The ASO test is primarily ordered to help determine whether a patient 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 sequelae, namely rheumatic fever and glomerulonephritis, can develop in some patients, especially young children. The test, therefore, is ordered if a patient presents with symptoms suggesting either of these conditions. Since the incidence of post-streptococcal complications has dropped in the U.S., so has the use of the ASO test.

The ASO test is ordered when a patient has symptoms that the doctor suspects may be due to an illness caused by a previous streptococcal infection. It is ordered when the symptoms emerge, usually in the weeks following a sore throat or skin infection. The test may be ordered twice over a period of 10-14 days to determine if the antibody level is rising, falling, or remaining the same.

ASO test results can be reported in several different ways; however, the interpretation is generally the same: the higher the result is, the more antibody that is present in the blood (unless a titer is performed, which is a ratio and therefore is interpreted differently).

The ASO antibody is either absent or present in very low concentrations in patients who have not had a recent streptococcal (strep) infection. Antibodies are produced about a week to a month after the 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 patient 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 minimal.

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 severity of the disease. If symptoms of rheumatic fever or glomerulonephritis are present, an elevated ASO level may be used to confirm the diagnosis.

Some antibiotics and corticosteroids may decrease ASO antibody levels.