Strep Throat or a Viral Sore Throat? Reading the Centor Score

The short answer#

Most sore throats are viral and settle on their own, so strep is the exception, not the rule. Group A strep causes roughly 5 to 15 percent of sore throats in adults and about 20 to 30 percent in children. The Centor score, and its age-adjusted McIsaac version, adds up four signs (no cough, tender front-of-neck lymph nodes, fever, and pus on the tonsils) plus age to estimate the odds of strep. A low score means strep is unlikely, so neither a test nor an antibiotic is needed. A higher score is a reason to swab and test, not an automatic reason for antibiotics.

Key points#

• Viruses cause the large majority of sore throats. Only about 1 in 10 adults and up to 3 in 10 children with a sore throat actually have strep.
• Cough, a runny nose, hoarseness, red eyes, or mouth ulcers point toward a virus and away from strep.
• The modified Centor (McIsaac) score sums four clinical signs plus age to separate low-risk throats from higher-risk ones.
• A score of 0 to 1 usually means skip both testing and antibiotics. A score of 2 or more is a reason to run a rapid strep test or throat culture.
• The 2025 IDSA guideline uses scoring mainly to find people who do not need testing at all, and reserves testing for those with a real chance of strep.
• Antibiotics shorten strep symptoms by only about 16 hours on average and mostly matter for preventing uncommon complications.
• Blood tests such as procalcitonin and CRP cannot reliably tell strep from a viral throat.

Why are most sore throats viral, not strep?#

The same viruses behind colds, flu, and mononucleosis inflame the throat. That is why a sore throat arriving with a cough, runny nose, hoarse voice, red eyes, or small mouth ulcers is almost always viral. Group A strep behaves differently. It tends to come on suddenly with painful swallowing, fever, and swollen tonsils, usually without a cough, and it is most common in children aged 5 to 15. Even so, the symptoms overlap enough that no single sign settles the question by itself.

What is the Centor score, and how does the McIsaac version work?#

The Centor score turns those signs into a simple tally. You get one point each for tonsillar swelling or pus, tender and swollen anterior cervical (front-of-neck) lymph nodes, a fever above 38 C, and the absence of cough. The McIsaac modification then adjusts for age, which matters because strep is far more common in school-aged children than in older adults.

The total sorts a throat into a rough probability of strep: about 1 to 2.5 percent at 0 or below, 5 to 10 percent at 1, 11 to 17 percent at 2, 28 to 35 percent at 3, and roughly 51 to 53 percent at 4 or more. In plain terms, even the highest band is close to a coin flip, which is exactly why the score guides testing rather than replacing it.

When should a sore throat be tested for strep?#

Testing earns its keep in the middle and higher bands. A score of 0 to 1 puts the odds of strep low enough that guidelines advise skipping both the swab and the antibiotic. A score of 2 or more is where a rapid antigen detection test, with a backup throat culture in children when the rapid test is negative, changes what you do next. A positive rapid test confirms strep; in adults a backup culture is generally unnecessary.

The 2025 IDSA guideline reframes the scoring systems around what they do best: identifying people with a low probability of strep, in whom further testing is unlikely to help. The guideline also advises against testing when the picture is clearly viral, with features like cough, runny nose, hoarseness, or mouth ulcers. There are sensible exceptions. Someone with a household contact who has confirmed strep, a history of rheumatic fever, or signs of a more serious infection such as an abscess or a scarlet-fever rash should be considered for testing even with a low score.

One common shortcut does not work: routine blood markers. Procalcitonin and CRP rise with many infections and cannot separate strep from a viral throat, so they are not a substitute for the swab. Our companion piece, What Procalcitonin and CRP Can and Cannot Decide About Antibiotics, walks through why these markers help in some settings and mislead in others.

Do I need antibiotics for a sore throat?#

For a viral sore throat, no. Antibiotics do nothing against viruses, and about 82 percent of untreated sore throats are gone within a week regardless. For confirmed strep, the honest answer is that antibiotics help, but modestly. On average they shorten symptoms by roughly 16 hours. The number needed to treat to relieve one person's sore throat by day three is under 6 overall and about 3.7 for people who are culture-positive for strep, which is another way of saying the benefit is real but small for any single patient.

The stronger case for treating confirmed strep is preventing uncommon complications. In older trials, antibiotics cut acute rheumatic fever by roughly two-thirds and reduced peritonsillar abscess and ear infections. Those trials mostly predate 1975, when complication rates ran much higher, so the absolute benefit today in high-income settings is smaller than the relative numbers suggest. When treatment is indicated, penicillin or amoxicillin for 10 days remains first-line because the spectrum is narrow, side effects are few, and cost is low; clindamycin is an option for a genuine penicillin allergy.

Weighing that modest upside against side effects and the broader cost of resistance is the whole point of testing before treating. Treating every sore throat empirically drives avoidable antibiotic use and, over time, resistance in the population. Our article on How Antibiotic Resistance Emerges and What Stewardship Actually Does explains why holding antibiotics for the throats that need them protects the drugs for everyone.

What complications are worth knowing about?#

Most of these are uncommon, but they explain why strep is worth catching. Scarlet fever, a sandpaper-textured rash with a strawberry tongue, appears in fewer than 10 percent of strep pharyngitis cases and is treated the same way as the underlying strep. Peritonsillar abscess (quinsy) is a collection of pus beside a tonsil that causes severe, one-sided throat pain. Acute rheumatic fever, which can damage heart valves, is now rare in the United States and concentrates in children aged 5 to 15. A kidney reaction called post-streptococcal glomerulonephritis can follow strep by a week or two and may show up as dark urine or facial puffiness.

When to seek care#

Some sore throats are emergencies regardless of the Centor score. Seek urgent or emergency care for any of the following:

• Trouble breathing, noisy or high-pitched breathing, or leaning forward and drooling to breathe (possible epiglottitis, a medical emergency).
• Inability to swallow your own saliva, drooling, or a muffled "hot potato" voice.
• Severe throat pain much worse on one side, trouble fully opening the mouth, or visible one-sided swelling (possible peritonsillar abscess).
• A stiff neck, a widespread rash, or a fever that is high or not improving.
• Symptoms that keep worsening after 3 days, do not improve after about a week, or return with dark urine or facial swelling.
• Signs of dehydration, marked drowsiness, or a young child who cannot drink.

For diagnosis and treatment of your own sore throat, see a licensed clinician.