How can I rule out streptococcal pharyngitis?

How to Rule Out Streptococcal Pharyngitis

A negative rapid antigen detection test (RADT) is sufficient to rule out strep throat in adults, but in children and adolescents you must follow up with a throat culture if the RADT is negative. 1, 2

Testing Strategy by Age Group

Adults

• Perform RADT when clinical features suggest possible strep (sudden onset sore throat, fever, tonsillar exudate, tender anterior cervical lymph nodes, absence of cough) 1, 2
• A negative RADT alone rules out strep pharyngitis—no backup throat culture is needed 1, 3
• This approach is justified because adults have only 5-10% prevalence of strep pharyngitis and extremely low risk of acute rheumatic fever 1, 3
• The RADT has ≥95% specificity, making false positives rare, though sensitivity is 80-90% 1, 2

Children and Adolescents (Ages 3-18)

• Perform RADT first when clinical features suggest strep 1, 2
• Always send a backup throat culture if RADT is negative because the test misses 10-20% of true infections in this age group 1, 2
• This two-step approach is mandatory due to higher strep prevalence (20-30%) and greater risk of acute rheumatic fever in children 1, 2
• Treatment can be initiated within 9 days of symptom onset and still prevent rheumatic fever, so waiting 24-48 hours for culture results is acceptable 1, 3

Children Under Age 3

• Do not test for strep unless an older sibling has confirmed strep infection 2, 3
• Strep pharyngitis and rheumatic fever are rare in this age group 2, 3

When NOT to Test (Strong Viral Features Present)

Skip testing entirely when these viral features are present, as they effectively rule out strep pharyngitis:

• Cough 1, 2, 4
• Rhinorrhea/coryza 1, 2, 4
• Conjunctivitis 1, 2, 4
• Hoarseness 1, 2, 4
• Oral ulcers 2, 4
• Diarrhea 1

The presence of these features strongly indicates viral pharyngitis, and testing will likely identify asymptomatic strep carriers rather than true infection 1, 2

Proper Specimen Collection Technique

Swab both tonsillar surfaces (or tonsillar fossae) and the posterior pharyngeal wall—do not touch other areas of the mouth 1, 2

Common pitfalls that cause false-negative results:

• Recent antibiotic use shortly before or at the time of specimen collection 1, 2
• Inadequate swabbing technique 1, 2
• Testing patients who have already started antibiotics 1

Throat Culture Details (When Indicated)

• Incubate cultures at 35-37°C for 18-24 hours before initial reading 1
• Re-examine plates at 48 hours if negative at 24 hours, as additional overnight incubation at room temperature identifies many positive cultures that would otherwise be missed 1
• The number of streptococcal colonies on the plate cannot reliably distinguish true infection from carrier state 1, 2

What NOT to Do

• Do not test or treat asymptomatic household contacts, even with a history of recurrent infections—up to one-third of household members may be asymptomatic carriers, and prophylactic treatment does not reduce subsequent infection rates 1, 3
• Do not perform routine follow-up testing after completing appropriate antibiotic treatment in asymptomatic patients—positive post-treatment tests often reflect carrier status rather than treatment failure 3
• Do not prescribe antibiotics based on clinical appearance alone without laboratory confirmation—exudates and white patches occur with viral infections too 1, 3, 4
• Do not order backup throat culture in adults with negative RADT—it wastes resources and is not indicated 3

Clinical Scoring Systems

The modified Centor criteria can help determine who needs testing:

• Fever (1 point)
• Tonsillar exudates (1 point)
• Tender anterior cervical lymphadenopathy (1 point)
• Absence of cough (1 point)
• Age 3-14 years (1 point) or age 15-44 years (0 points) or age ≥45 years (-1 point)

Score 0-1: No testing needed 1, 4 Score 2-3: Perform RADT or throat culture 1, 4 Score ≥4: Consider RADT with possible empiric treatment 4

However, even with high clinical scores, microbiologic confirmation is mandatory before prescribing antibiotics because clinical features alone cannot reliably distinguish strep from viral pharyngitis 1, 4