A throat culture is a technique for identifying disease bacteria in materialtaken from the throat. Most throat cultures are done to rule out infections caused by beta-hemolytic streptococci, which cause strep throat. Hemolytic means that these streptococci destroy red blood cells. These organisms are GroupA streptococci, specifically Streptococcus pyogenes. Since most sorethroats are caused by viral infections rather than by S. pyogenes, a correct diagnosis is important to prevent unnecessary use of antibiotics and to begin treatment of strep infections as soon as possible. Group A streptococcal infections are potentially life-threatening, often involving other partsof the body in addition to the throat. Besides causing sore throat (pharyngitis), streptococci can also cause scarlet fever, rheumatic fever, kidney disease, or abscesses around the tonsils.
Throat cultures can also be used to identify other disease organisms that arepresent in the patient's throat; and to identify people who are carriers ofthe organisms that cause meningitis and whooping cough. Besides their use indiagnosis, throat cultures are sometimes used to test antibiotics for their effectiveness in treating different infections.
Throat cultures should be taken before the patient is given any antibiotic medications. In addition, the patient's immunization history should be checkedto evaluate the possibility that diseases other than strep are causing the sore throat. The care provider should wash the hands carefully after taking thespecimen to prevent the spread of any infectious organisms.
A throat culture test should be done on anyone who has symptoms of a strep throat. These symptoms include a sore throat that may be accompanied by a fever, body aches, and loss of appetite. Age is a consideration, in that strep throat is more common in children than in adults. The tonsils and the back of the throat often appear red, swollen, and streaked with pus. These symptoms usually appear 1-3 days after being exposed to group A strep. Because strep is highly contagious, family members and close contacts of patients diagnosed with strep throat should also have throat cultures performed if they show signsof the disease.
The specimen for throat culture is obtained by wiping the patient's throat with a cotton swab. The patient is asked to tilt the head back and open the mouth wide. With the tongue depressed and the patient saying "ah," the care provider wipes the back of the throat and the tonsils with a sterile swab. The swab is applied to any area that appears either very red or discharging pus. The swab is removed gently without touching the teeth, gums, or tongue. It is then placed in a sterile tube for immediate delivery to a laboratory. Obtaining the specimen takes less than 30 seconds. Laboratory results are usually available in two to three days. The swabbing procedure may cause gagging but isnot painful. The doctor makes a note for the laboratory to indicate if any disease organisms other than strep are suspected, because some require specialgrowth conditions in the laboratory. The patient does not need to avoid foodor fluids before the test. Recent gargling or treatment with antibiotics, however, will affect the culture results. The laboratory should be notified if the patient has been recently taking antibiotic medications.
S. pyogenes is cultured on a growth medium called blood agar. Agar isa gel that is made from the cell walls of red algae. Blood agar is made fromagar gel and sheep's blood. When the throat swab reaches the laboratory, it is wiped across a blood agar plate. The plate is allowed to incubate for 24-48hours to allow the growth of bacteria. If the organism is a Group A hemolytic streptococcus, the area immediately around the bacterial colony will be cleared of red blood cells. Hemolytic streptococci dissolve (lyse) red blood cells, leaving a clear zone surrounding the colony.
So-called instant strep tests are now available to help diagnose strep throat. They can be used in the doctor's office and take about 10-30 minutes to perform. Instant tests detect an antigen associated with the streptococcus. If an instant throat test is negative, however, a standard throat culture can beperformed to verify the results.