Centor Score Calculator (Modified / McIsaac)
Estimate the probability that a sore throat is caused by group A streptococcus (strep) with the modified Centor score, also called the McIsaac score. Four clinical features each add a point — fever, absent cough, swollen tender neck nodes and tonsillar exudate or swelling — and age adjusts the total. Choose the age band and tick the features present to get the score, the approximate chance of strep and the testing recommendation.
About the modified Centor score
The Centor criteria estimate the probability of group A streptococcal pharyngitis; the McIsaac modification adds an age adjustment and extends use to children. Sources: Centor RM et al., Medical Decision Making 1981; McIsaac WJ et al., CMAJ 1998 and JAMA 2004.
Frequently asked questions
What are the Centor criteria?
The four original Centor criteria each score one point: a temperature over 38 °C (history of fever), absence of cough, swollen tender anterior cervical (neck) lymph nodes, and tonsillar swelling or exudate. The McIsaac modification adds age: plus one point for ages 3–14, no change for 15–44, and minus one point for 45 and over. The total ranges from −1 to 5.
What does each score mean?
The approximate chance of group A strep rises with the score: roughly 1–2.5% at 0 or below, 5–10% at 1, 11–17% at 2, 28–35% at 3, and 51–53% at 4 or 5. These are probabilities, not certainties — the score guides whether to test rather than giving a diagnosis on its own. This calculator shows the matching probability band and the suggested action for whatever total you build. Even at 4 or 5 roughly half of sore throats are still not streptococcal, which is why a positive test rather than a high score is what justifies antibiotics.
How does the score guide testing and antibiotics?
A common approach is: score 0–1, no testing or antibiotics (a viral cause is likely); score 2–3, do a rapid antigen test (and throat culture in children if negative) and treat only if positive; score 4–5, test and treat if positive. Guidelines increasingly favour test-guided treatment over empiric antibiotics even at high scores, to avoid unnecessary antibiotic use. This calculator mirrors that pathway, showing “No testing or antibiotics” at 0–1, “Rapid strep test; treat if positive” at 2–3, and “Test and treat if positive” at 4–5. Local guidance differs, so check the pathway used where you practise before acting on the suggestion.
What is the difference between Centor and McIsaac?
The Centor score uses the four clinical criteria alone (0–4). The McIsaac score, sometimes called the modified Centor score, adds the age adjustment, extending the range to −1 to 5 and making it usable from age 3. This calculator uses the McIsaac version, which is the one most guidelines reference.
When should the score not be used?
The modified Centor score is intended for acute sore throat in otherwise well patients and does not apply to those with signs of serious infection such as difficulty breathing, drooling, a muffled voice or severe one-sided swelling, who need urgent assessment. It also does not detect other causes such as infectious mononucleosis. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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