Wednesday, August 01, 2018

by IRWIN Z. ROTHENBERG, MBA, MS, CLS(ASCP) 

Strep tests are used to determine if a person with a sore throat (pharyngitis) has strep throat, an infection of the throat and tonsils caused by the bacteria Streptococcus pyogenes, also called Group A Streptococcus (GAS), or if the sore throat is caused by a virus.  The majority of sore throats (70%-85%)¹ are actually viral in nature, and will resolve without treatment within a few days.  

However, it is important to determine when sore throats are caused by GAS, since this is the most common bacterial cause of acute pharyngitis, responsible for 5%–15% of sore throat visits in adults and 20%–30% in children. It is most common in children ages 5 to 15 years old  (over 50 % of the cases occur within this age bracket²); it is very contagious; and needs to be identified as soon as possible, and treated with antibiotics³.  

Accurate diagnosis of streptococcal pharyngitis followed by appropriate antimicrobial therapy is important for the prevention of acute rheumatic fever; for the prevention of suppurative complications (e.g.  peritonsillar abscess, cervical lymphadenitis, mastoiditis, and, possibly, other invasive infections); to improve clinical symptoms and signs; for the rapid decrease in contagiousness; for the reduction in transmission of GAS to family members, classmates, and other close contacts of the patient; to allow for the rapid resumption of usual activities; and for the minimization of potential adverse effects of inappropriate antimicrobial therapy⁴.

Fortunately, most streptococcal infections are now routinely diagnosed and treated through rapid strep testing, these complications are rare in the United States, but they do still occur⁵.

Establishing the Diagnosis of GAS Pharyngitis⁶

1. Swabbing the throat and testing for GAS pharyngitis by rapid strep test and/or throat culture should be performed because clinical features alone do not reliably discriminate between GAS and viral pharyngitis except when overt viral features like runny nose, cough, oral ulceration, and/or hoarseness are present. In children and adolescents, negative rapid strep tests should be backed up by a throat culture; however, positive rapid strep tests do not necessitate a back-up culture because they are highly specific.

2. The routine use of back-up throat cultures for those with a negative rapid strep test is not generally recommended for adults, because of the low incidence of GAS pharyngitis, and because the risk of subsequent acute rheumatic fever is exceptionally low. Physicians who wish to ensure they are achieving maximal sensitivity in diagnosis may (of course) continue to use conventional throat culture or back up negative rapid strep tests with a culture.

3. Anti-streptococcal antibody titers are not recommended in the routine diagnosis of acute pharyngitis as they reflect past but not current events.

Who Should Undergo Testing for GAS Pharyngitis?⁷

1. Testing for GAS pharyngitis usually is not recommended for children or adults with acute pharyngitis with clinical and epidemiological features that strongly suggest a viral basis (e.g, cough, runny nose, hoarseness, and oral ulceration).

1. Diagnostic studies for GAS pharyngitis are not indicated for children under three years old because acute rheumatic fever is rare in children under three years old, and the incidence of streptococcal pharyngitis and the classic presentation of streptococcal pharyngitis are uncommon in this age group. Selected children under three years old who have other risk factors, such as an older sibling with GAS infection, may be considered for testing

1. Follow-up post treatment throat cultures or rapid strep testing are not recommended routinely but may be considered in special circumstances 

1. Diagnostic testing or empiric treatment of asymptomatic household contacts of patients with acute streptococcal pharyngitis is not routinely recommended

If the results of the rapid strep test are positive, further testing is not necessary, and treatment can be started immediately.

Testing Guidelines

l. The Rapid Strep Test (RST)

Description⁸

A major disadvantage of throat cultures is the delay (overnight or longer) in obtaining results. RSTs have been developed for the identification of GAS pharyngitis directly from throat swabs, with shorter turnaround time. Rapid identification and treatment of patients with GAS pharyngitis can reduce the risk of spread, allowing the patient to return to school or work sooner, and can reduce the acute associated morbidity. The use of RSTs for certain populations (e.g, patients in emergency departments) was reported to significantly increase the number of patients appropriately treated for streptococcal pharyngitis, compared with traditional throat cultures.

RSTs currently available are highly specific (approximately 95%) when compared with blood agar plate cultures. False-positive test results are highly unusual, and therefore therapeutic decisions can be made with confidence on the basis of a positive test result. Unfortunately, the sensitivity of most of these tests is 70%–90%, compared with blood agar plate culture.

The first RSTs used latex agglutination methods, were relatively insensitive, and had unclear end points. Newer tests based on enzyme immunoassay techniques offer increased sensitivity and a more sharply defined end point. 

The practitioner should be aware that some of these rapid strep tests are not waived, and therefore, require proper certification or accreditation of the physician's laboratory. Neither conventional throat culture nor RSTs accurately differentiate acutely infected persons from asymptomatic streptococcal carriers with viral pharyngitis. Nevertheless, they allow physicians to withhold antibiotics from the great majority of patients with sore throats for whom results of culture or RST are negative. This is of extreme importance, because nationally up to 70% of patients with sore throats seen in primary care settings receive prescriptions for antimicrobials, while only 20%–30% are likely to have GAS pharyngitis.

Since the sensitivities of the various RSTs are <90% in most studied populations of children and adolescents, and because the proportion of acute pharyngitis due to GAS in children and adolescents is sufficiently high (20%–30%), a negative RADT should be accompanied by a follow-up or back-up throat culture in children and adolescents, while this is not necessary in adults under usual circumstances, as noted above.

Procedure⁹

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.

Limitations¹⁰

1. 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.
2. The test will not detect viral causes of sore throat.
3. 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. A positive culture requires antibiotics, nevertheless.
4. Most rapid strep tests have a sensitivity of 90%, meaning that the test will be positive in 90 of 100 patients who are documented to have strep throat via throat culture obtained at the same time. Since 10 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.
5. 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.

ll.  The Throat Culture

Culture of a throat swab on a sheep-blood agar plate has historically been the standard for the documentation of the presence of GAS pharyngitis in the upper respiratory tract and for the confirmation of the clinical diagnosis of acute streptococcal pharyngitis. If performed correctly, culture of a single throat swab on a blood agar plate is 90%–95% sensitive for detection of GAS pharyngitis.  A major disadvantage of throat cultures is the delay (overnight or longer) in obtaining results.   This spurred the development and adoption of rapid strep testing directly from throat swabs, with shorter turnaround time¹¹. 

Limitations¹²

Several variables affect the accuracy of throat culture results. For example, the manner in which the swab is obtained has an important impact on the yield of streptococci. Throat swab specimens should be obtained from the surface of either tonsils (or tonsillar fossae) and the posterior pharyngeal wall. Other areas of the oral pharynx and mouth are not acceptable sites. Healthcare professionals who try to obtain a throat swab from an uncooperative child without immobilizing the neck may obtain a specimen that is neither adequate nor representative. In addition, false-negative results may be obtained if the patient has received an antibiotic shortly before the throat swab is obtained.

Another variable that can affect the throat culture result is the duration of incubation. Once plated, a culture should be incubated at 35°C–37°C for 18–24 hours before reading. Additional incubation overnight at room temperature may identify a number of additional positive throat culture results. Thus, although initial therapeutic decisions may be made on the basis of overnight culture, it is advisable to reexamine plates at 48 hours that yield negative results at 24 hours.

The clinical significance of the number of GAS colonies on the throat culture plate is problematic. Although patients with true acute GAS pharyngitis are likely to have more strongly positive cultures than patients who are streptococcal carriers (i.e, individuals with chronic GAS colonization of the pharynx), there is too much overlap in this regard to permit accurate differentiation on this basis alone.

Summary of The Testing Protocol for Group A Streptococcal Pharyngitis¹³

REFERENCES 

¹ Rapid Strep Test.   Mersch, J.  MD, MedicineNet.com Newsletter.  2018.  https://www.medicinenet.com/rapid_strep_test/article.htm

² Guideline for The Diagnosis and Management of Acute Pharyngitis.  Alberta Medical Association. 2008 Update. 

http://www.topalbertadoctors.org/download/368/acute_pharyngitis_guideline.pdf

³ LabTestsOnline.  An AACC Publication.  January 2018. https://labtestsonline.org/tests/strep-throat-test

⁴ Clinical Practice Guideline for the Diagnosis and Management of Group A Streptococcal Pharyngitis: 2012 Update by the Infectious Diseases Society of America.  Shulman S., Bisno, A., Clegg, H., Gerber, M., Kaplan, E., Lee, G., Martin, J.,  and Van Beneden, C.  September 9,  2012.    https://academic.oup.com/cid/article/55/10/e86/321183

⁵ Rapid Strep Test.   Mersch, J.  MD, MedicineNet.com Newsletter.  2018.  https://www.medicinenet.com/rapid_strep_test/article.htm

⁶ Clinical Practice Guideline for the Diagnosis and Management of Group A Streptococcal Pharyngitis: 2012 Update by the Infectious Diseases Society of America.  Shulman S., Bisno, A., Clegg, H., Gerber, M., Kaplan, E., Lee, G., Martin, J.,  and Van Beneden, C.  September 9,  2012.    https://academic.oup.com/cid/article/55/10/e86/321183

⁷ Ibid.

⁸ Ibid.

⁹ Rapid Strep Test.   Mersch, J.  MD, MedicineNet.com Newsletter.  2018.  https://www.medicinenet.com/rapid_strep_test/article.htm

¹0 Ibid.

¹1 Clinical Practice Guideline for the Diagnosis and Management of Group A Streptococcal Pharyngitis: 2012 Update by the Infectious Diseases Society of America.  Shulman S., Bisno, A., Clegg, H., Gerber, M., Kaplan, E., Lee, G., Martin, J.,  and Van Beneden, C.  September 9,  2012.    https://academic.oup.com/cid/article/55/10/e86/321183

¹2 Ibid.

¹3 University of Washington Department of Pediatrics. Sore Throat – Clinical Guidelines. Wright, J. Nov 8 2012.

https://depts.washington.edu/uwgenped/images/sore-throat-flowchart.png

IRWIN Z. ROTHENBERG, MBA, MS, CLS(ASCP) 

Irwin Z. Rothenberg, MBA, MS, CLS(ASCP), Technical Writer /Quality Advisor, COLA Resources, Inc.