The ASO test is primarily ordered by itself or along with an anti-DNase B to help determine whether a person has had a recent streptococcal infection. In most cases, strep infections are identified and treated with antibiotics and the infections resolve. In cases where they do not cause identifiable symptoms and/or go untreated, however, post-streptococcal complications (), namely and glomerulonephritis, can develop in some patients, especially young children. The test, therefore, is ordered if a person presents with symptoms suggesting rheumatic fever or glomerulonephritis and has had a recent history of sore throat or a confirmed streptococcal infection. Since the incidence of post-streptococcal complications has dropped in the U.S., so has the use of the ASO test
What does the test result mean?
ASO antibodies are produced about a week to a month after an initial strep infection. ASO levels peak at about 4 to 6 weeks after the illness and then taper off but may remain at detectible levels for several months after the strep infection has resolved.
If the test is negative or if ASO is present in very low concentrations, then the person tested most likely has not had a recent strep infection. This is especially true if a sample taken 10 to 14 days later is also negative or low level and if an anti-DNase B test is also negative. A small percentage of those with a post-streptococcal complication will not have an elevated ASO.
If the ASO level is high or is rising, then it is likely that a recent strep infection has occurred. ASO levels that are initially high and then decline suggest that an infection has occurred and may be resolving.
The ASO test does not predict if complications will occur following a streptococcal infection, nor do they predict the type or severity of the disease. If symptoms of rheumatic fever or glomerulonephritis are present, an elevated ASO level may be used to help confirm the diagnosis.