Rapid Strep Test (cont.)
John Mersch, MD, FAAP
Dr. Mersch received his Bachelor of Arts degree from the University of California, San Diego, and prior to entering the University Of Southern California School Of Medicine, was a graduate student (attaining PhD candidate status) in Experimental Pathology at USC. He attended internship and residency at Children's Hospital Los Angeles.
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
In this Article
- Overview of streptococcal throat infection
- What is a rapid strep test?
- Why is identification of streptococcal infection important?
- What is the traditional test for strep throat?
- Why is the rapid test better?
- How is a rapid strep test done?
- What are the limitations of the rapid strep test?
Why is the rapid test better?
By comparison with culture methods, a rapid strep test is much quicker and can produce results within minutes.
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 pharyngitis are not identified by the rapid strep test.
- The test will not detect viral causes of pharyngitis.
- A positive test response occurs when a reaction between a protein on the surface of strep bacteria and chemicals in the test materials exists. 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.
REFERENCE: Fauci, Anthony S., et al. Harrison's Principles of Internal Medicine. 17th ed. United States: McGraw-Hill Professional, 2008.
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