Throat Culture

A throat culture is used primarily to isolate and identify pathogens. Culture results are considered in relation to the patient's clinical status, recent antimicrobial therapy, and amount of normal flora.

Purpose

• To isolate and identify group A beta-hemolytic streptococci (Strepto­coccus pyogenes) - allowing early treatment of pharyngitis - and to prevent sequelae, such as rheumatic heart disease and glomerulonephritis
• To screen asymptomatic carriers of pathogens, especially Neisseria menin­gitidis
• Rarely, to identify Corynebacterium diphtheriae and Bordetella pertussis
• To identify Candida albicans, although direct potassium hydroxide preparation usually provides the same information faster

Patient preparation

• Explain to the patient that this test is used to identify microorganisms that may be causing his symptoms or to screen for asymptomatic carriers.
• Inform him that he needn't restrict food or fluids before the test.
• Tell him that a specimen will be collected from his throat and who will collect the specimen and when.
• Describe the procedure, and warn him that he may gag during the swabbing.
• Check the patient's history for recent antimicrobial therapy. Determine immunization history if pertinent to preliminary diagnosis.

Equipment

Gloves, sterile swab and culture tube with transport medium or commercial collection and transport system

Procedure and posttest care

• Tell the patient to tilt his head back and close his eyes.
• With the throat well illuminated, check for inflamed areas, using a tongue blade.
• Swab the tonsillar areas from side to side; include inflamed or purulent sites.
• Do not touch the tongue, cheeks, or teeth with the swab.
• Immediately place the swab in the culture tube.
• If a commercial sterile collection and transport system is used, crush the ampule and force the swab into the medium to keep the swab moist.
• Note recent antimicrobial therapy on the laboratory slip; label the specimen with the patient's name, the doctor's name, the date and time of collection, and the origin of the specimen; indicate the suspected organism, especially C. diphtheriae (requires two swabs and special growth medium), B. pertussis (requires a nasopharyngeal culture and a special growth medium), and N. meningitidis (requires enriched selective media).
• Nonculture antigen testing methods can be used to detect group A streptococcal antigen in as little as 10 minutes. Cultures are then performed on negative specimens.

Precautions

• Procure the throat specimen before beginning antimicrobial therapy.
• Wear gloves when performing the procedure and handling specimens.
• Send the specimen to the laboratory immediately. Unless a commercial sterile collection and transport system is used, keep the container upright during transport.
• To protect the specimen and prevent its exposure to pathogens, use aseptic technique during the procedure, and observe proper precautions when sending the specimen to the laboratory.

Clinical Alert: Laryngo spasm may occur after the culture is obtained if the patient has epiglottitis or diphtheria. Keep resuscitation equipment nearby.

Normal findings

Normal throat flora include nonhemolytic and alpha-hemolytic streptococci, Neisseria species, staphylococci, diphtheroids, some hemophilus, pneumococci, yeasts, and enteric gram-negative rods.

Abnormal findings

Pathogens that may be cultured include group A beta-hemolytic streptococci (S. pyogenes), which can cause scarlet fever and pharyngitis; C. albicans, which can cause thrush; C. diphtheriae, which can cause diphtheria; and B. pertussis, which can cause whooping cough. The laboratory report should indicate the prevalent organisms and the quantity of pathogens cultured.

Interfering factors

• Failure to report recent or current antimicrobial therapy on the laboratory slip (possible false-negative)
• Failure to use the proper transport medium
• More than a 15-minute delay in sending the specimen to the laboratory