ASO Titer Cut-off Values
The upper limit of normal for ASO titer is approximately 200-250 IU/mL in children and adolescents, though this varies by geographic location, age, and local streptococcal prevalence patterns. 1, 2
Standard Cut-off Values by Population
- Children (5-15 years): The upper limit of normal is 239 IU based on studies of healthy children with no recent streptococcal infection 2
- General pediatric population: Values of 333 units or more should be considered abnormal, as 15-20% of healthy children may have titers between 250-333 units 3
- Adults: Generally lower than children, with most laboratories using 200 IU as the upper limit of normal 4
Critical Interpretation Considerations
ASO titers must be interpreted in clinical context, not as standalone diagnostic criteria. 1, 4
- ASO begins rising approximately 1 week after streptococcal infection and peaks between 3-6 weeks, with sensitivity of 80-90% for detecting prior infection 1
- Titers can remain elevated for several months after uncomplicated infections, making timing crucial for interpretation 1
- Anti-DNase B testing should be performed concurrently, as it has higher sensitivity (90-95%) and rises 1-2 weeks after infection, peaking at 6-8 weeks 1
Geographic and Temporal Variability
The normal upper limit varies significantly by geographic location and can change annually within the same population. 3
- Annual variations can show up to six-dilution differences in normal upper limits between different years in the same population 3
- Local streptococcal prevalence patterns directly influence population baseline titers 3, 2
- Physicians must consider local epidemiological data when interpreting results 3
Clinical Application for Post-Streptococcal Complications
ASO testing is indicated only for confirming previous streptococcal infection in suspected post-streptococcal complications (acute rheumatic fever, post-streptococcal glomerulonephritis), NOT for diagnosing acute pharyngitis. 1
When to Order ASO Testing:
- Acute rheumatic fever: Patients with migratory arthritis, carditis, chorea, erythema marginatum, or subcutaneous nodules 1
- Post-streptococcal glomerulonephritis: Patients with hematuria, proteinuria, edema, and hypertension 1
- Never for: Routine pharyngitis evaluation or as sole indication for tonsillectomy 1, 5
Common Pitfalls to Avoid
- Elevated ASO does not correlate with ESR, CRP, or rheumatoid factor and can be found in various non-streptococcal conditions 4
- Only 14% of patients with elevated ASO have positive streptococcal cultures, so elevated titer alone does not confirm active infection 4
- Specificity is only 12% when using ASO alone without confirming Group A beta-hemolytic streptococci in tissue culture 5
- Performing tonsillectomy based solely on elevated ASO without documented GABHS in tonsillar tissue is not evidence-based 5