What is the role of Antistreptolysin O (ASO) test in diagnosing and managing streptococcal infections?

Antistreptolysin O (ASO) Testing: Clinical Role and Interpretation

Primary Indication: Confirming Past Streptococcal Infection in Post-Streptococcal Complications

ASO testing should be used exclusively to confirm previous streptococcal infection in patients with suspected post-streptococcal complications (acute rheumatic fever or acute glomerulonephritis), not for diagnosing acute pharyngitis. 1, 2

When to Order ASO Testing

Appropriate Clinical Scenarios

• Suspected acute rheumatic fever with clinical features including migratory arthritis, carditis, chorea, erythema marginatum, or subcutaneous nodules 2
• Suspected post-streptococcal glomerulonephritis presenting with hematuria, proteinuria, edema, and hypertension 2
• Confirming prior GAS infection in patients with possible nonsuppurative complications when the timing of acute infection is uncertain 1

When NOT to Order ASO Testing

• Never for diagnosing acute pharyngitis - ASO titers reflect past immunologic events and cannot determine whether current pharyngeal GAS represents true infection versus carrier state 1
• Not for routine follow-up after treated streptococcal pharyngitis 1
• Not for screening asymptomatic patients 1

Optimal Testing Strategy

Timing and Interpretation

• ASO rises approximately 1 week after infection and peaks at 3-6 weeks, with sensitivity of 80-90% 1, 2
• Anti-DNase B rises 1-2 weeks after infection and peaks at 6-8 weeks, with sensitivity of 90-95% 2, 3
• Both antibodies can remain elevated for several months after even uncomplicated infections 1, 2

Recommended Testing Approach

Order both ASO and anti-DNase B simultaneously for maximum diagnostic accuracy - this combination achieves 95.5% sensitivity and 88.6% specificity for post-streptococcal disease, superior to either test alone 3. If only ASO is initially obtained and remains normal, anti-DNase B should be added 1.

Obtain at least two sequential measurements rather than relying on a single titer, as interpretation of isolated values is prone to misdiagnosis 4.

Critical Interpretation Pitfalls

Age-Related Normal Values

• School-age children have higher baseline ASO titers than adults 1
• Laboratory personnel and physicians commonly misinterpret results by failing to account for age-specific normal ranges 1
• The 80th percentile defines the upper limit of normal, which varies by population and age 5, 6

Test Limitations

• Single ASO measurements have poor diagnostic performance - sensitivity and specificity of only 65.9% and 81.9% respectively when using traditional "upper limit of normal" cutoffs 5
• Newer latex agglutination and nephelometric assays are not well standardized against traditional neutralization assays, creating potential interpretation problems 1
• 5% of healthy adults over age 60 may have undetectable ASO titers, so absence doesn't exclude prior streptococcal exposure 7

Clinical Decision Algorithm

1. Patient presents with suspected post-streptococcal complication (rheumatic fever or glomerulonephritis)
2. Order both ASO and anti-DNase B simultaneously 3
3. If both negative, obtain repeat titers in 2-3 weeks to detect rising titers 4
4. If either elevated or rising, confirms recent streptococcal infection and supports diagnosis of post-streptococcal complication 1, 2
5. Initiate treatment to eradicate streptococcal infection with penicillin V (500 mg 2-3 times daily for 10 days) or amoxicillin (50 mg/kg once daily for 10 days) to prevent permanent cardiac valve damage and mortality 2

The key principle: ASO testing is a retrospective diagnostic tool for post-streptococcal complications, not a real-time test for acute infection. 1