What are the diagnostic methods for Streptococcal (Strep) infection?

From the Guidelines

Strep throat is best diagnosed through a combination of clinical evaluation and laboratory testing, with a rapid strep test being the most direct method, and a throat culture as a backup for negative results, especially in children and adolescents, as recommended by the most recent guidelines 1. The diagnosis of strep throat involves several key steps, including:

• Clinical evaluation to identify symptoms such as sudden onset of sore throat, pain when swallowing, fever above 101°F, swollen tonsils with white patches or streaks, swollen lymph nodes in the neck, and absence of cough or runny nose.
• Use of the Centor criteria (fever, tonsillar exudates, tender anterior cervical lymphadenopathy, and absence of cough) to assess the likelihood of strep throat, with higher scores indicating greater probability.
• Laboratory testing, starting with a rapid antigen detection test (RADT) to quickly identify group A streptococcal antigens, which has a high specificity of approximately 95% compared to blood agar plate culture 1.
• If the RADT is negative but strep is still suspected, a throat culture may be performed, which takes 24-48 hours but is more accurate, especially in children and adolescents, as it can identify a considerable number of positive throat culture results that would not otherwise have been identified 1.
• The use of a backup culture is generally not necessary in adults because the incidence of the illness and the risk of subsequent rheumatic fever are low in adults, but it can be considered 1. Some important considerations in the diagnosis of strep throat include:
• The sensitivity of RADTs, which can range from 70% to 90% compared to blood agar plate culture, and the potential for false-negative results, especially in patients who are merely Streptococcus carriers and not truly infected 1.
• The importance of proper technique in obtaining a throat swab, including sampling from the surface of both tonsils (or tonsillar fossae) and the posterior pharyngeal wall, to ensure accurate results 1.
• The potential for spectrum bias, which refers to the phenomenon that, with a greater pretest probability of GAS pharyngitis, the sensitivities of RADTs and throat culture are greater, and the need to consider this when interpreting test results 1.

From the Research

Diagnosis of Strep

To diagnose strep, several methods can be used, including:

• Throat culture, which is considered the diagnostic standard 2, 3
• Rapid antigen detection testing, which has improved sensitivity and specificity 2, 3, 4
• Rapid strep test, which can help identify streptococcal pharyngitis when patients have severe symptoms or when special situations warrant early detection 2
• Clinical decision rules, such as the modified Centor score, to assess the risk of group A beta-hemolytic streptococcal infection 5, 3

Diagnostic Criteria

The following criteria can be used to diagnose strep:

• Sore throat
• Temperature greater than 100.4 degrees F (38 degrees C)
• Tonsillar exudates
• Cervical adenopathy
• Fever
• Tonsillar exudate
• Cervical lymphadenitis
• Patient ages of 3 to 15 years increase clinical suspicion 5, 3

Testing Methods

The following testing methods can be used to diagnose strep:

• Throat culture, which is the most rational use for most patients 2
• Rapid antigen detection testing, which can be used to identify streptococcal pharyngitis when patients have severe symptoms or when special situations warrant early detection 2, 4
• Automated rapid Strep A test, which has high diagnostic accuracy and can reduce antibiotic prescriptions for children 4

Treatment

The following treatments can be used for strep:

• Penicillin, which is the treatment of choice due to its cost, narrow spectrum of activity, and effectiveness 3
• Amoxicillin, which is equally effective and more palatable 3
• Erythromycin and first-generation cephalosporins, which are options in patients with penicillin allergy 3
• Antibiotics, which should only be prescribed when a diagnosis of strep is confirmed or suspected based on clinical criteria and testing results 5, 6