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Antistreptolysin O (ASO) titer

Also Known As:  ASOT, ASO Titer, Antistreptolysin O Titer, ASLO

Anti-streptolysin O (ASO or ASLO) is the antibody made against streptolysin O, an immunogenic, oxygen-labile streptococcal hemolytic exotoxin produced by most strains of group A and many strains of groups C and G Streptococcus bacteria. The “O” in the name stands for oxygen-labile; the other related toxin being oxygen-stable streptolysin-S. The main function of streptolysin O is to cause hemolysis (the breaking open of red blood cells) — in particular, beta-hemolysis.

Why Get Tested:

  • To diagnosis,  exposure to Streptococcal infection.
  • To diagnose poststreptococcal diseases like:
    • Glomerulonephritis.
    • Rheumatic fever.
    • Bacterial Endocarditis.
    • Scarlet fever.
  • Significant level was seen in glomerulonephritis and rheumatic fever.

When Get Tested:

When you have symptoms such as fever, chest pain, fatigue and shortness of breath that suggest rheumatic fever or symptoms such as fluid accumulation (edema) and dark urine that are associated with glomerulonephritis, especially when you recently may have had a group A streptococcal (GAS) infection that was not diagnosed and treated appropriately

Normal Value:

  • Adult / elderly = ≥ 160 Todd unit/mL or <200 IU.
    • 6 months to 2 years = ≤50 Todd units/mL.
    • 2 to 4 years = ≤ 160 Todd units /mL
    • 5 to 12 years  = 170 to 330 Todd units/mL.
  • If it is above 200 then it is a significant level.
  • Greater than 500 Todd unite indicate Acute rheumatic fever or acute post-streptococcal glomerulonephritis.
  • Serial assay of ASO should be advised for rising titer.

Positive Increase in.

  • Streptococcal infections.
  • Bacterial Endocarditis.
  • Post-streptococcal glomerulonephritis (50 to 70 % of the patients shows no high titer).
  • Rheumatic fever.
  • Scarlet fever.
  • Streptococcal pyoderma. 
    • ASO is often not raised in these diseases.

Important Factors

  1. An increased titer may be found in the healthy carrier.
  2. Antibiotic therapy may suppress the antibody response.
  3. Increased Lipoprotein level inhibit Streptolysin O and produce false high ASO titer.




Possible References Used


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