Centor Score Calculator
Calculate the Modified Centor (McIsaac) score to assess the probability of Group A Streptococcal pharyngitis and guide testing and treatment decisions for sore throat.
Clinical Criteria
<15 years: +1 point; 15-44 years: 0 points; ≥45 years: -1 point
Clinical Findings
Assessment Results
Modified Centor Score Guide
| Score | Strep Probability | Recommendation | Management |
|---|---|---|---|
| -1 or 0 | 1-2.5% | No testing/antibiotics | Symptomatic care only |
| 1 | 5-10% | Usually no testing | Supportive treatment |
| 2 | 11-17% | Rapid strep test | Treat if positive |
| 3 | 28-35% | Rapid strep test | Treat if positive, consider culture if negative |
| 4-5 | 51-53% | Test or empiric treatment | Consider treating without testing |
Important Notes:
- Modified Centor adds age adjustment: <15 years (+1), 15-44 years (0), ≥45 years (-1)
- Rapid antigen detection test (RADT) has ~85-95% sensitivity, ~95% specificity
- Negative RADT in children/adolescents should be confirmed with throat culture
- Most pharyngitis cases are viral and do not require antibiotics
Antibiotic Treatment Options
| Antibiotic | Dosage (Adult) | Duration | Notes |
|---|---|---|---|
| Penicillin V | 500 mg PO BID-TID | 10 days | First-line, narrow spectrum |
| Amoxicillin | 500 mg PO BID or 1000 mg daily | 10 days | Preferred, better absorption |
| Cephalexin | 500 mg PO BID | 10 days | If penicillin allergy (not anaphylaxis) |
| Azithromycin | 500 mg day 1, then 250 mg daily | 5 days | If penicillin anaphylaxis |
| Benzathine Penicillin G | 1.2 million units IM once | Single dose | If compliance concerns |
Understanding Strep Throat
What is Group A Streptococcal Pharyngitis?
Strep throat is a bacterial infection caused by Group A Streptococcus (Streptococcus pyogenes). It accounts for 20-30% of pharyngitis cases in children and 5-15% in adults. Unlike viral pharyngitis, strep requires antibiotic treatment to prevent complications.
Why Use the Centor Score?
The Centor Score helps distinguish bacterial from viral pharyngitis, reducing unnecessary antibiotic prescriptions. Most sore throats are viral and resolve without antibiotics. The score identifies patients who benefit from testing or treatment versus those who need only symptomatic care.
Clinical Features Favoring Strep
- Sudden onset: Rapid development of symptoms over hours
- Tonsillar exudates: White/yellow patches on enlarged, red tonsils
- Tender cervical nodes: Painful anterior neck lymph nodes
- Fever: Usually >38°C (100.4°F)
- No cough: Absence of cough, rhinorrhea, or viral symptoms
- Headache and abdominal pain: Common in children
- Petechiae on palate: Small red spots on roof of mouth
Clinical Features Favoring Viral Infection
- Gradual onset of symptoms
- Cough, rhinorrhea (runny nose), congestion
- Hoarseness or laryngitis
- Conjunctivitis (pink eye)
- Diarrhea
- Viral exanthem (rash)
- Discrete ulcers (suggests viral or herpangina)
Complications of Untreated Strep
While most strep infections resolve spontaneously, antibiotics reduce complications:
- Suppurative (early): Peritonsillar abscess, cervical lymphadenitis, otitis media, sinusitis
- Non-suppurative (late): Acute rheumatic fever (2-3 weeks post-infection), post-streptococcal glomerulonephritis (1-3 weeks), PANDAS (pediatric autoimmune neuropsychiatric disorder)
Antibiotics started within 9 days prevent rheumatic fever. They also reduce symptom duration by ~1 day and decrease transmission.
Testing Methods
- Rapid Antigen Detection Test (RADT): Results in 5-10 minutes, 85-95% sensitivity, ~95% specificity. Negative RADT in children should be backed up with culture.
- Throat Culture: Gold standard, 90-95% sensitivity, requires 24-48 hours. Used when RADT negative in high-risk patients.
- Molecular testing (PCR): High sensitivity/specificity, rapid results, but expensive and may detect carriers.
When to Return to School/Work
Patients are contagious until 24 hours after starting antibiotics. Without antibiotics, contagiousness lasts 2-3 weeks. Return to school/work is safe after:
- At least 24 hours of antibiotic therapy
- Fever resolved without fever-reducing medication
- Feeling well enough to participate
Frequently Asked Questions
Can you have strep throat without fever?
Yes, though less common. About 10-15% of strep throat cases occur without fever, especially in adults. The Centor score accounts for this – you can still have a moderate score (2-3) without fever if you have the other criteria (exudates, tender nodes, no cough). Testing is still recommended with scores of 2-3.
Why not just treat everyone with a sore throat with antibiotics?
Most sore throats (70-85%) are viral and don't benefit from antibiotics. Unnecessary antibiotic use contributes to antibiotic resistance, causes side effects (allergic reactions, diarrhea, yeast infections), increases healthcare costs, and doesn't speed recovery for viral infections. Selective testing and treatment based on Centor score reduces inappropriate antibiotic use by 50-80%.
What if my rapid strep test is negative but I still feel sick?
Negative rapid tests have good negative predictive value, especially in adults. If you're an adult with a negative test, strep is very unlikely and you have a viral infection requiring only symptomatic care. In children and adolescents, a throat culture should confirm negative RADT results. If symptoms are severe or worsening despite a negative test, see your doctor for re-evaluation.
How long does strep throat last without treatment?
Untreated strep throat symptoms typically last 3-5 days, though some cases persist longer. Antibiotics reduce symptom duration by about 1 day (from ~3-4 days to ~2-3 days). However, the main reasons to treat are preventing complications (rheumatic fever, abscess) and reducing transmission to others. Most people feel better within 2-3 days of starting antibiotics.
Can you get strep throat more than once?
Yes, strep infections don't provide long-lasting immunity, and there are many different strains of Group A Streptococcus. Recurrent infections can occur, especially in children and those with close contact with carriers. If someone has frequent strep infections (≥5 in one year, or ≥3 per year for 2 years), evaluation for chronic carrier state or tonsillectomy consideration may be warranted.
What are strep carriers and do they need treatment?
Strep carriers are people who have Group A Strep bacteria in their throat without active infection or symptoms. About 10-15% of school-age children are carriers. Carriers don't need treatment because they: 1) are unlikely to spread strep to others, 2) are not at risk for complications, and 3) may remain carriers despite multiple antibiotic courses. Treat only if symptomatic.
When should I see a doctor for a sore throat?
See a doctor if you have: severe sore throat with difficulty swallowing or breathing, drooling, inability to open mouth fully (trismus), severe one-sided throat pain (possible abscess), high fever >103°F (39.4°C), symptoms lasting >1 week, recurrent sore throats, or if you're immunocompromised. Also seek care if Centor score suggests testing/treatment is needed (score ≥2).
What's the difference between tonsillitis and pharyngitis?
Pharyngitis is inflammation of the pharynx (throat), while tonsillitis specifically involves inflammation of the tonsils. Strep pharyngitis often causes tonsillitis as well. The terms are sometimes used interchangeably, but pharyngitis can occur without significant tonsillar involvement, and tonsillitis can occur from other causes. The Centor score applies to pharyngitis with or without tonsillitis.
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