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Strep Throat Diagnosis

How Diagnostic Tests for Strep Throat Work

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Updated March 16, 2009

Strep throat, also known as streptococcal pharyngitis, is caused by bacterial infection of the inside of the throat. It is caused by the bacterial species Streptococcus pyogenes, also known as Group A Strep.

Symptoms of strep throat may include sore throat, fever, swollen lymph nodes, headache, and sometimes vomiting or rash. Accurate diagnosis of strep throat is important for preventing long-term complications, such as heart and kidney disease.

Diagnosis of strep throat cannot be based solely on clinical signs and symptoms, and additional tests are required for positive identification of the Streptococcus pyogenes bacteria that cause the disease.

Clinical Strep Throat Diagnosis

Again, clinical diagnosis based on patients’ symptoms is not sufficient for definitive diagnosis of strep throat. Doctors base their decision of whether or not to run laboratory diagnostic tests on four conditions:

  1. Fever higher than 99.9 degrees
  2. Patches of pus in the throat
  3. Swollen lymph nodes in the neck
  4. Absence of cough

Studies have indentified certain predictors of strep throat. One prediction rule uses these four potential features. If all four conditions are present, the probability of strep throat is 56%. In the presence of three conditions, the likelihood is 32%; two conditions, 15%; and one condition, 6%. If no conditions are present, the probability of strep is less than 2.5%, and your doctor will not likely recommend further diagnostic testing.

Throat Cultures for Strep Throat

The throat culture is the diagnostic gold standard, with the main disadvantage being a 24 to 48 hour delay in culture results. The throat culture is performed by culturing swabs of bacteria collected from the back of the throat. The bacteria are allowed to grow on special plates called sheep blood agar plates. Streptococcus pyogenes has a unique feature which allows it to break open red blood cells. When the bacteria is grown on sheep blood agar (made up of sheep blood mixed with bacteria growth media), a clearing of the red blood cells on the petri dish indicates positive identification of the Streptococcus pyogenes bacteria.

Rapid Antigen Detection Tests for Strep Throat

It used to take two days to diagnose strep throat, but now doctors can do it within minutes by using a rapid test.

The rapid antigen detection tests are performed by swabbing the back of the throat for the Strep pyogenes bacteria. The bacterial swab is then subjected to either enzyme or acid techniques to extract bacterial antigens, which are then measured by various assays (lab tests).

Why do doctors still do throat cultures if rapid tests are available? Rapid antigen tests, while fast, have much lower sensitivity than throat cultures, so false-negatives may be reported. It has been recommended that a negative result from a rapid antigen test should be followed up with a throat culture.

When Should I Call the Doctor?

If you or your child has a sore throat with fever or exhibit any of the other characteristic symptoms, contact your doctor or pediatrician to see if you should get tested for Strep throat.

Source:

Rakel and Bope. Conn’s Current Therapy 2008, 60th ed. ©2008 Saunders. An Imprint of Elsevier.

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