GAS bacterial pharyngitis — diagnose with RADT/culture and treat with penicillin to prevent rheumatic fever.
Also known as: GAS pharyngitis, strep throat, group A strep, streptococcal pharyngitis
Overview
Acute pharyngitis caused by Streptococcus pyogenes (group A beta-hemolytic Streptococcus, GAS). Distinguished from far more common viral pharyngitis by clinical features supported by rapid antigen detection test (RADT) and/or throat culture.
Epidemiology
GAS causes 15-30% of pharyngitis in children and 5-15% in adults. Most common in children 5-15 years. Peak in late winter/early spring. Spread by respiratory droplets; incubation 2-5 days. Asymptomatic carriage 5-20% in school-age children.
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Question 1EENTEasy
A 9-year-old girl presents with a 2-day history of sore throat, fever, and painful swallowing, without cough or rhinorrhea. Exam shows tonsillar exudates, tender anterior cervical lymphadenopathy, and a temperature of 38.8 C. A rapid streptococcal antigen test is positive. She weighs 30 kg and has no drug allergies. Which of the following is the most appropriate first-line antibiotic?
AClindamycin
BAzithromycin
CAmoxicillin
DTrimethoprim-sulfamethoxazole
ECiprofloxacin
Reveal answer & full explanation
Correct answer: C — Amoxicillin
AClindamycin
BAzithromycin
CAmoxicillin✓
DTrimethoprim-sulfamethoxazole
ECiprofloxacin
Why amoxicillin is correct
This is group A streptococcal (GAS) pharyngitis, confirmed by a positive rapid antigen test in a child meeting Centor/McIsaac criteria (fever, tonsillar exudates, tender anterior cervical adenopathy, absence of cough)
Group A Streptococcus has never developed penicillin resistance, so a narrow-spectrum penicillin is first-line
In non-allergic children, amoxicillin (50 mg/kg once daily, max 1000 mg, for 10 days) is the IDSA-preferred regimen because once-daily dosing and palatable suspension improve adherence
Adequate treatment shortens symptoms, reduces transmission, lowers the risk of suppurative complications such as peritonsillar abscess, and prevents acute rheumatic fever
Why the others are wrong
Azithromycin — reserved for severe penicillin allergy; rising GAS macrolide resistance limits it as a routine first choice
Trimethoprim-sulfamethoxazole — does not reliably eradicate GAS and does not prevent rheumatic fever, so it is inappropriate for pharyngitis
Ciprofloxacin — a fluoroquinolone with poor streptococcal activity and pediatric cartilage-toxicity concerns, making it the wrong class entirely
Clindamycin — an IDSA-listed alternative for patients with penicillin allergy, but in a non-allergic child it offers no advantage over narrow-spectrum amoxicillin and adds a higher risk of C. difficile infection and growing GAS clindamycin resistance
Question 2EENTEasy
A 12-year-old male has 2 days of sore throat, fever 39°C, tender anterior cervical lymph nodes, and tonsillar exudate. He has no cough or coryza. Which of the following is the most appropriate next step in management?
AEmpiric oral amoxicillin
BRapid strep test or throat culture
CMonospot heterophile antibody test
DAcetaminophen and reassurance
EDelayed antibiotic prescription
Reveal answer & full explanation
Correct answer: B — Rapid strep test or throat culture
AEmpiric oral amoxicillin
BRapid strep test or throat culture✓
CMonospot heterophile antibody test
DAcetaminophen and reassurance
EDelayed antibiotic prescription
Why Rapid strep test or throat culture is correct
This patient scores 4 on the Centor criteria: tonsillar exudate, tender anterior cervical lymph nodes, fever >38°C, and absence of cough
Modified Centor (McIsaac) criteria also incorporate age
With ≥3 Centor criteria, testing (rapid antigen detection test +/- culture) is indicated
Rapid strep is highly specific: a positive result warrants treatment
A negative rapid strep should be confirmed by throat culture in children (more sensitive) but not in adults
Why the others are wrong
Acetaminophen and reassurance — appropriate for 0-1 Centor criteria where no testing or treatment is needed; this patient has 4 criteria
Empiric oral amoxicillin — empiric amoxicillin without testing risks a rash if the cause is EBV mononucleosis rather than strep
Monospot heterophile antibody test — anchoring on mononucleosis in an adolescent with exudative pharyngitis: the vignette offers none of the clues that prompt EBV testing (posterior cervical lymphadenopathy, splenomegaly, marked fatigue), while fever, exudate, tender anterior nodes, and no cough make GAS the diagnosis to test for first; heterophile antibodies are also often falsely negative in the first week of illness
Delayed antibiotic prescription — a back-up prescription to fill only if symptoms persist is an antibiotic-stewardship strategy for self-limited respiratory infections, which makes it appealing, but it neither confirms nor excludes GAS; with 4 Centor criteria in a child, US guidelines call for testing first and treating only a confirmed infection
Additional high-yield points
Score of 2: test if available
Treatment: penicillin V or amoxicillin for 10 days, or single-dose IM benzathine penicillin; cephalexin or clindamycin if penicillin-allergic
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Crowded living conditions (schools, military barracks)
Caregiver/parent of school-age child
Pathophysiology
S. pyogenes (Lancefield group A) adheres to pharyngeal epithelium via M protein, lipoteichoic acid, and fibronectin-binding proteins. Streptolysins and pyrogenic exotoxins cause local inflammation and systemic symptoms. Suppurative complications (peritonsillar abscess, otitis) arise from contiguous spread. Nonsuppurative sequelae (acute rheumatic fever, post-streptococcal glomerulonephritis) reflect immune-mediated cross-reactivity.
Modified Centor (McIsaac) score components: fever >38, tonsillar exudate, tender anterior cervical adenopathy, absence of cough, age 3-14 (+1) or 15-44 (0) or ≥45 (-1).
Differential diagnosis
Viral pharyngitis (adenovirus, rhinovirus, coronavirus) — Cough, rhinorrhea, conjunctivitis, hoarseness, viral exanthem — argue AGAINST GAS; supportive care
Infectious mononucleosis (EBV) — Adolescent/young adult with fatigue, posterior cervical lymphadenopathy, palatal petechiae, splenomegaly, atypical lymphocytes; heterophile (Monospot) or EBV serology; rash is common (~30%) if amoxicillin given
Acute HIV (seroconversion) — Sore throat, fever, lymphadenopathy, mucocutaneous ulcers, rash; HIV RNA or 4th-gen antibody/antigen test
Lemierre syndrome — Persistent fever after pharyngitis, unilateral neck pain/swelling, internal jugular vein thrombophlebitis with septic pulmonary emboli; usually Fusobacterium necrophorum
Kawasaki disease (children <5) — ≥5 days fever + 4 of 5 criteria (conjunctivitis, mucositis, rash, extremity changes, cervical lymphadenopathy); echo for coronary aneurysms
Diagnostic workup
Diagnostic criteria
IDSA 2012: confirm GAS pharyngitis with positive RADT or throat culture in symptomatic patients with clinical features suggestive of GAS (IDSA sets no score cutoff; McIsaac ≥2 is a common testing threshold); do not test those with overt viral features. Test of cure NOT routine.
Labs
Rapid antigen detection test (RADT) — sensitivity ~85%, specificity >95%
Throat culture (gold standard, ~90-95% sensitive) — required to confirm negative RADT in children/adolescents per IDSA; not necessary in adults given low GAS prevalence and minimal rheumatic fever risk
Do NOT test asymptomatic patients (high carriage rate, false positives)
Do NOT test patients with clear viral features (cough, coryza, conjunctivitis, hoarseness)
ASO and anti-DNase B titers — for documenting prior GAS infection in rheumatic fever evaluation, not acute diagnosis
Heterophile (Monospot) or EBV-specific serology if mononucleosis suspected
Imaging
Not required for uncomplicated GAS pharyngitis
Lateral neck XR or CT if epiglottitis, retropharyngeal abscess, or deep neck infection suspected
Diagnostic algorithm
Modified Centor Criterion
Points
Fever >38°C (100.4°F)
+1
Absence of cough
+1
Tender anterior cervical lymphadenopathy
+1
Tonsillar exudate or swelling
+1
Age 3-14 years
+1
Age 15-44 years
0
Age ≥45 years
-1
Score 0-1: no testing, no antibiotic
—
Score 2-3: RADT; treat if positive
—
Score 4-5: RADT; treat if positive; culture if RADT negative in children/adolescents
—
Modified Centor (McIsaac) score for risk stratification of GAS pharyngitis.
Treatment
First-line
Penicillin V 500 mg PO BID-TID × 10 days (adults) OR amoxicillin 50 mg/kg/day (max 1000 mg) once daily or divided BID × 10 days (children) — narrow-spectrum, low cost, no documented resistance
Penicillin G benzathine 1.2 million units IM × 1 (adults; 600,000 units if <27 kg) — single-dose option, ensures adherence
Supportive care: acetaminophen or NSAIDs, saltwater gargles, hydration
Treatment within 9 days of symptom onset prevents acute rheumatic fever; symptomatic improvement within 24-48 h
Return to school/work after 12-24 h of antibiotic therapy and afebrile
Severe penicillin allergy: azithromycin × 5 days OR clindamycin × 10 days (check local susceptibility; about 1 in 3 US invasive GAS isolates were macrolide- and clindamycin-nonsusceptible in 2022)
Recurrent symptomatic GAS: confirm true infection (not carrier with intercurrent viral illness); consider clindamycin or amox-clav for re-treatment
Tonsillectomy considered for ≥7 episodes/year, ≥5/year × 2 years, or ≥3/year × 3 years (Paradise criteria) plus documentation in each
Cough, coryza, conjunctivitis, hoarseness, oral ulcers → VIRAL. Do not test or treat for strep.
McIsaac (modified Centor) score guides testing: 0-1 → no test, no antibiotic; ≥2 → RADT (culture backup if negative in children/adolescents); treat only confirmed GAS.
Amoxicillin given to mono often produces a non-IgE-mediated maculopapular rash (~30% in modern series; 80-100% in historical ampicillin reports) — not a true penicillin allergy.
Antibiotics prevent acute rheumatic fever but NOT post-streptococcal glomerulonephritis.
No documented GAS resistance to penicillin — penicillin remains first-line; macrolide resistance rising.
Test-of-cure not recommended unless symptomatic recurrence or prior rheumatic fever.
References
IDSA 2012 — Shulman ST et al. Clinical Practice Guideline for the Diagnosis and Management of Group A Streptococcal Pharyngitis (Update). Clin Infect Dis 2012;55(10):e86-e102
AAP Red Book 2024 — American Academy of Pediatrics. Group A Streptococcal Infections. In: Red Book 2024
AHA — Gerber MA et al. Prevention of Rheumatic Fever and Diagnosis and Treatment of Acute Streptococcal Pharyngitis. Circulation 2009;119(11):1541-1551
JAMA 2025 (CDC ABCs) — Gregory CJ et al. Invasive Group A Streptococcal Infections in 10 US States. JAMA 2025;333(17):1498-1507
Pediatrics 2013 — Chovel-Sella A et al. Incidence of rash after amoxicillin treatment in children with infectious mononucleosis. Pediatrics 2013;131(5):e1424-e1427
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