Throat Swab and Rapid Strep Test

A throat swab is used to diagnose group A beta haemolytic streptococcus (Streptococcus pyogenes, GAS) pharyngitis, and occasionally other pathogens (group C and G streptococci, Arcanobacterium haemolyticum, Corynebacterium diphtheriae, Neisseria gonorrhoeae, Fusobacterium necrophorum, Candida, and respiratory viruses by PCR).

The rapid antigen detection test (RADT) detects the group A carbohydrate antigen by lateral flow immunoassay or optical immunoassay in 5 to 15 minutes at the point of care, with specificity of 95 to 98% and sensitivity of 70 to 90%. Culture on sheep blood agar (incubated for 18 to 48 hours) is the reference method, with sensitivity of 90 to 95%.

Molecular tests (nucleic acid amplification) have sensitivity and specificity of 95 to 99% and are increasingly used.

Sore throat accounts for 1 to 2% of consultations. Most are viral (70 to 85% of adults, 60 to 70% of children). GAS causes 5 to 15% of adult and 20 to 30% of childhood sore throat, peaking at 5 to 15 years in winter and spring.

Treating GAS pharyngitis shortens illness by about 1 day, reduces suppurative complications (peritonsillar abscess) and prevents acute rheumatic fever (which is the main reason in endemic regions, where it affects over 40 million people with rheumatic heart disease). Post-streptococcal glomerulonephritis is not prevented by antibiotics.

Indications. Sore throat with features of bacterial infection (sudden onset, fever, tonsillar exudate, tender anterior cervical nodes, absence of cough) assessed by a clinical score (Centor or FeverPAIN). Sore throat in a child over 3 years with exposure to GAS. Suspected scarlet fever. Recurrent tonsillitis. Contacts in outbreaks.

Suspected diphtheria (grey pseudomembrane, bull neck, travel or incomplete vaccination, special culture and PCR). Suspected gonococcal pharyngitis (sexual exposure). Not for sore throat with cough, hoarseness, coryza or conjunctivitis, which is mostly viral, nor for children under 3 years (rheumatic fever is rare).

Specimen. Swab of both tonsils and the posterior pharyngeal wall, avoiding the tongue, cheeks and uvula, with the patient's mouth open and tongue depressed. A second swab is collected when both RADT and culture are planned. Swabs are placed in transport medium and delivered within 24 hours (room temperature or refrigerated). Special requests for diphtheria (swab from beneath the membrane, with laboratory warning), gonorrhoea (NAAT swab) and other organisms.

Reference values. RADT: negative. Culture: normal throat flora, no beta haemolytic streptococci. Molecular: GAS not detected.

Interpretation.

• Positive RADT with compatible symptoms: GAS pharyngitis, treated without waiting for culture (specificity is high, and the test can be used alone to start antibiotics).
• Negative RADT in adults: GAS is excluded without confirmatory culture in many practices, because sensitivity is high enough and the risk of rheumatic fever in adults is very low in high income countries.
• Negative RADT in children and adolescents: confirm by throat culture or molecular test, because of lower RADT sensitivity and the greater risk of rheumatic fever (in endemic regions the culture or PCR may also be applied to adults).
• Culture growth of beta haemolytic colonies: bacitracin sensitivity, latex agglutination or MALDI-TOF identifies group A. Groups C and G beta haemolytic streptococci cause similar illness, but their treatment is debated, and they do not cause rheumatic fever.
• Carriers: 5 to 20% of school age children carry GAS in the throat, and a positive test in a child with a viral illness cannot separate infection from carriage. Antibiotics are not needed for carriers, and eradication is only for special situations (outbreaks of rheumatic fever, recurrent invasive disease).
• Anti-streptolysin O and anti-DNase B antibodies document recent streptococcal infection retrospectively in suspected rheumatic fever or glomerulonephritis, but have no role in acute pharyngitis.
• Fusobacterium necrophorum, an anaerobe causing sore throat in adolescents and young adults with a risk of Lemierre syndrome, is detected by PCR or anaerobic culture.

Critical values and findings needing immediate action.

• Sore throat with stridor, drooling, muffled voice, trismus, uvular deviation, neck swelling or inability to swallow: airway emergency (epiglottitis, retropharyngeal or peritonsillar abscess, deep neck infection), urgent imaging and ENT or anaesthetic review.
• Grey adherent pseudomembrane with bleeding on removal, bull neck and cardiac or neurological signs: respiratory diphtheria, isolate, send swabs under the membrane, give diphtheria antitoxin and antibiotics on clinical suspicion, and notify public health urgently.
• Sore throat followed by joint pain, carditis, chorea, rash or subcutaneous nodules: acute rheumatic fever (positive throat culture or antibody evidence), echocardiography, and secondary prophylaxis.
• Sore throat with rash and shock, or neck pain with sepsis after sore throat in a young adult: toxic shock and Lemierre syndrome, urgent intravenous antibiotics.
• Unilateral severe sore throat with fever in a young adult: peritonsillar abscess, needle aspiration or drainage.

Interferences and pitfalls.

• Poor swabbing technique (touching the tongue or cheeks, insufficient pressure) lowers sensitivity of every method, and recent antibiotics lower the yield.
• RADT sensitivity is lower in light infection, and reading outside the stated time causes errors. Quality control and staff training matter.
• Many positive cultures in sore throat reflect carriage, particularly in children.
• Clinical scores (Centor 3 or 4, FeverPAIN 4 or 5) improve the chance of GAS to 35 to 55%, whereas a score of 0 to 1 gives under 10%, and testing is limited to those with higher scores.
• Antibiotics for viral pharyngitis promote resistance and carry adverse effects, and delayed prescriptions are an option for borderline cases.
• Penicillin remains first line, and macrolide and clindamycin resistance are increasing in some regions, so alternatives are chosen by local susceptibility. Treatment of 10 days of penicillin V is needed for eradication and prevention of rheumatic fever, and a single dose of intramuscular benzathine penicillin is used in endemic and adherence limited settings.
• Infectious mononucleosis (exudative tonsillitis with posterior cervical nodes) is not treated with aminopenicillins, because of rash. Test for Epstein-Barr virus when the pharyngitis is prolonged.
• Throat cultures do not detect viruses, and respiratory viral PCR panels are used when relevant.

Special groups.

• Children: peak incidence between 5 and 15 years. Scarlet fever (sandpaper rash, strawberry tongue, Pastia lines) is treated as GAS pharyngitis.
• Adolescents and young adults: Fusobacterium and Epstein-Barr virus are considered when sore throat persists.
• Pregnancy: penicillin is used, and group A streptococcus in puerperal sepsis is a serious risk.
• Rheumatic fever survivors: secondary prophylaxis every 3 to 4 weeks with benzathine penicillin for years.
• Regions: rheumatic heart disease burdens low income populations (Pacific, Indigenous Australian, sub-Saharan African, South Asian), and active case finding and sore throat treatment in children are priority prevention steps.

Follow up tests. Throat culture after a negative RADT in children, anti-streptolysin O and anti-DNase B titres (retrospective), Epstein-Barr virus serology and heterophile test, full blood count, C-reactive protein, throat PCR panels for viruses and Fusobacterium, diphtheria culture and toxin PCR, echocardiography and ECG in suspected rheumatic fever, urinalysis and blood pressure for post-streptococcal glomerulonephritis, and cervical imaging for deep neck infection.