No Step Strep
When I started my pediatric practice in 1986, we tested patients for strep throat by performing a throat culture, which was placed in a small office incubator for 48 hours. Typically, we put patients on an antibiotic pending culture results and would stop antibiotics if the culture proved negative. In my first year of practice, an interesting new technology arrived—rapid antigen detection tests (RADTs). These tests were reasonably accurate and enabled us to make a diagnosis at the time of the visit.
In the United States there are 15 million visits to primary care physicians for pharyngitis each year. Twenty percent to 30% of pharyngeal infections in children are attributed to group A streptococcus (GAS), also known as Streptococcus pyogenes. In contrast, only 5% to 15% of throat infections in adults are caused by GAS.
In distinguishing strep pharyngitis from viral infections, one needs to focus on the symptoms and signs in the presenting patient. Strep infections are not accompanied by symptoms typical of a viral illness, ie, cough, ulcers on the pharynx, and rhinorrhea. Symptoms of strep pharyngitis often include fever, enlarged and painful cervical nodes, sore throat, often headache, as well as abdominal pain with or without nausea and vomiting. Signs often associated with strep infections are tonsillar inflammation or exudate, petechial on the soft palate, and enlarged tender lymph nodes.
Some pediatricians use the “strep score” to help identify strep pharyngitis. This score was first developed by Centor and later modified by McIsaac to take the patient’s age into consideration. If you use the McIsaac score, patients are assigned 1 point each if: 1) they are aged between 3 and 14 years; 2) do not have a cough; 3) have swelling or exudate of the tonsils; 4) have tender anterior cervical nodes; or 5) have a fever exceeding 38°C (100.4° F). A score of 0 or 1 indicates that strep is unlikely and no strep test should be performed. A score of 4 or 5 indicates that a strep infection is more likely and strep testing should be performed. Only half of patients with a high strep score prove to have a positive culture.
Also complicating the diagnosis of strep pharyngitis is that 15% to 20% of children are “carriers” of strep in the oropharynx and do not get strep infections, but present with a viral pharyngitis, test positive for strep, and receive antibiotic treatment. Carriers of strep do not manifest an immune response to strep that would be expected in infection. Antibody testing (anti-deoxyribonuclease B, antistreptolysin O) can distinguish carriage from true infection in suspected cases.
Treatment of strep infections with penicillin or alternative antibiotics shorten the course of the infections slightly; prevent suppurative complications such as peritonsillar abscess or lymphadenitis; prevent rheumatic fever and rheumatic heart disease; and reduce the spread of the illness in schools and households. However, rheumatic fever is very rare in first world countries with an incidence of less than 1 per 100,000 infections. In other parts of the world, rheumatic fever continues to be a major cause of heart disease in children, so the approach to the diagnosis and treatment of strep can vary by country.
Before the availability of RADTs in the 1980s, pediatricians employed several methods of testing swabs for strep. Studies found that using a 2-swab sample is more reliable in detecting strep compared with a single swab. Additionally, placing the swab in Todd Hewitt broth for 2 hours before plating often increases the likelihood of generating a positive culture, especially when there are few bacteria on the throat swab
In the 1980s, latex agglutination tests were the first commercially available tests to speed the diagnosis of strep infections. Specificity was excellent (false positive rate was low), but sensitivity was often poor (false negative rate was high), so negative tests were always backed up with a throat culture. These were eventually replaced by enzyme linked immunoassays.
The most popular rapid strep tests on the market today are lateral flow immunoassays. In this test, antibody specific to GAS carbohydrate cell wall antigen is coated on the test line region of a test strip or membrane. During testing, the extracted throat swab specimen reacts with an antibody to GAS that is coated onto particles. The mixture migrates up the strip or membrane to react with the antibody to GAS on the strep and generate a colored line in the test region. The presence of this line in the test region indicates a positive result, whereas its absence indicates a negative result. To serve as a procedural control, a colored line will always appear in the control region if the test has been performed properly. If a control line does not appear, the test result is not valid. Keep in mind that false negative tests are associated with poor specimens, uncooperative patients, inexperience, or testing a patient early in the course of a strep infection.
NOVAmed Ltd, an innovative medical technology company based in Israel has developed a patented “No Step” lateral flow strep test. One simply uses the supplied swab to obtain a specimen, places the swab in the cartridge and releases reagents with a push of the blue cap of the device. One repositions the swab after one minute and reads the test 5 minutes after sample introduction.
As the test sample flows through the device, labeled antibody-dye conjugate binds to the strep A carbohydrate antigen forming an antibody-antigen complex. When this complex binds to the anti-Strep A antibody at the test region it produces a purple color band on the membrane. In the absence of Strep A there is no line in the test region. The reaction mixture continues to flow through the device and unbound conjugate binds to the reagents in the control region producing a purple color band indicating the the device/reagents performed correctly.
The device is available over the counter at pharmacies in Israel and the company is now seeking FDA approval for distribution in the United States.
The package insert can be found here: https://irp-cdn.multiscreensite.com/e82d5079/files/uploaded/IFU-%20No-Step%20Strep%20A%20Test-R-6009-v.22-28.11.18.pdf