Centor / McIsaac Score Calculator (sore throat)

Understanding the Centor / McIsaac score — concept and principle

The Centor score, and its McIsaac modification that adds an age adjustment, estimates the chance that a sore throat is caused by group A streptococcus (strep throat) rather than a virus. It helps decide who should have a rapid antigen test or throat culture and who can be managed without testing or antibiotics.

How it is calculated

• Temperature above 38 °C: +1
• Absence of cough: +1
• Tender, swollen anterior cervical (front neck) lymph nodes: +1
• Tonsillar swelling or exudate: +1
• Age 3–14: +1; age 15–44: 0; age 45 or over: −1

The McIsaac score ranges from −1 to 5. The calculator maps it to an approximate likelihood of streptococcal infection: 0 or less about 1–2.5%, 1 about 5–10%, 2 about 11–17%, 3 about 28–35%, 4 or more about 51–53%.

The principle

Centor and colleagues (1981) found that these four findings were the most useful in distinguishing streptococcal from other causes of sore throat in adults. Cough and a runny nose point to viral infection, so the absence of cough counts in favour of strep. McIsaac and colleagues (1998) noted that strep pharyngitis is much more common in school-age children and less common in older adults, so they added a point for age 3–14 and subtracted one for 45 or over. Each point raises the pretest probability, which then guides whether testing is worthwhile.

Worked example

1. Defaults: no fever above 38 °C, cough present, no tender neck nodes, no tonsil swelling or exudate, age 15–44.
2. McIsaac score = 0 + 0 + 0 + 0 + 0 = 0.
3. A score of 0 or less corresponds to a strep likelihood of about 1–2.5%.
4. For comparison, a 10-year-old with fever, no cough and tonsillar exudate scores 4: about 51–53%.

How to read the result

At low scores (0–1) the chance of strep is small, and testing and antibiotics are usually not needed. Testing is usually considered at 2–3 and above, with antibiotics guided by the test result. Even at the highest score, about half of patients do not have strep, which is why many guidelines prefer testing over treating on the score alone. Local guidance differs between countries.

Common mistakes

• Prescribing antibiotics on the score alone without a confirming test where testing is available.
• Forgetting the age adjustment, which changes the result for children and older adults.
• Ignoring red flags such as drooling, difficulty swallowing or breathing, or a muffled voice.

Frequently asked questions

What is the difference between Centor and McIsaac?

Centor uses the four clinical findings only. McIsaac adds the age adjustment, so its range runs from −1 to 5.

Why does a cough lower the chance of strep?

Cough is typical of viral respiratory infections and is uncommon in streptococcal pharyngitis, so its absence favours strep.

Is the score useful in children under 3?

Streptococcal pharyngitis is uncommon in that age group and the score was not designed for it.

Sources: Centor R. M. et al., Medical Decision Making, 1981 · McIsaac W. J. et al., CMAJ, 1998