This Centor score calculator applies the Centor criteria with the McIsaac age adjustment to estimate the probability that a sore throat is group A streptococcal — and what testing that probability usually triggers.




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Four Signs and a Birthday: Sorting Strep From Viral

Part of our Health Calculators collection.

Worked Example

  • 9-year-old: fever, exudate, tender nodes, no cough: 4 clinical points + 1 age = 5 → top probability band (~56%+) — testing still standard in children, treatment follows the result in most guidelines
  • 40-year-old with cough, no fever, mild redness: cough present kills the “absence” point → 0 + 0 = 0 → ~2.5–7%: the score's most useful output is permission to not swab and not prescribe
  • Why “absence of cough” counts: viral pharyngitis brings cough, coryza and hoarseness as traveling companions; strep classically arrives alone with fever and exudate — the negative finding is real evidence

Score Discipline

  • The score estimates probability; the swab decides treatment. Even at 4–5, roughly half of throats aren't strep — hence test-first norms especially in adults.
  • McIsaac's age adjustment exists because base rates differ: strep causes ~20–30% of children's sore throats vs 5–15% of adults'.
  • Centor says nothing about the dangerous mimics — peritonsillar abscess, epiglottitis, Lemierre's: drooling, trismus, unilateral swelling or toxicity bypass scoring entirely.
  • Local guidelines differ (US IDSA vs European) on adult testing thresholds; the probability table is the shared ground.

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Frequently Asked Questions

How is the Centor score calculated?

One point each for fever > 38 °C, absence of cough, tender anterior cervical nodes, and tonsillar swelling/exudate — plus the McIsaac age adjustment: +1 for ages 3–14, 0 for 15–44, −1 for 45+. Range −1 to 5. This calculator applies the modified (McIsaac) version.

What probability of strep does each score mean?

Pooled published estimates: −1/0 → roughly 1–7%, 1 → ~6–12%, 2 → ~14–17%, 3 → ~30–35%, 4 → ~51–56%, 5 → ~56%+. Even the maximum score is barely better than a coin flip — the argument for confirmatory testing baked into the guidelines.

Does a high Centor score mean antibiotics right away?

Usually it means test (rapid antigen, with culture backup conventions varying by age), then treat positives. Some adult guidelines permit empiric treatment at 4+, others don't — regional guidance governs. The score's settled job is ruling the low end OUT of testing and antibiotics.

Why does the score ask about age?

Because base rates drive everything: streptococcal pharyngitis is a school-age disease (20–30% of sore throats) and comparatively uncommon after 45 (5–15% in adults overall, lower with age). McIsaac folded that epidemiology into the score as +1/0/−1.

Can Centor be used in children under 3?

Not really — strep pharyngitis is uncommon and presents differently under 3, and rheumatic fever risk is very low; guidelines generally advise against routine testing there except with a known household contact. The score was derived in ages 3+ (McIsaac) and adults (Centor).

Is this Centor calculator medical advice?

No — an educational implementation of a published score. Sore throats with red-flag features (drooling, breathing difficulty, dehydration, unilateral swelling) need prompt in-person care regardless of score.