Synonym
Tubes
- Sterile rayon or dacron swab with a plastic shaft in a transport container/tube (provided by lab)
- Specimens: May require a single swab or two throat swabs (Rapid strep test with automatic culture) depending upon the laboratory
Additional information
- Throat swab collection:
- The patient is instructed to tilt the head back and close eyes
- Oral cavity/throat should be well illuminated, check for inflamed areas by depressing the patient's tongue with a tongue blade
- Rotate the swab firmly and gently over the posterior pharynx, both tonsils or fossae, and on areas of inflammation or exudation
- Avoid touching the teeth, cheeks, tongue, or lips with the swab
- Immediately place swab in transport container/tube (provided by lab)
- Because most patients gag or cough, the collector should wear a facemask for protection
- Immediately send the specimen to the lab
- Note the date, time, patient’s details, and recent antimicrobial therapy (specimen before beginning microbial therapy is preferred) on lab request form
Info
- Rapid streptococcus test is performed to detect the presence of group A beta hemolytic streptococcal antigen from throat swab
- Group A b-hemolytic streptococcal (GABHS) throat infection is the most common cause of bacterial pharyngitis with serious sequelae such as acute rheumatic fever, acute glomerulonephritis, peritonsillar abscess, and Streptococcal toxic shock syndrome
- The various methods to detect streptococcal antigen involve extraction of antigen from a throat swab and detection of this antigen with a streptococcal antibody, and include:
- Enzyme immunoassay
- Latex agglutination
- Optical immunoassay
- Double antibody sandwich immunoassay
- Immunochromatographic detection
Clinical
- The clinical utility of rapid streptococcal test includes:
- Evaluation and diagnosis of suspected streptococcal pharyngeal infection
- Rapid determination of streptococcal infection to initiate early treatment and prevent complications
- To differentiate between streptococcal pharyngitis and other similar illnesses or infections (e.g. viral pharyngitis, influenza, infectious mononucleosis)
- Detection of streptococcal asymptomatic carriers especially in children of school going age to prevent spread of infection
- Rapid detection of strep throat in family members or close contacts of affected person
- GABHS pharyngitis is a highly contagious infection that spreads by sneezing, coughing, or shaking hands especially in children between that ages of 5-15 years
- Acute streptococcal pharyngitis may clinically present as:
- Abrupt onset of fever
- Sore throat
- Headache
- Malaise
- Abdominal pain
- Difficulty in swallowing
- Pharyngeal/tonsillar inflammation with exudates (gray-white patches)
- Tender anterior cervical lymph nodes
- Sandpaper like consistency, scarletiniform rash
- There usually should be an absence of rhinorrhea, cough, hoarseness, or diarrhea (these usually indicate a viral etiology to the illness)
- An untreated or delayed treatment of streptococcal throat infection can increase the patient's risk of developing:
- Scarlet fever
- Rheumatic fever
- Acute glomerulonephritis
- Peritonsillar abscess
- Meningitis
- Streptococcal toxic shock syndrome (STSS)
- A rapid strep test takes about 5-15 minutes to perform and has a lab specificity of 97-98% and sensitivity of 70%
- The excellent specificity of this test, and short time to perform the test, aids in early diagnosis and initiation of therapy to reduce the duration and severity of symptoms during the acute phase, and typically prevents serious sequelae of GABHS infection
- A negative rapid test or false negative results (4-30%) should be confirmed with a throat culture (gold standard)
Additional information
- In cases of pharyngitis, the frequency of the pharyngitis being due to GABHS varies substantially by age group:
- <3 years: 3-15%
- Children: 35-50%
- 15 to 35: 10-20%
- Over 35: 5%
- Streptococcal pharyngitis occurs more commonly during winter and spring seasons
- Rapid antigen detection is not sensitive for Group C and G streptococci or other bacterial pathogens
- Factors interfering with test results include
- Recent or current antimicrobial therapy (possible false negative)
- Using antiseptic mouthwashes before collection of throat swab
- Contamination with oral flora
- Swabs containing cotton, calcium alginate, or wooden of shafts
- Improper collection technique, inadequate sample collection, or swab placed in transport medium containing charcoal
- Heavy growth of Staphylococcus aureus in patient’s throat (false positive result)
- Insufficient antigen obtained from the swab extraction procedure (false negative result)
- Related laboratory tests include:
Nl Result
- Rapid Streptococcus Test: Negative
- Confirmatory culture: Negative for Group A Beta Hemolytic Streptococcus
High Result
Conditions associated with a positive result on the rapid streptococcal test include:
- GABHS infection of the pharynx (e.g. presence of streptococcal pharyngitis)
- Rheumatic fever
- Scarlet fever
- Streptococcal glomerulonephritis
Low Result
A negative result for rapid streptococcal testing should be considered in the clinical scenario and pre-test probability of the condition being due to GABHS.
It is common practice to follow negative rapid streptococcal tests with culture to confirm a negative result (in some settings a false negative rate of up to 30% can occur).
References
- Abu-Sabaah AH et al. Better diagnosis and treatment of throat infections caused by group A beta-haemolytic streptococci. Br J Biomed Sci. 2006;63(4):155-8.
- ARUP Laboratories®. Streptococcus (Group A) Rapid with Reflex to Culture. [Homepage on the internet]©2007. Last accessed on July 2, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0028903.jsp
- EdmonsonMB et al.Relationship between the clinical likelihood of group a streptococcal pharyngitis and the sensitivity of a rapid antigen-detection test in a pediatric practice. Pediatrics. 2005 Feb;115(2):280-5.
- eMedicine from WebMD®. Pharyngitis. [Homepage on the Internet] ©1996-2006. Last updated on April 21, 2005. Last accessed on July 2, 2007. Available at URL: http://www.emedicine.com/EMERG/topic419.htm
- Health enotes®. Rapid Streptococcus Antigen Tests. [Homepage on the Internet]©2007. Last accessed on July 2, 2007. Available at URL: http://health.enotes.com/nursing-encyclopedia/rapid-streptococcus-antigen-tests
- Lab Tests online®. Strep Throat. [Homepage on the Internet]©2001-2007. Last reviewed on January 20, 2006. Last accessed on July 2, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/strep/glance.html
- Mirza A et al. Throat culture is necessary after negative rapid antigen detection tests. Clin Pediatr (Phila). 2007 Apr;46(3):241-6.
- Vincent MT et al. Pharyngitis. Am Fam Physician. 2004 Mar 15;69(6):1465-70. Available at URL: http://www.aafp.org/afp/20040315/1465.html