The Anti-Streptolysin O (ASO) test detects antibodies against Streptolysin O, a toxin produced by Group A Streptococcus (GAS) bacteria. This test helps identify a recent or past streptococcal infection, which may lead to complications like rheumatic fever or post-streptococcal glomerulonephritis (PSGN).
✅ Diagnose recent streptococcal infections (if symptoms have resolved)
✅ Evaluate post-streptococcal complications (rheumatic fever, PSGN)
✅ Differentiate between viral and bacterial causes of symptoms
| ASO Level (IU/mL) | Interpretation |
|---|---|
| < 200 IU/mL (Adults) | Normal (No recent streptococcal infection) |
| < 250 IU/mL (Children) | Normal |
| 200-400 IU/mL | Possible recent streptococcal infection |
| > 400 IU/mL | Strongly suggests a recent or past streptococcal infection |
🔹 ASO levels peak 3-6 weeks after infection and decline after 6-12 months.
🔹 A rising ASO titer (two tests taken 2-4 weeks apart) confirms an active or recent infection.
🔹 ASO may be negative in some cases (e.g., skin infections caused by GAS).
Rheumatic Fever (RF) → ASO is often elevated; diagnosis also requires Jones Criteria (joint pain, heart inflammation, nodules, etc.).
Post-Streptococcal Glomerulonephritis (PSGN) → ASO can be elevated, but anti-DNase B is more reliable.
Scarlet Fever & Tonsillitis → ASO may rise, but throat cultures confirm acute infections.
✔ Throat Culture / Rapid Strep Test → Detects active infection.
✔ Anti-DNase B Test → More sensitive for PSGN and skin infections.
✔ C-Reactive Protein (CRP) & ESR → Assess inflammation in suspected rheumatic fever.
✔ Kidney Function Tests (Urinalysis, Creatinine) → Evaluate PSGN.
🔹 Assess symptoms (joint pain, fever, rash, kidney issues).
🔹 Consider antibiotics (penicillin, amoxicillin) if an active infection is present.
🔹 Monitor for complications (heart, kidney, or joint involvement).
🔹 Follow up with a rheumatologist or nephrologist for post-streptococcal syndromes.