Estimate the probability that a sore throat is caused by group A streptococcus (GAS), using the 4 Centor criteria (Centor et al., 1981) with the McIsaac age adjustment (1998). Score range −1 to 5.
Estimated probability of group A streptococcal infection: (McIsaac validation; absolute rates vary by setting).
| McIsaac score | Strep probability | Interpretation |
|---|---|---|
| −1 or 0 | 1–2% | Low — routine testing generally not recommended |
| 1 | 5–10% | Low — routine testing generally not recommended |
| 2 | 11–17% | Moderate — clinicians often consider a rapid antigen test or throat culture |
| 3 | 28–35% | Elevated — testing usually recommended |
| 4 | 51–53% | High — clinicians usually test first or consider empiric treatment |
| 5 | 51–53% | High — clinicians usually test first or consider empiric treatment |
Probabilities from the McIsaac validation (CMAJ 1998/2000); a large US validation (Fine et al., 2012, n=206,870) found the same ordering with slightly higher absolute rates (e.g. score 4 ≈ 55%). Testing guidance summarized from the 2025 IDSA guideline — educational information only.
A 30-year-old patient has fever ≥38 °C, tonsillar exudate, and tender anterior cervical lymph nodes — but does cough (so the “absence of cough” point is not earned).
The Centor score is a clinical prediction rule developed by Centor and colleagues in 1981 to estimate the probability that a sore throat is caused by group A streptococcus (GAS). It sums 4 clinical criteria, each worth 1 point, giving an original score from 0 to 4.
Each counts 1 point: (1) history of fever or a measured temperature of at least 38°C (100.4°F); (2) absence of cough; (3) tonsillar exudate or swelling; (4) swollen and tender anterior cervical lymph nodes. Cough counts against strep because it suggests a viral cause.
McIsaac and colleagues (1998) adjusted the score for patient age, because strep throat is more common in children and rare in older adults: ages 3–14 add 1 point, ages 15–44 add 0, and ages 45 and over subtract 1 point. The resulting McIsaac score ranges from -1 to 5.
In the McIsaac validation cohorts, the observed probability of group A streptococcal infection was: score -1 or 0, 1–2%; score 1, 5–10%; score 2, 11–17%; score 3, 28–35%; score 4 or 5, 51–53%. A large US validation of 206,870 patients found the same ordering with slightly higher absolute rates (for example, score 4 around 55%).
No. Even at the highest scores, roughly half of patients do not have group A streptococcal pharyngitis — about 45% of patients scoring above 4 points in validation studies tested negative. The score is a probability estimate, not a diagnosis, which is why clinicians confirm with a test.
Current guidelines (summarized from the 2025 IDSA guideline) generally suggest: scores 0–1 — no routine strep testing; score 2 — a rapid antigen test or throat culture before any treatment decision; scores 3–5 — testing first, or the clinician may consider empiric treatment. High-risk patients may be tested even at low scores. This is educational information; the decision belongs to a clinician.
The score is not validated under age 3, and group A streptococcal pharyngitis is uncommon in this age group, so routine testing is generally not recommended there. This calculator flags ages under 3 as not applicable.
It is a prediction rule, not a diagnostic test: symptoms of strep and viral sore throats overlap substantially. It performs best at ruling out strep at low scores. It does not account for rheumatic fever history, local outbreaks, exposure history, or complications — a clinician weighs all of these.
Robert M. Centor and colleagues derived the original 4-criterion score in 1981 in a US emergency department. Warren J. McIsaac and colleagues added the age adjustment in 1998, deriving it from 521 patients in a Toronto family practice and validating it in 621 patients across 49 Ontario communities.
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