Wound Culture

Performed to confirm infection, a wound culture is a microscopic analysis of a specimen from a lesion. Wound cultures may be aerobic, for detection of organisms that usually appear in a superficial wound, or anaerobic, for organisms that need little or no oxygen and appear in areas of poor tissue perfusion, such as postoperative wounds, ulcers, and compound fractures. Indications for wound culture include fever as well as inflammation and drainage in damaged tissue.

Purpose

• To identify an infectious microbe in a wound

Patient preparation

• Explain to the patient that this test is used to identify infectious microbes.
• Describe the procedure, informing the patient that a drainage specimen from the wound is withdrawn by a syringe or removed on sterile cotton swabs.
• Tell him who will collect the specimen.

Equipment

Sterile cotton swabs and sterile culture tube or commercial sterile collection and transport system (for aerobic culture); sterile cotton swabs or sterile 10-ml syringe with 21g needle, and special culture tube containing carbon dioxide or nitrogen (for anaerobic culture); sterile gloves; alcohol sponges; sterile gauze; povidone-iodine solution.

Procedure and posttest care

• Put on gloves, prepare a sterile field, and clean the area around the wound with antiseptic solution.
• For an aerobic culture, express the wound and swab as much exudate as possible, or insert the swab deeply into the wound and gently rotate. Immediately place the swab in the aerobic culture tube.
• For an anaerobic culture, insert the swab deeply into the wound, gently rotate, and immediately place the swab in the anaerobic culture tube. Or insert the needle into the wound, aspirate 1 to 5ml of exudate into the syringe, and ediately inject the exudate into the robic culture tube. If the needle is covered with a rubber stopper, the aspirate may be sent to the laboratory in syringe.
• Record on the laboratory slip recent antimicrobial therapy, the source of the specimen, and the suspected organism. Label the specimen container appropritely with the patient's name, the doctor's name, the facility number, the wound site, and the time of specimen collection. Dress the wound.

Precautions

• Clean the area around the wound thoroughly to limit contamination of me culture by normal skin flora, such as diphtheroids, Staphylococcus epidermidis, and alpha-hemolytic streptococci. Do not clean the area around a perineal wound.
• Make sure no antiseptic enters the wound.
• Obtain exudate from the entire wound, using more than one swab if necessary.
• Because some anaerobes die in the presence of even a small amount of oxygen, place the specimen in the culture tube quickly, take care that no air enters the tube, and check that double stoppers are secure.
• Keep the specimen container upright, and send it to the laboratory within 15 minutes to prevent growth or deterioration of microbes.
• Wear gloves during the procedure and when handling the specimen, and take necessary isolation precautions when sending the specimen to the laboratory.

Normal findings

Normally, no pathogenic organisms are present in a clean wound.

Abnormal findings

The most common aerobic pathogens for wound infection include taphylo­coccus aureus, group A beta-hemolytic streptococci, Proteus, Escherichia coli and other Enterobacteriaceae, and some Pseudomonas species; the most common anaerobic pathogens include some Clostridium and Bacteroides species.

Interfering factors

• Failure to report recent or current antimicrobial therapy (possible false-negative)
• Failure to use proper collection technique
• Failure to use the proper transport medium, allowing the specimen to dry and the bacteria to deteriorate