Antistreptolysin O (ASO) Titer

Group A streptococci produce several extracellular products including streptolysin O, hyaluronidase, streptokinase, desoxyribonuclease (DNAase), and nicotinamide adenine dinucleotidase (NADase). Although acute streptococcal infections of the pharynx are easily documented by culture, such an infection may not be clinically apparent and the patient may not present until after the onset of glomerulonephritis or acute rheumatic fever. Serologic detection of antibodies to these streptococcal antigens may provide evidence of previous streptococcal infection. The most commonly performed test is the Antistreptolysin O (ASO) titer.

The presence of antibody in the patient's serum will inhibit the hemolytic activity of streptolysin O. The test result is reported as the reciprocal of the highest dilution which inhibits hemolysis and is referred to as Todd units. A rise in titer of two or more dilution increments between acute and convalescent specimens is considered significant. When only one specimen is available, an "upper limit of normal" must be determined. The lowest titers, usually < 50 Todd units, are seen in children 6 months to 2 years old. The highest titers are seen in school children with up to 80% having titers between 166 to 333 Todd units. Ninety five percent of healthy adults will have ASO titers of < 200 Todd units. In adults, a titer > 200 Todd units is suggestive of antecedent streptococcal infection. Eighty percent of patients with rheumatic fever and 90% of patients with post‑streptococcal glomerulonephritis will have ASO titers > 333 Todd units.Only 25% of patients with streptococcal pyoderma with or without glomerulonephritis will have elevated ASO titers. However, 60% will have elevated anti‑DNAase B titers.

Reference range is 0 – 200 IU/mL.

Specimen requirement is one SST tube of blood.

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