What is the most appropriate initial lab test to order for a patient presenting with symptoms of a sore throat to diagnose streptococcal infection?

Rapid Antigen Detection Test (RADT) for Streptococcal Pharyngitis

The most appropriate initial lab test for diagnosing streptococcal pharyngitis is the rapid antigen detection test (RADT), which provides results within minutes and has excellent specificity (≥95%), allowing confident treatment decisions based on positive results. 1, 2

Test Selection Algorithm

First-Line Testing

• Perform RADT as the initial diagnostic test when clinical features suggest possible Group A Streptococcal (GAS) pharyngitis, including sudden onset sore throat, fever >100.4°F, tonsillar exudates, tender anterior cervical lymphadenopathy, or absence of viral features (cough, coryza, conjunctivitis). 1, 2

• RADT has approximately 95% specificity, meaning false-positive results are highly unusual, so therapeutic decisions can be made with confidence on positive results without need for culture confirmation. 1

• RADT sensitivity ranges from 70-90% compared to throat culture, which is why negative results require different management strategies based on patient age. 1

Age-Specific Follow-Up Testing

For Children and Adolescents (3-18 years):

• Always perform backup throat culture when RADT is negative, as the sensitivity of 80-90% means 10-20% of true infections will be missed, and this age group has higher prevalence (20-30%) of GAS pharyngitis plus risk of rheumatic fever. 1, 2

• Collect a dual pharyngeal swab initially—one for immediate RADT and one for culture if RADT is negative, discarding the second swab if RADT is positive. 1

For Adults:

• Backup throat culture after negative RADT is generally not necessary due to lower incidence of streptococcal infection (5-10%) and extremely low risk of acute rheumatic fever in adults. 1, 2

For Children Under 3 Years:

• Testing is not routinely recommended due to low incidence of GAS pharyngitis and rare occurrence of rheumatic fever, except when risk factors exist such as an older sibling with GAS infection. 2

Throat Culture Specifications

When throat culture is indicated (negative RADT in children or as primary test):

• Obtain specimens from both tonsillar surfaces (or tonsillar fossae) and posterior pharyngeal wall—other oropharyngeal areas are not acceptable sampling sites. 1

• Incubate at 35-37°C for 18-24 hours before initial reading, then re-examine plates at 48 hours if negative at 24 hours, as additional overnight incubation at room temperature identifies additional positive cultures. 1

• Transport swabs at room temperature within 2 hours using appropriate swab transport devices. 1

Clinical Decision Support

When to Test

• Use clinical scoring (Centor criteria) to determine testing necessity: patients with 3-4 criteria should undergo RADT, while those with 0-2 criteria are unlikely to have streptococcal infection and testing may lead to unnecessary antibiotic use. 1, 2

• Do not test when overt viral features are present, including conjunctivitis, coryza, cough, diarrhea, hoarseness, discrete ulcerative stomatitis, or viral exanthem. 2

Common Pitfalls to Avoid

• Recent antibiotic use causes false-negative results—both RADT and culture may be negative if the patient received antibiotics shortly before or at the time of specimen collection. 1, 2

• Do not test asymptomatic household contacts of patients with GAS pharyngitis, as testing or empiric treatment of contacts is not recommended. 2

• The number of GAS colonies on culture plates cannot reliably distinguish true infection from carrier state—although true acute GAS pharyngitis patients tend to have more strongly positive cultures than carriers, there is too much overlap for accurate differentiation. 1

• Do not use anti-streptococcal antibody titers for diagnosis of acute pharyngitis, as they reflect past rather than current immunologic events and have no value in diagnosing acute infection. 1, 2

Test Performance Considerations

• Proper swabbing technique is crucial for accuracy—inadequate sampling is a major cause of false-negative results in both RADT and culture. 1, 2

• Spectrum bias affects test sensitivity—RADTs and throat cultures have greater sensitivity when performed in patients with higher pretest probability of GAS pharyngitis. 1, 2

• Neither RADT nor throat culture accurately differentiates acutely infected persons from asymptomatic streptococcal carriers with intercurrent viral pharyngitis, but they allow physicians to withhold antibiotics from the majority of patients with negative results. 1