Antistreptolysin O antibodies will be raised after infection with streptococci. Levels greater than 200 units per millilitre are considered significant (although cut off levels will vary between laboratory undertaking the test).
Measurement can be useful diagnostically if recent infection with streptococci is known to be important in aetiology. Diseases include:
- rheumatic fever
- post streptococcal glomerulonephritis
- scarlet fever
The antibody level appears as early as one week following streptococcal infection. The titre level rises rapidly by 3-4 weeks and then declines quickly; the antibody level may remain raised for months.
Note that even in severe streptococcal infection there will be an elevated ASOT titre in only 70-80% of patients.
False positive results are associated with tuberculosis, liver disease (e.g. active viral hepatitis) and bacterial contamination.
If the specimen is lipaemic or contaminated then the latex agglutination method may result in a false positive result.
If required then other streptococcal antigens may be tested e.g. antistreptococcal hyaluronidase and antideoxyribonuclease.