Featured Article
Technology Spotlight
Please check out our Clinical Diagnostics Assay section for more information or to find manufacturers that sell these products
Group A beta-hemolytic streptococcus (GAS, Streptococcus pyogenes) is associated with a wide range of infections and other illnesses, and is a significant worldwide cause of acute pharyngitis and skin infections. Although most infections are usually short-lived and benign, if left untreated, some can worsen and become life-threatening. The rapid diagnosis and treatment of group A streptococcal infections have been shown to shorten the illness, reduce the severity of symptoms and risk of abscess formation, and decrease the occurrence of delayed complications such as acute rheumatic fever (ARF) and acute glomerulonephritis.
GAS accounts for up to to 30% of acute pharyngitis cases in children and 10% in adults; therefore, differentiating between viral and GAS infections is important for their management and can greatly decrease the unnecessary use of antibiotics. Because of the risks in delaying diagnosis, physicians have often relied on clinical scoring systems when deciding to administer antibiotics. However, a recent review of published articles found that no system could be used to diagnose GAS pharyngitis with 85% probability. Therefore, the 2012 update to guidelines from the Infectious Diseases Society of America (IDSA) stipulate that the diagnosis of GAS pharyngitis requires microbiological confirmation by a rapid antigen detection test (RADT) and/or culture of a throat swab.
Diagnosing GAS Pharyngitis: Rapid Antigen Detection Test Versus Culture
The gold standard for diagnosing streptococcal pharyngitis is culturing a throat swab specimen on sheep blood agar (SBA). The sensitivity of culture is 90% to 95% if swabbing and culture are performed correctly. Although cultures require at least 18 hours of incubation at 37 °C, significantly more positive cultures can be identified at 48 hours. By contrast, the point-of-care rapid antigen detection tests (RADTs) allow the identification of group A streptococcal antigen on a throat swab in a matter of minutes. Most of the current RADTs have specificities ≥95%, and sensitivities ranging from 70% to 93%. Given the high specificity of all RADTs, patients with positive RADTs can start treatment immediately, whereas according to the 2012 guidelines of the IDSA, negative RADTs in adolescents and children should be backed up by culture. Culture is not necessary in adults, because of the low incidence of GAS pharyngitis and low risk for ARF in these patients.
Rapid Strep Test Kits: RADTs Versus DNA Probe
Rapid strep tests include RADTs such as the chromatographic immunoassays, which identify cell wall carbohydrates and provide results in minutes; and a test that employs a chemiluminescent DNA probe to identify a GAS-specific sequence. However, the DNA probe assay requires equipment not available in most outpatient settings and takes up to two hours to complete.
Clia-waived immunoassays include:
- The QuickVue In-Line Strep A Test from Quidel Corporation (San Diego, CA) allows for the rapid detection of GAS antigen directly from patient throat swab specimens. It is a lateral-flow color-based chromatographic immunoassay utilizing in-the-device antigen extraction. Two-color results provide clear readability. The 25-test kit package is stored at room temperature and includes positive and negative external controls. The manufacturer reported specificities and sensitivities of 94%–99% and 87%–92%, respectively, using SBA culture as the reference.
- The OSOM® Ultra Strep A Test from Sekisui Diagnostics (Framingham, MA), is a color-based chromatographic immunoassay that detects GAS antigen directly from a throat swab, providing results within six minutes. The throat swab is placed in a tube containing reagents, antigen is extracted, and a test strip is dipped into the extraction solution. The solution migrates along the membrane of the test strip and the GAS antigen is captured by antibody. The 50-test kit is stored at room temperature and includes positive and negative controls. The manufacturer reported a specificity and sensitivity of 97.8% (95% CI 96.6%–99%) and 96% (95% CI 94.4%–97.6%), respectively, using SBA culture as the reference.
- Alere Inc. (Waltham, MA) offers both an Acceava® Strep A Twist® cassette and Acceava Strep A Dipstick for rapid detection of GAS antigen from throat swabs. Both are color-based chromatographic immunoassays that yield results in 5 minutes. The Strep A Twist cassette utilizes a simple-to-use on-board extraction chamber and has a reported specificity and sensitivity of 94% and 90%, respectively. The Strep A Dipstick 50-test kit includes extraction tubes and has a reported specificity and sensitivity of 95% and 97%, respectively.
DNA probe assay:
- The GASDirect Test for Group A Streptococcus from Hologic Gen-Probe Inc. (San Diego, CA) is a DNA probe assay that uses nucleic acid hybridization for the qualitative detection of GAS RNA directly from throat swabs. Hologic Gen-Probe claims that this test is equivalent to culture and that therefore the GASDirect Test can be used to back up a negative rapid antigen assay or as a stand-alone test. The assay employs a single-stranded DNA probe with a chemiluminescent label. The probe hybridizes to ribosomal RNA, which is measured in the Gen-Probe luminometer.
Conclusion
The RADTs are widely used and have even been applied to the diagnosis of GAS skin infections. However, assessments on the impacts of rapid testing on antibiotic prescription have focused on pharyngitis. The use of antibiotics in children and adolescents with pharyngitis has trended downward over the last decade. RADTs used in emergency departments have been reported to significantly increase the number of patients who were appropriately treated for GAS pharyngitis, and a study of pediatricians has found that 72% of those without access to laboratory tests were more likely to prescribe antibiotics compared with 28% with access to tests. Quick strep tests have improved the practice of medicine by reducing the unnecessary use of antibiotics.
Please check out our Clinical Diagnostics Assay section for more information or to find manufacturers that sell these products