What is the use of Antistreptolysin O (ASO) titre in clinical practice?

Clinical Use of Antistreptolysin O (ASO) Titre

The primary clinical use of Antistreptolysin O (ASO) titre is for confirming recent Group A Streptococcal (GAS) infection in patients with suspected nonsuppurative complications such as acute rheumatic fever or acute glomerulonephritis, not for diagnosing acute pharyngitis. 1

Mechanism and Interpretation

ASO titre measures antibodies produced against streptolysin O, an extracellular antigen produced by Group A Streptococcus. Key characteristics include:

• Timing: ASO titres begin to rise approximately 1 week after infection and peak 3-6 weeks after infection 1
• Duration: Elevated titres may persist for several months after uncomplicated GAS infections 1
• Normal values: Vary by age - higher in school-age children than adults 1
• Upper limit of normal (ULN): Varies geographically, with studies suggesting values around 262.4 IU/ml in some regions 2

Primary Clinical Applications

1. Diagnosis of Post-Streptococcal Complications

• Acute Rheumatic Fever (ARF)

  • ASO titre provides evidence of preceding GAS infection required for ARF diagnosis according to Jones criteria 3
  • Most valuable when paired with clinical manifestations (carditis, polyarthritis, chorea, etc.) 3
  • Particularly important in cases with subtle or subclinical presentations

• Post-Streptococcal Glomerulonephritis

  • Helps establish recent GAS infection as the trigger for nephritis 1

2. Sequential Testing Approach

For optimal diagnostic value:

• If the initial ASO test is negative but clinical suspicion remains high, an anti-DNase B test should be performed 1, 4
• Anti-DNase B titres begin to rise 1-2 weeks and peak 6-8 weeks after infection 1
• Combined testing increases diagnostic sensitivity for confirming recent GAS infection 4

Important Limitations and Pitfalls

1. Not for acute pharyngitis diagnosis

   • ASO titres reflect past, not present immunologic events 1
   • Cannot distinguish between true infection and carrier state 1
   • Throat culture or rapid antigen detection test is preferred for acute pharyngitis 1

2. Single measurement limitations

   • A single elevated ASO titre has limited diagnostic value (sensitivity ~66%, specificity ~82%) 5
   • Ideally, paired samples showing a rising titre provide more conclusive evidence 4

3. Interpretation challenges

   • Age-specific normal ranges must be considered - children have higher normal values than adults 1
   • Geographic variations in ULN exist, potentially leading to misclassification of ARF cases 6
   • Elevated ASO can occur in conditions unrelated to post-streptococcal diseases 7

Best Practices for ASO Testing

1. When to order:

   • For suspected nonsuppurative complications of GAS (ARF, glomerulonephritis)
   • Not for routine evaluation of acute pharyngitis or sore throat

2. Optimal testing approach:

   • Order both ASO and anti-DNase B tests when possible 1, 4
   • Consider sequential samples 2-4 weeks apart if timing of infection is unclear
   • Interpret results in context of clinical presentation and local ULN values

3. Result interpretation:

   • Rising titres are more diagnostically valuable than a single elevated result
   • Consider age-appropriate normal ranges when interpreting results
   • Use in conjunction with clinical criteria for ARF or other post-streptococcal conditions

By understanding these principles, clinicians can appropriately utilize ASO testing to confirm recent GAS infection in patients with suspected post-streptococcal complications while avoiding misdiagnosis from overreliance on single measurements.