What is the treatment for a child or young adult with an elevated Anti-Streptolysin O (ASO) titre, indicating recent or past streptococcal infection, and no history of penicillin allergy?

Treatment for Elevated ASO Titre

An isolated elevated ASO titre does not require antibiotic treatment, as it only indicates past streptococcal infection, not active disease requiring therapy. 1

Understanding What ASO Titre Actually Means

The critical distinction is that ASO testing reflects past immunologic events, not present infection:

• ASO begins rising approximately 1 week after infection and peaks at 3-6 weeks, making it impossible to identify acute infection 1, 2
• Elevated titers persist for several months after even uncomplicated streptococcal infections 1, 2
• ASO cannot distinguish between active infection and chronic carrier state with concurrent viral illness 2
• Normal ASO levels are higher in school-age children than adults, leading to frequent misinterpretation 1, 2

When ASO Testing Is Actually Indicated

ASO testing should only be used to confirm previous streptococcal infection in patients with suspected post-streptococcal complications:

• Suspected acute rheumatic fever (migratory arthritis, carditis, chorea, erythema marginatum, subcutaneous nodules) 1, 3
• Suspected post-streptococcal glomerulonephritis (hematuria, proteinuria, edema, hypertension) 3
• If ASO is not elevated in these scenarios, obtain anti-DNase B testing for confirmation 1

Treatment Algorithm Based on Clinical Scenario

Scenario 1: Elevated ASO + Current Pharyngitis Symptoms

Perform throat culture or rapid antigen detection test (RADT) to diagnose acute infection 1, 2:

• If positive for GAS: Treat with penicillin V 500 mg PO 2-3 times daily for 10 days (adolescents/adults) or 250 mg 2-3 times daily for children <27 kg 1, 4
• Alternative: Amoxicillin 50 mg/kg once daily (maximum 1 g) for 10 days, preferred in young children due to better palatability 1, 4
• If negative for GAS: No antibiotic treatment; the elevated ASO reflects prior infection and current symptoms are likely viral 2

Scenario 2: Elevated ASO + Signs of Acute Rheumatic Fever

Immediate antibiotic treatment is mandatory to prevent permanent cardiac valve damage and mortality 3:

• First-line: Penicillin V 500 mg PO 2-3 times daily for 10 days OR amoxicillin 50 mg/kg once daily for 10 days 1, 3, 4
• Penicillin allergy (non-anaphylactic): First-generation cephalosporin (cephalexin or cefadroxil) for 10 days 4
• Penicillin allergy (anaphylactic): Clindamycin 20 mg/kg/day divided in 3 doses (maximum 1.8 g/day) for 10 days 1, 4
• After acute treatment: Initiate continuous prophylaxis with benzathine penicillin G 1.2 million units IM every 3-4 weeks for 5 years or until age 21 (whichever is longer) without carditis, or 10 years/until age 21 with carditis 3, 5

Scenario 3: Elevated ASO + No Current Symptoms

No treatment is indicated 1, 2:

• The elevated titre represents resolved past infection
• Antibiotics do not lower ASO titers and provide no benefit 2
• Monitor for development of post-streptococcal complications over the next 2-8 weeks if the infection was recent 1

Critical Treatment Principles

Why 10-Day Duration Is Non-Negotiable

• 10-day treatment is essential for maximum pharyngeal eradication and prevention of acute rheumatic fever 1, 4, 6
• Shorter courses dramatically increase therapeutic failure and rheumatic fever risk (exception: azithromycin 5-day course) 4
• Even when started up to 9 days after symptom onset, penicillin effectively prevents rheumatic fever 1

Why Penicillin Remains First-Line

• No GAS resistance to penicillin has ever been documented worldwide 1, 4, 5
• Narrow spectrum minimizes selective pressure on resistant flora 4
• Cost-effective with proven long-standing efficacy 1, 4

Common Pitfalls to Avoid

• Never use ASO titre alone to decide antibiotic treatment or tonsillectomy 2
• Never treat elevated ASO without confirming active infection via throat culture or RADT 1, 2
• Up to 20% of asymptomatic school-age children are GAS carriers during winter/spring and may have viral pharyngitis with positive GAS testing 2
• Do not prescribe azithromycin or clarithromycin as first-line; reserve for true penicillin allergy due to 5-8% macrolide resistance rates 4
• Do not shorten treatment below 10 days except for azithromycin 5-day regimen 4, 6

Penicillin Allergy Alternatives

For patients with documented penicillin allergy:

• Non-anaphylactic allergy: Cephalexin or cefadroxil for 10 days 1, 4
• Anaphylactic/immediate hypersensitivity: Clindamycin (20 mg/kg/day divided TID, max 1.8 g/day) for 10 days 1, 4
• Alternative macrolides: Azithromycin 12 mg/kg once daily (max 500 mg) for 5 days OR clarithromycin 15 mg/kg/day divided BID (max 250 mg BID) for 10 days 1, 4
• Clindamycin resistance remains very low at approximately 1% in the United States 4