotic the patient may be taking may need to be changed based on the
sensitivity report.