What is the purpose and application of the strep score system in diagnosing streptococcal pharyngitis?

Strep Score Systems for Diagnosing Streptococcal Pharyngitis

Purpose and Clinical Reality

Clinical scoring systems for streptococcal pharyngitis help identify which patients need laboratory testing, but they cannot replace bacteriologic confirmation because even experienced physicians cannot diagnose group A streptococcal pharyngitis with certainty based on clinical features alone. 1, 2

The fundamental limitation is that clinical scoring systems predict positive throat cultures or rapid antigen detection tests (RADTs) only ≤80% of the time, and even when patients present with all clinical features suggestive of streptococcal pharyngitis, only 35-50% are actually confirmed to be group A streptococcus positive. 1, 2

How Scoring Systems Work

Clinical scoring systems incorporate specific features that increase the probability of group A streptococcal infection:

• Age 5-15 years (higher risk group with 20-30% prevalence vs. 5-10% in adults) 2, 3
• Winter/early spring season (peak incidence period) 4
• Fever ≥38.3°C (suggests bacterial rather than viral etiology) 3, 4
• Tender enlarged anterior cervical lymph nodes (lymphadenitis) 1, 3
• Tonsillopharyngeal erythema, edema, or exudate 3, 4
• Absence of viral upper respiratory symptoms (no conjunctivitis, rhinorrhea, or cough) 1, 3, 4

A score of 5-6 points predicted positive cultures in 59-75% of children, while the combination of age 5-15 years, fever, and absence of upper respiratory symptoms predicted positive cultures in 72% of patients. 4

Appropriate Application in Clinical Practice

When to Use Scoring Systems

• Use scoring systems to determine who needs testing, not to make treatment decisions without laboratory confirmation 1, 3
• Patients with obvious viral features (cough, coryza, conjunctivitis, diarrhea) do not need testing and should be coded for acute pharyngitis 1, 2
• Patients with clinical features that do not clearly exclude streptococcal infection should be tested regardless of their score 3

Testing Algorithm Based on Score and Age

For children and adolescents:

• Perform RADT if clinical features suggest possible streptococcal infection 3
• Always confirm negative RADT with throat culture due to test sensitivity <90% and higher prevalence of streptococcal infection in this age group 5, 3
• Treatment within 9 days of symptom onset still prevents acute rheumatic fever, so waiting for culture results is safe 5

For adults:

• Perform RADT if clinical features suggest possible streptococcal infection 3
• A negative RADT alone is sufficient to rule out streptococcal pharyngitis—no backup culture needed due to low prevalence (5-10%) and extremely low rheumatic fever risk 5, 3

Critical Pitfalls to Avoid

• Never treat based on clinical score alone without laboratory confirmation, even with high scores 1, 2, 6
• Do not assume high scores mean streptococcal infection—studies show that patients with the highest symptom scores (3-4 points) had pharyngitis due to group C streptococcus, group G streptococcus, Mycoplasma pneumoniae, Chlamydia pneumoniae, influenza viruses, and other pathogens, not just group A streptococcus 6
• Recognize that up to 70% of patients with sore throats receive antibiotics, yet only 20-30% actually have group A streptococcal pharyngitis, highlighting massive overtreatment when scoring systems are used without laboratory confirmation 2, 3
• Do not test or treat asymptomatic household contacts, even with documented exposure—up to one-third of households include asymptomatic carriers, and prophylaxis has not been shown to reduce subsequent infection rates 5

Impact on Antibiotic Prescribing

Using scoring systems combined with RADTs significantly reduces inappropriate antibiotic use:

• Paediatricians without access to laboratory tests prescribed antibiotics in 72.2% of cases versus only 28.2% when tests were available 7
• A pragmatic clinical decision rule in low-resource settings allowed for 35-55% antibiotic reduction with 88% specificity 8
• The key is using scores to select patients for testing, then basing treatment decisions on test results, not scores 7