Estimate the probability of group A streptococcal pharyngitis using clinical criteria and age adjustment. Guides testing and treatment decisions.
The Centor score is a clinical prediction rule that estimates the probability of group A streptococcal (GAS) infection in patients with acute pharyngitis. It uses four clinical criteria: presence of tonsillar exudate, tender anterior cervical lymphadenopathy, fever greater than 38°C (100.4°F), and absence of cough. Each criterion is assigned 1 point, for a maximum score of 4. The score helps clinicians decide whether to perform rapid antigen detection testing (RADT) or throat culture, and whether antibiotics are warranted.
The McIsaac modification adds an age adjustment to the original Centor score. This accounts for the fact that the likelihood of GAS infection varies with age. Children aged 3–14 years have a higher pre-test probability of GAS infection and receive +1 point. Adults aged 15–44 receive no age adjustment. Adults aged 45 and older receive −1 point, as GAS infection becomes much less likely in this group. The modified score ranges from −1 to +5.
RADT is recommended for scores of 2 or higher, as the probability of GAS rises significantly at these thresholds. RADT is rapid (5–10 minutes), sensitive (~90–95%), and specific (~95–99%). For negative RADT in young children (age <16 years), follow-up with throat culture is recommended per IDSA guidelines, as RADT can miss 5–10% of GAS cases. For scores ≤1, testing is not routinely recommended due to low pre-test probability.
No — especially in children. The IDSA 2012 guideline recommends confirming GAS with RADT or throat culture before starting antibiotics, even in children with high symptom scores. Empiric treatment without testing risks unnecessary antibiotic exposure, promotes resistance, and provides no clinical benefit if the cause is viral. For high-risk scores (≥4), RADT is recommended, and if positive, antibiotics are indicated. If RADT is negative in a child <16 years, backup throat culture should be performed before treating.
Penicillin V (oral) or amoxicillin are first-line agents for GAS pharyngitis in both children and adults. Standard dosing for children is amoxicillin 12.5 mg/kg twice daily × 10 days (max 500 mg/dose). For penicillin-allergic patients without prior anaphylaxis, a first-generation cephalosporin (e.g., cephalexin) can be used. For true IgE-mediated allergy, macrolides (e.g., azithromycin) are alternatives, though resistance is increasing. Treatment duration is always 10 days to eradicate the organism and prevent acute rheumatic fever (ARF).
Acute rheumatic fever (ARF) is a serious autoimmune sequela of untreated or inadequately treated group A streptococcal infection. It typically occurs 2–4 weeks after pharyngitis and can cause carditis (valvulitis, pericarditis, heart failure), arthritis, chorea, and subcutaneous nodules. ARF can lead to permanent rheumatic heart disease with mitral stenosis, requiring valve replacement. Full 10-day antibiotic courses are essential to prevent ARF. ARF incidence has declined dramatically in developed nations due to prompt antibiotics but remains common in low-income countries.
The McIsaac score has a negative predictive value (NPV) of 90–99% when the score is ≤1 — meaning if your score is low, GAS is very unlikely and testing is not recommended. However, it is not perfect for ruling in GAS (positive predictive value ~50% at score 4), so testing with RADT/culture is still essential before empiric antibiotics, even with high scores. The score is best used as a framework for deciding who needs testing, not as a definitive diagnosis.