Medical AICentor (McIsaac) streptococcal pharyngitis calculator
Four clinical criteria (tonsillar exudate, tender anterior cervical nodes, fever by history, absence of cough), each worth 1 point, plus the McIsaac age adjustment, estimate the likelihood of group A streptococcal pharyngitis and frame the decision to test or treat.
How to measure each input
- Tonsillar exudate or swelling
- Inspect the tonsils with a good light. Either visible exudate (pus) or notable tonsillar enlargement satisfies the criterion.
- Anterior cervical nodes
- Palpate the anterior cervical chain (front of the neck). The point requires tender or enlarged nodes there, not posterior or generalised lymphadenopathy.
- Fever
- A history of fever above 38 C (100.4 F) counts even if the patient is afebrile at the visit. A measured temperature above 38 C also qualifies.
- Absence of cough
- Score this when there is no cough. Cough, coryza, hoarseness, and conjunctivitis suggest a viral cause and argue against strep, so their absence is what adds the point.
- Age band
- Use chronological age. The McIsaac modification adds 1 point for ages 3 to 14, no points for 15 to 44, and subtracts 1 point for 45 and older, reflecting how strep prevalence falls with age.
Interpretation
| Band | Meaning |
|---|---|
| 0 or less, very low probability | Group A strep is unlikely (roughly 1 to 2.5% in McIsaac's validation). Testing and antibiotics are generally not recommended; treat symptomatically. |
| 1, low probability | Low probability (around 5 to 10%). Testing is optional; many patients can be managed symptomatically without a test. |
| 2 to 3, intermediate probability | Intermediate probability (around 11 to 35%). Perform a rapid antigen detection test or throat culture and treat only if positive. |
| 4 to 5, high probability | Higher probability (around 50% or more). US (IDSA) guidance still favours confirming with a rapid antigen test or culture before antibiotics rather than empiric treatment; some other guidance considers empiric treatment in this band. |
Pitfalls, exclusions and caveats
- The score estimates probability of group A strep, not severity. It does not detect or address peritonsillar abscess, epiglottitis, or other airway emergencies, which need separate urgent assessment.
- Do not apply it to children under 3 years, in whom classic streptococcal pharyngitis is uncommon and the rules were not validated.
- US (IDSA) practice is to confirm with a rapid antigen detection test or throat culture before antibiotics, even at high scores, rather than treating empirically.
- A negative rapid antigen test in children and adolescents should generally be backed up by throat culture because of lower test sensitivity in that group; routine back-up culture is not advised in adults.
- It does not distinguish streptococcal carriage with an intercurrent viral illness from true infection, so clinical context still matters.
- Suspect infectious mononucleosis (Epstein-Barr virus) in adolescents and young adults with exudative pharyngitis, posterior cervical nodes, and fatigue, as antibiotics are not indicated and aminopenicillins can cause a rash.
1 point each: tonsillar exudate or swelling + tender or swollen anterior cervical lymph nodes + history of fever > 38 C + absence of cough. McIsaac age modification: age 3 to 14 = +1, 15 to 44 = 0, >= 45 = -1. Total -1 to 5.The Centor score was described by Centor et al. (1981) and the age modification by McIsaac et al. (1998, 2004). This implementation is an educational tool and is not affiliated with the original authors or any guideline body.
Frequently asked
What is the difference between the Centor score and the McIsaac modification?
The original Centor score uses four clinical criteria for a total of 0 to 4. The McIsaac modification adds an age adjustment: +1 for ages 3 to 14, 0 for 15 to 44, and -1 for 45 and older, giving a range of -1 to 5. The adjustment improves accuracy across age groups because strep prevalence falls with age.
Should I prescribe antibiotics for a high Centor score without testing?
US (IDSA) guidance recommends confirming group A strep with a rapid antigen detection test or throat culture before antibiotics, even at a high score, rather than treating empirically. Some other national guidance permits empiric treatment at high scores, so follow your local guideline.
When should I not use this score?
Do not use it in children under 3 years, and do not rely on it when there are red flags for airway compromise or deep-neck infection such as drooling, trismus, muffled voice, or severe unilateral swelling, which require urgent evaluation regardless of the score.
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