Blood Culture

A blood culture is performed to isolate and aid 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 incubating it.

Purpose

• To confirm bacteremia
• To identify the causative organism in bacteremia and septicemia

Patient preparation

• Explain to the patient that this procedure is used to help identify the organism causing his symptoms.
• Inform him that he needn't restrict food or fluids before the test.
• Tell him how many samples the test will require and who will perform the venipunctures and when.
• Inform him that he may experience transient discomfort from the needle punctures and the tourniquet. Reassure him that collecting each sample usually takes less than 5 minutes.

Equipment

Gloves; tourniquet; small adhesive bandages; alcohol swabs; povidoneiodine swabs; 10- to 20-ml syringe for an adult, 6-ml syringe for a child; three or four sterile needles; two blood culture bottles, one vented (aerobic) and one unvented (anaerobic), with nutritionally enriched broths and sodium polyethanol sulfonate added, or bottles with resin or a lysis-centrifugation tube

Procedure and posttest care

• 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 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, one syringe of 2 to 6 mI for a child, or 0.1 to 1 ml for neonates, infants, and small children.
• Clean the diaphragm tops of the culture bottles with alcohol or iodine, 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, such as Bactec resin medium or Antimicrobial Removal Device, add blood to the resin in the bottles and invert them gently to mix.
• If you're using the lysis-centrifugation technique (Isolator), draw the blood directly into a special collection and processing tube.
• Indicate the tentative diagnosis on the laboratory slip, and note any current or recent antimicrobial therapy.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Wear gloves when performing the procedure and handling specimens.
• Send each specimen to the laboratory immediately after collection.
• Don't draw blood from an existing I.V. catheter. Use a vein below an I.V. catheter or in the opposite arm.

Normal findings

Normally, blood cultures are negative for pathogens.

Abnormal findings

Positive blood cultures don't necessarily confirm pathologic septicemia. Mild, transient bacteremia may occur during the course of many infectious diseases or may complicate other disorders. Persistent, continuous, or recurrent bacteremia reliably confirms the presence of serious infection. To detect most causative agents, blood cultures are ideally drawn on 2 consecutive days.

Isolation of most organisms takes about 72 hours; negative cultures are held for 1 or more weeks before being reported negative.

Common blood pathogens include Streptococcus pneumoniae and other Streptococcus species, Haemophilus injluenzae, Staphylococcus aureus, Pseudomonas aeruginosa, Bacteroidaceae, Brucella, Enterobacteriaceae, coliform bacilli, and Candida albicans. Although 2% to 3% of cultured blood samples are contaminated by skin bacteria, such as Staphylococcus epidermidis, diphtheroids, and Propionibacterium, these organisms may be clinically significant when isolated from multiple cultures or trom immunocompromised patients.

Interfering factors

• Failure to report recent or current antimicrobial therapy on the laboratory slip (possible false-negative)
• Failure to use proper collection technique
• Removal of culture bottle caps at the bedside (possible prevention of anaerobic growth)
• Use of incorrect bottle and media (possible prevention of aerobic growth)