A blood culture is performed to isolate and aid in the identification of the pathogens in bacteremia (bacterial invasion of the bloodstream) and septicemia (systemic spread of such infection). It requires inoculating a culture medium with a blood sample and in cubating it. Blood cultures can identify about 67% of pathogens within 24 hours and up to 90% within 72 hours.
Bacteremia may be transient, intermittent, or continuous. The timing of sample collection for blood cultures varies; it usually depends on the suspected type of bacteremia (intermittent or continuous) and on whether drug therapy needs to be started regardless of test results.
• Previous or current antimicrobial therapy (possible false negative)
• Removal of culture bottle caps at the bedside (possible prevention of anaerobic growth)
• Use of the incorrect bottle and media (possible prevention of aerobic growth)
• Avoid drawing blood from an existing IV catheter.
• Use a vein below an IV catheter or in the opposite arm.
Before the Test
• Confirm the patient’s identity using two patient identifiers according to facility policy.
• Explain to the patient that the blood culture procedure is used to help identify the organism causing his symptoms.
• Inform the patient that he doesn’t need to restrict food and fluids.
• Advise the patient that he may experience slight discomfort from the tourniquet and needle punctures.
During the Test
• Put on gloves.
• Clean the venipuncture site with an alcohol swab and then with an iodine swab, working in a circular motion from the site outward.
• Wait at least 1 minute for the patient’s skin to dry, and remove the residual iodine with an alcohol swab or remove the iodine after venipuncture.
• Apply the tourniquet.
• Perform a venipuncture; draw 10 to 20 mL of blood for an adult.
• Clean the diaphragm tops of the culture bottles with alcohol or iodine (or other antiseptic agent per facility
policy), and change the needle on the syringe.
• If broth is used, add blood to each bottle until a 1:5 or 1:10 dilution is obtained. For example, add 10 mL of
blood to a 100-mL bottle. (The size of the bottle varies, depending on facility procedure.)
• If a special resin is used, add blood to the resin in the bottles and invert them gently to mix.
• If the lysis–centrifugation technique (Isolator) is used, draw the blood directly into a special collection and
• Indicate the tentative diagnosis on the laboratory request as well as current or recent antimicrobial therapy.
• Send each sample to the laboratory immediately after collection.
After the Test
• Apply direct pressure to the venipuncture site until bleeding stops.
• Assess the venipuncture site for hematoma formation; if one develops, apply direct pressure.
• Prepare to initiate antimicrobial therapy, as ordered.
Negative for pathogens
Elevated Levels (Positive Cultures)
• Mild, transient bacteremia infections
• Infections due to Streptococcus pneumoniae and other Streptococcus species, Haemophilus influenzae, Staphylococcus aureus, Pseudomonas aeruginosa, Bacteroides, Brucella, Enterobacteriaceae, coliform bacilli, and Candida albicans
• Staphylococcus epidermidis, diphtheroids, and Propionibacterium (in immunocompromised patients)
• Mycobacterium tuberculosis and M. avium complex (in patients with human immunodeficiency virus infection)
• Institute appropriate infection control precautions as indicated by the causative organism.
• Adhere to standard precautions at all times.
• Expect to collect samples over the course of 2 consecutive days
Brunner & Suddarth’s (2010). Handbook of laboratory and Diagnostic Test. New York: Lippincott Williams & Wilkins