Anti-Streptolysin O (ASO) Test
The Anti-Streptolysin O (ASO) Test measures antibodies against streptolysin O, a toxin produced by Group A Streptococcus (GAS) bacteria. It helps diagnose recent or past streptococcal infections, which can lead to complications like rheumatic fever or glomerulonephritis.
Why is the ASO Test Done?
✅ To Detect Recent Streptococcal Infections
- Helps confirm a recent Group A Streptococcus infection when symptoms are unclear.
✅ To Diagnose Post-Streptococcal Complications
- Rheumatic Fever (RF): Affects the heart, joints, skin, and brain.
- Post-Streptococcal Glomerulonephritis (PSGN): Causes kidney inflammation.
- Scarlet Fever: Rash and fever following strep throat.
✅ To Differentiate Between a Past and Active Infection
- A high ASO titer suggests recent infection, but it does not confirm an active infection.
Test Procedure
🩸 Blood Sample Collection:
- A blood sample is drawn from a vein.
- No fasting required.
- Results available in 2–5 days.
Normal Range & Interpretation
| Result |
ASO Level (Todd Units/mL) |
Interpretation |
| Normal (Negative) |
< 200 IU/mL (Adults)
< 150 IU/mL (Children) |
No recent strep infection |
| Borderline |
200 – 300 IU/mL |
Possible mild infection; may need repeat testing |
| Positive (Elevated) |
> 300 IU/mL |
Suggests a recent streptococcal infection |
🔹 ASO levels start rising 1–3 weeks after infection, peak at 3–5 weeks, and return to normal in 6 months.
🔹 A single high ASO level does not confirm an active infection—repeat testing may be needed.
What Do Abnormal ASO Test Results Mean?
🔴 High ASO Levels Suggest:
- Recent or past Group A Streptococcus infection
- Possible complications such as:
- Rheumatic Fever (fever, joint pain, heart inflammation)
- Glomerulonephritis (blood in urine, swelling, high blood pressure)
- Scarlet Fever (red rash, fever, sore throat)
🟢 Normal/Low ASO Levels:
- No recent strep infection or antibodies have declined over time.
- Does not rule out an active infection, as some people don’t produce ASO antibodies.
Next Steps If ASO Is Elevated?
🔬 Additional Tests for Confirmation:
- Throat Culture or Rapid Strep Test – Detects an active Group A Streptococcus infection.
- Anti-DNase B Test – Detects past strep infections, especially in kidney disease cases.
- C-Reactive Protein (CRP) & Erythrocyte Sedimentation Rate (ESR) – Measures inflammation levels.
- Urinalysis & Kidney Function Tests – If post-strep glomerulonephritis is suspected.
- Echocardiogram (ECG/EKG) – If rheumatic heart disease is suspected.
✅ Treatment Options If Positive:
- Antibiotics (Penicillin, Amoxicillin, or Erythromycin) – If an active infection is present.
- Anti-Inflammatory Medications (NSAIDs, Steroids) – If complications like rheumatic fever occur.
- Long-Term Antibiotic Prophylaxis – For recurrent rheumatic fever cases.
Key Takeaways
🔹 ASO test detects past or recent Group A Streptococcus infections.
🔹 Elevated ASO suggests a recent infection and may indicate complications.
🔹 A single high ASO level does NOT confirm an active infection—other tests may be needed.
🔹 If positive, further tests and possible treatment are required, depending on symptoms.
Anti-Streptolysin O (ASO) Test
The Anti-Streptolysin O (ASO) Test measures antibodies against streptolysin O, a toxin produced by Group A Streptococcus (GAS) bacteria. It helps diagnose recent or past streptococcal infections, which can lead to complications like rheumatic fever or glomerulonephritis.
Why is the ASO Test Done?
✅ To Detect Recent Streptococcal Infections
- Helps confirm a recent Group A Streptococcus infection when symptoms are unclear.
✅ To Diagnose Post-Streptococcal Complications
- Rheumatic Fever (RF): Affects the heart, joints, skin, and brain.
- Post-Streptococcal Glomerulonephritis (PSGN): Causes kidney inflammation.
- Scarlet Fever: Rash and fever following strep throat.
✅ To Differentiate Between a Past and Active Infection
- A high ASO titer suggests recent infection, but it does not confirm an active infection.
Test Procedure
🩸 Blood Sample Collection:
- A blood sample is drawn from a vein.
- No fasting required.
- Results available in 2–5 days.
Normal Range & Interpretation
| Result |
ASO Level (Todd Units/mL) |
Interpretation |
| Normal (Negative) |
< 200 IU/mL (Adults)
< 150 IU/mL (Children) |
No recent strep infection |
| Borderline |
200 – 300 IU/mL |
Possible mild infection; may need repeat testing |
| Positive (Elevated) |
> 300 IU/mL |
Suggests a recent streptococcal infection |
🔹 ASO levels start rising 1–3 weeks after infection, peak at 3–5 weeks, and return to normal in 6 months.
🔹 A single high ASO level does not confirm an active infection—repeat testing may be needed.
What Do Abnormal ASO Test Results Mean?
🔴 High ASO Levels Suggest:
- Recent or past Group A Streptococcus infection
- Possible complications such as:
- Rheumatic Fever (fever, joint pain, heart inflammation)
- Glomerulonephritis (blood in urine, swelling, high blood pressure)
- Scarlet Fever (red rash, fever, sore throat)
🟢 Normal/Low ASO Levels:
- No recent strep infection or antibodies have declined over time.
- Does not rule out an active infection, as some people don’t produce ASO antibodies.
Next Steps If ASO Is Elevated?
🔬 Additional Tests for Confirmation:
- Throat Culture or Rapid Strep Test – Detects an active Group A Streptococcus infection.
- Anti-DNase B Test – Detects past strep infections, especially in kidney disease cases.
- C-Reactive Protein (CRP) & Erythrocyte Sedimentation Rate (ESR) – Measures inflammation levels.
- Urinalysis & Kidney Function Tests – If post-strep glomerulonephritis is suspected.
- Echocardiogram (ECG/EKG) – If rheumatic heart disease is suspected.
✅ Treatment Options If Positive:
- Antibiotics (Penicillin, Amoxicillin, or Erythromycin) – If an active infection is present.
- Anti-Inflammatory Medications (NSAIDs, Steroids) – If complications like rheumatic fever occur.
- Long-Term Antibiotic Prophylaxis – For recurrent rheumatic fever cases.
Key Takeaways
🔹 ASO test detects past or recent Group A Streptococcus infections.
🔹 Elevated ASO suggests a recent infection and may indicate complications.
🔹 A single high ASO level does NOT confirm an active infection—other tests may be needed.
🔹 If positive, further tests and possible treatment are required, depending on symptoms.
