Is an ASO Titer of 280 IU/mL Considered High?
An ASO titer of 280 IU/mL is mildly elevated above the typical upper limit of normal (200-240 IU/mL), but this finding alone has no clinical significance for diagnosing acute streptococcal infection and should NOT guide treatment decisions. 1
Critical Context: ASO Testing Is NOT for Acute Infection Diagnosis
ASO titers reflect past immunologic events, not current infection, and are completely inappropriate for diagnosing acute pharyngitis or deciding whether to treat with antibiotics. 2 The antibody response begins rising approximately 1 week after infection and peaks between 3-6 weeks, making it impossible to identify an acute infection in progress. 1, 3
When ASO Testing Is Actually Indicated
ASO testing has only TWO legitimate clinical uses:
- Confirming prior streptococcal infection in patients with suspected acute rheumatic fever (presenting with carditis, polyarthritis, chorea, subcutaneous nodules, or erythema marginatum) 2, 3
- Confirming prior streptococcal infection in patients with suspected post-streptococcal glomerulonephritis (presenting with hematuria, proteinuria, edema, and hypertension) 3, 4
Interpreting Your Result of 280 IU/mL
Normal Reference Ranges Vary by Age and Geography
- Children ages 5-15 years: Upper limit of normal is approximately 239-256 IU/mL 5, 6
- Adults ages 30-70 years: Upper limit of normal is approximately 210 IU/mL 5
- School-age children naturally have higher baseline ASO levels than adults, so age-appropriate interpretation is essential 1, 4
Your Titer of 280 Is Only Marginally Elevated
A titer of 280 IU/mL is just slightly above the upper limit of normal and could represent:
- Recent past streptococcal infection (within the last 3-6 months), as titers can remain elevated for several months after uncomplicated infections 1, 3
- Normal variation in a healthy individual, as approximately 5-8% of healthy people have elevated ASO titers without any clinical significance 7, 8
- Chronic carriage state with previous viral infections, where the elevated titer reflects old infections rather than current disease 1
Critical Pitfall: Do NOT Use ASO to Diagnose or Treat Acute Infection
If you have current symptoms of pharyngitis or tonsillitis, the ONLY appropriate diagnostic tests are:
- Rapid antigen detection test (RADT) for immediate results 2, 1
- Throat culture as the gold standard for identifying active Group A Streptococcus 2, 1
An elevated ASO cannot distinguish between active infection and chronic carrier state with a concurrent viral infection. 1 Up to 20% of asymptomatic school-age children are streptococcal carriers during winter and spring, and they can develop viral pharyngitis while testing positive for streptococcus. 1
What This Result Does NOT Mean
- It does NOT mean you need antibiotics 1
- It does NOT mean you have an active streptococcal infection 2, 1
- It does NOT correlate with inflammatory markers like ESR or CRP 7
- It does NOT indicate need for tonsillectomy 1
When to Be Concerned About Elevated ASO
You should only be concerned about an elevated ASO if you have clinical features of post-streptococcal complications:
- Acute rheumatic fever symptoms: migratory joint pain, new heart murmur, involuntary movements (chorea), skin rash (erythema marginatum), or subcutaneous nodules 2, 3
- Post-streptococcal glomerulonephritis symptoms: dark/cola-colored urine, facial swelling, high blood pressure, or reduced urine output 3
In these specific scenarios, an ASO of 280 would provide supporting evidence of recent streptococcal infection, and combined testing with anti-DNase B would detect up to 98% of proven cases. 1, 4
Bottom Line for Clinical Practice
A titer of 280 IU/mL is marginally elevated but clinically meaningless without appropriate clinical context. If you ordered this test to evaluate sore throat or decide about antibiotics, the test was ordered inappropriately and should be ignored. 2, 1 If you have current throat symptoms, obtain a throat culture or rapid strep test instead. 2, 1