Type of test : Blood

 Normal findings

 Negative

 Test explanation and related physiology

Blood cultures are obtained to detect the presence of bacteria in the blood (bacteremia). An episode of bacteremia is usually accompanied by chills and fever; thus the blood culture should be drawn when the patient manifests these signs to increase the chances of growing bacteria on the cultures. It is important that at least two culture specimens be obtained from two different sites. If one produces bacteria and the other does not, it is safe to assume that the bacteria in the first culture may be a contaminant and not the infecting agent. When both cultures grow the infecting agent, bacteremia exists and is caused by the organism that is growing in the culture.

If the patient is receiving antibiotics during the time that the cultures are drawn, the laboratory should be notified. Resin may be added to the culture medium to negate the antibiotic effect in inhibiting growth of the offending bacteria in the culture. If cultures are to be performed while the patient is on antibiotics, the blood culture specimen should be taken shortly before the next dose of the antibiotic is administered. All cultures preferably should be performed before antibiotic therapy is initiated.

Culture specimens drawn through an IV catheter are frequently contaminated, and tests using them should not be performed unless catheter sepsis is suspected. Most organisms require approximately 24 hours to grow in the laboratory, and a preliminary report can be given at that time. Often, 48 to 72 hours is required for growth and identification of the organism. Anaerobic organisms may take longer to grow.

Interfering factors

• Contamination of the blood specimen, especially by skin bacteria, may occur.

Procedure and patient care

Before

* Explain the procedure to the patient.

* Tell the patient that no fasting is required.

During

 • Carefully prepare the venipuncture site with povidone-iodine (Betadine). Allow the skin to dry.

• Clean the tops of the Vacutainer tubes or culture bottles with povidone-iodine and allow them to dry. Some laboratories suggest cleaning with 70% alcohol after cleaning with povidone-iodine and air drying.

• Collect approximately 10 to 15 mL of venous blood by venipuncture from each site in a 20-mL syringe.

• Discard the needle on the syringe and replace it with a second sterile needle before injecting the blood sample into the culture bottle.

• Inoculate the anaerobic bottle first if both anaerobic and aerobic cultures are needed.

 • Mix gently after inoculation.

• Label the specimen with the patient’s name, date, time, and tentative diagnosis.

 • Indicate on the laboratory slip any medications that may affect test results.

After

• Transport the culture bottles immediately to the laboratory (or at least within 30 minutes).

• Notify the physician of any positive results so that appropriate antibiotic therapy can be initiated.

Abnormal findings

 Bacteremia