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How is a rapid strep test done?
Obtaining a specimen is the same whether your doctor will do a throat culture or rapid test for strep. A cotton swab (similar to a Q-tip) is quickly rubbed over both tonsils as well as the back wall of the mouth (the posterior pharynx). It is important to avoid contact with other structures inside the mouth such as the tongue or cheeks. The swab is then placed in a specialized container and the rapid test performed. Many people find that obtaining the swab produces a gagging sensation. However, since the entire swabbing process lasts less than five seconds this inconvenience is minimal.
What are the limitations of the rapid strep test?
There are several manufactures of rapid strep tests. Each manufacturer has designed their test to respond only to the presence of the particular streptococcal bacteria (Group A) responsible for strep throat. Other bacteria which are less much less likely to cause sore throats are not identified by the rapid strep test.
- The test will not detect viral causes of sore throat.
- A positive test response occurs when a reaction occurs between a protein on the surface of strep bacteria and chemicals in the test materials. Either living or dead strep bacteria will produce a positive reaction.
- Most rapid strep tests have a sensitivity of 95%, meaning that the test will be positive in 95 of 100 patients who are documented to have strep throat via throat culture obtained at the same time. Since 5 of 100 patients with strep throat will be missed using a rapid strep test, all negative swab specimens should be sent for culture to confirm the absence of strep bacteria.
- A positive culture requires antibiotics.
- The rapid strep test has a 98% specificity. This means that 98 of 100 positive tests correctly indicate the presence specifically of Group A streptococcus bacteria; 2 of 100 positive results are "false positives" - indicative of similarities between various surface proteins found on strep bacteria and other non-strep bacteria found in the mouth.
Fauci, Anthony S., et al. Harrison's Principles of Internal Medicine. 17th ed. United States: McGraw-Hill Professional, 2008.
The Red Book - 29th Edition 2012 Report of the Committee on Infectious Diseases. American Academy of Pediatrics; pg: 668 - 675.