From the Guidelines
A strep culture should be obtained in children and adolescents with symptoms suggestive of strep throat, and in adults with a negative rapid antigen detection test (RADT) when the clinical features alone do not reliably discriminate between GAS and viral pharyngitis, as recommended by the Infectious Diseases Society of America 1. To get a proper strep culture, the healthcare provider will use a long cotton swab to collect a sample from the back of the throat and tonsils, specifically from the surface of either tonsils (or tonsillar fossae) and the posterior pharyngeal wall, as other areas of the oral pharynx and mouth are not acceptable sites 1. For accurate results, patients should avoid using antiseptic mouthwash or taking antibiotics before the test, as this may lead to false-negative results 1. The sample is then sent to a laboratory where it's placed on a special medium to allow any streptococcal bacteria to grow, typically taking 24-48 hours for results, with the culture being incubated at 35°C–37°C for 18–24 hours before reading, and reexamined at 48 hours if the initial result is negative 1. While rapid strep tests provide faster results (within minutes), they're less sensitive than cultures, so a negative rapid test may be followed by a culture for confirmation, especially in children and adolescents, as recommended by the Infectious Diseases Society of America 1. Strep cultures are important because they help determine if symptoms are caused by Group A Streptococcus bacteria, which requires antibiotic treatment (typically penicillin or amoxicillin for 10 days) to prevent complications like rheumatic fever or kidney inflammation, and even if symptoms improve, patients should complete the full course of antibiotics if prescribed. Key considerations for obtaining a strep culture include:
- Using a long cotton swab to collect a sample from the back of the throat and tonsils
- Avoiding antiseptic mouthwash or antibiotics before the test
- Incubating the culture at 35°C–37°C for 18–24 hours before reading, and reexamining at 48 hours if the initial result is negative
- Considering a throat culture in children and adolescents with symptoms suggestive of strep throat, and in adults with a negative RADT when the clinical features alone do not reliably discriminate between GAS and viral pharyngitis.
From the Research
Guidelines for Getting a Strep Culture
- The decision to get a strep culture should be based on clinical suspicion, with factors such as fever, tonsillar exudate, cervical lymphadenitis, and patient age increasing the likelihood of group A beta-hemolytic streptococcal pharyngitis 2.
- Rapid antigen detection testing (RADT) can be used to diagnose streptococcal pharyngitis, but results should be confirmed with a throat culture in patients with negative RADT results, especially in children and adolescents 3, 4, 2.
- Throat culture is considered the diagnostic standard for streptococcal pharyngitis, although RADT has improved sensitivity and specificity 5, 2.
- The modified Centor score can be used to help physicians decide which patients need no testing, throat culture/rapid antigen detection testing, or empiric antibiotic therapy 5, 6.
Indications for Throat Culture
- Patients with severe symptoms or special situations that warrant early detection of streptococcal pharyngitis may require a throat culture 4.
- Children and adolescents with a negative rapid antigen test result should have a throat culture to confirm the diagnosis 2.
- Patients with worsening symptoms after appropriate antibiotic initiation or with symptoms lasting 5 days after the start of treatment should be reevaluated and may require a throat culture 2.
Specimen Collection
- Good results from rapid strep tests depend on the quality of the specimen, emphasizing the importance of proper specimen collection 4.