The ASO test is primarily used to help determine whether a recent strep infection with group A Streptococcus:

• Is the cause of a person's or glomerulonephritis, a form of kidney disease
• Caused rheumatic fever in a person with signs and symptoms

The test may be ordered by itself or along with an anti-DNase B, another test used to detect recent strep infections.

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, complications (sequelae), namely rheumatic fever and glomerulonephritis, can develop in some people, 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.

The ASO test is ordered when a person has symptoms that a health practitioner suspects may be due to an illness caused by a previous strep infection.It is ordered when the symptoms emerge, usually in the weeks following a sore throat or skin infection when the bacteria are no longer present in the throat or on the skin.

Some symptoms of rheumatic fever may include:

• Fever
• Joint swelling and pain in more than one joint, especially in the ankles, knees, elbows and wrists, sometimes moving from one joint to another
• Small, painless nodules under the skin
• Rapid, jerky movements (Sydenham's chorea)
• Skin rash
• Sometimes the heart can become inflamed (carditis); this may not produce any symptoms but also may lead to shortness of breath, heart palpitations, or chest pain

Some symptoms of glomerulonephritis may include:

• Fatigue, decreased energy
• Decreased urine output
• Bloody urine
• Rash
• Joint pain
• Swelling (edema)
• High blood pressure

However, these symptoms can be seen in other conditions.

The test may be performed twice, with samples collected about two weeks apart, for acute and convalescent ASO titers.This is done to determine if the antibody level is rising, falling, or remaining the same.

ASO antibodies are produced about a week to a month after an initial strep infection. The amount of ASO antibody (titer) peaks at about 3 to 5 weeks after the illness and then tapers off but may remain detectable for several months after the strep infection has resolved.

A negative ASO or ASO that is present at very low titers means 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 (low titer of antibody) and if an anti-DNase B test is also negative (low titer of antibody). A small percentage of people with a complication related to a strep infection will not have an elevated ASO. This is especially true with glomerulonephritis that may develop after a skin strep infection.

An elevated titer of antibody (positive ASO) or an ASO titer that is rising means that it is likely that the person tested has had a recent strep infection. ASO titers that are initially high and then decline suggest that an infection has occurred and may be resolving.

The ASO test does not predict whether complications will occur following a strep infection, nor does it 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.

Use of some antibiotics and corticosteroids may decrease ASO antibody levels.