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What is a rapid strep test?
The rapid strep test is a quick and accurate diagnostic tool used to determine whether or not strep bacteria are present in the patient's throat. The same test may be used to evaluate for the presence of Streptococci in other infected areas (for example, perianal infection in either gender, or vulvar infection in pre-pubertal girls).
Streptococcal throat infections facts
- The incidence of sore throats (pharyngitis) varies with season, age of the patient, and geographic area.
- Sore throats can be caused by infection from either viruses or bacteria.
- Less than one-third of all sore throats is bacterial, the most common bacteria being Group A streptococcus (GAS).
- Children 5 years to 15 years of age are the most common age group infected by group A strep.
- Infection is most common during the winter/early spring season. This is likely due in part to the seasonal variation of strep bacteria concentration in the community as well as the higher likelihood of close proximity of individuals due to either weather conditions and/or school attendance during winter months.
The pediatric population may have different strep throat (GAS) symptoms:
- Infants (< 1 year of age) low grade fever (< 101 F, 38 C), thick purulent nasal discharge fussiness, decreased appetite, often following exposure to either daycare or older infected siblings
- Children (< 3 years of age) prolonged purulent nasal discharge, low grade fever, and enlarged and tender lymph nodes in the neck area
- Children (> 3 years of age) sudden onset sore throat, moderate fever (> 101 F, 38 C), headache, upset stomach, and enlarged and tender lymph nodes in the neck area. Other upper respiratory symptoms such as runny nose and cough are not usually associated with strep throat.
The majority of sore throats (70% to 85%) are caused by viruses. Representative examples are:
- EBV (Epstein-Barr virus or mononucleosis) and CMV (cytomegalic inclusion virus) infections may produce a mononucleosis symptom complex (sore throat, fever, disproportionate fatigue, tender and swollen neck lymph nodes, and commonly enlargement of the spleen and liver);
- adenovirus (which may be associated with conjunctivitis ("pink eye");
- influenza; and
- miscellaneous others - herpes, rhinovirus (cause of the common "cold"), etc.
Unfortunately, no single characteristic element of either the patient's history or physical examination discriminates between strep thraot (GAS) and non-strep sore throat. As a result, it is imperative to have laboratory evaluation since strep throat should be treated with antibiotics, while relief of symptoms remains the mainstay of treatment for viral sore throats.
Why is identification of streptococcal infection important?
Patients benefit both immediately and potentially in the long-term by rapid confirmation of streptococcal cause of their sore throat. Antibiotic treatment provides a quicker reduction of symptoms, shortens the duration of illness and quickly, and efficiently eliminates the possibility of spread of infection to others.
- Although rare, serious consequences of streptococcal infections do occur. Rheumatic fever is associated with heart, joint and nervous system damage and is preventable by rapid treatment of strep disease.
- Serious kidney disease that may result in kidney failure may also be a consequence of streptococcal infection.
- A throat infection due to strep needs antibiotic therapy initiated as rapidly as possible. The rapid strep test facilitates this goal.
What is the traditional test for strep throat?
The traditional test for a strep throat has been a throat culture. The major drawback of a throat culture is that the results take two to three days due to the time necessary to allow enough GAS bacteria to grow to enable accurate identification.
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 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.
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Jameson, JL, et al. Harrison's Principles of Internal Medicine, 20th ed. (Vol.1 & Vol.2). McGraw-Hill Education 2018.
The Red Book - 29th Edition 2012 Report of the Committee on Infectious Diseases. American Academy of Pediatrics; pg: 668 - 675.
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