Anti Streptolysin O (ASO) Titres

Antistreptolysin O (ASO) titer measures antibodies against streptolysin O, a toxin produced by group A Streptococcus. It helps detect recent strep infection—useful when assessing suspected rheumatic fever or post-streptococcal glomerulonephritis. Levels typically rise 1–3 weeks after infection, peak at 3–5 weeks, and may stay elevated for months. An elevated ASO supports but does not alone confirm diagnosis.

Antistreptolysin O (ASO) titers are typically considered normal when below about 200 IU/mL in adults; children may have slightly higher reference limits. Exact cutoffs vary by laboratory (commonly ~100–250 IU/mL). Clinically, a rising ASO titer or a significant elevation above the lab’s reference range—rather than a single borderline value—indicates recent streptococcal infection.

A high antistreptolysin O (ASO) titer indicates recent streptococcal exposure and can support a diagnosis of post‑streptococcal complications (e.g., rheumatic fever or glomerulonephritis). Interpretation requires clinical correlation—symptoms, timing, and other tests (throat culture, anti‑DNase B). Management may include antibiotics for active infection, monitoring for organ involvement, and specialist referral if complications are suspected.

ASO titer measures antibodies to streptococcal toxin; reference ranges vary by lab. Commonly: <200 IU/mL = negative/normal, 200–400 IU/mL = equivocal, and >400 IU/mL = elevated, suggesting recent streptococcal infection. Antibodies rise 1–3 weeks after infection, peak at 3–5 weeks, then decline over months. Interpret with clinical signs; a rising paired titer confirms recent infection.

A normal antibody titer means the antibody concentration is high enough to indicate immunity; exact "normal" values vary by pathogen, laboratory and test method. Labs usually report results as “positive/immune” or provide a reference cutoff. For example, hepatitis B surface antibody ≥10 mIU/mL is generally considered protective. Always review your lab’s reference range and discuss results with your clinician.

High antistreptolysin O (ASO) titre indicates recent or past streptococcal infection but does not itself cause heart disease. However, an autoimmune reaction after strep (rheumatic fever) can attack the heart and lead to rheumatic heart disease. ASO supports the diagnosis but must be interpreted with symptoms, examination, and other tests; an elevated ASO alone doesn’t confirm heart involvement.