ASO (Antistreptolysin O)
Definition
A blood test that measures antibodies formed against streptolysin O, a substance produced by the group A streptococcus bacterium.
Purpose
Ordered in patients with signs of a past group A streptococcal infection: strep throat, rheumatic fever, scarlet fever, endocarditis, glomerulonephritis and guttate psoriasis.
Results
- Negative, no strep infection
- Positive, you have recently had a strep infection, even if you had no symptoms
Normal Values
Laboratory-specific; the source does not give a numerical value.
Risks and Limitations
Two important points about timing:
1. Late persistence: ASO antibodies can remain in the blood weeks or months after the strep infection has passed. In other words, a positive result does not mean active infection. 2. Early false negative: a test that is negative the first time may be positive when repeated. The physician may repeat the test after 2 to 4 weeks.
Preparation: fast for 6 hours before the test.