The Antistreptolysin O (ASO) screen is a blood test that measures the concentration of antibodies produced in response to a specific bacterial toxin released by Group A Streptococcus (GAS) bacteria. GAS causes common infections like strep throat. The primary function of the ASO screen is not to diagnose an active strep infection, but rather to provide evidence of a recent infection. It helps determine if a strep infection occurred weeks or months ago, even if the initial illness was mild or went unnoticed.
When Doctors Order the ASO Screen
Doctors typically order an ASO screen when a patient presents with symptoms that suggest a complication following a recent GAS infection. The ASO test is used to investigate potential post-streptococcal conditions that develop after the initial infection has cleared.
The test provides supporting evidence for the diagnosis of two major post-streptococcal illnesses: Acute Rheumatic Fever (ARF) and Post-streptococcal Glomerulonephritis (PSGN). ARF is an inflammatory condition that can affect the joints, skin, brain, and heart, often appearing two to four weeks after an untreated strep infection. PSGN is an inflammation of the small filtering units in the kidneys, which may occur about one to three weeks following a strep throat or skin infection.
Since these complications develop after the bacteria are often gone, the ASO test confirms the preceding event: a recent strep exposure. A high or rising ASO level, when combined with the specific symptoms of ARF or PSGN, helps link the current illness to a past GAS infection. This information is valuable because many strep infections that trigger these conditions are asymptomatic or so mild that the patient does not recall having been sick.
How the Test Measures Antibodies
The ASO screen measures the immune response to Streptolysin O, a specific toxin produced by Group A Streptococcus bacteria. When GAS infects the body, the immune system creates Antistreptolysin O antibodies to neutralize it. The test is performed on a standard blood sample, typically drawn from a vein in the arm.
The laboratory procedure measures the concentration, or titer, of these specific antibodies in the blood serum. ASO antibodies usually begin to appear about one week after the infection starts. Their levels generally peak between three and five weeks following the initial illness.
After peaking, the antibody levels gradually decrease over several months, but they can remain detectable for a long time. Doctors may order a second ASO test two to four weeks after the first, known as serial testing, to determine if the titer is rising or falling.
Decoding Your ASO Titer Results
The result of the ASO screen is reported as a titer, a numerical value representing the concentration of Antistreptolysin O antibodies in the blood. A titer above the established normal range suggests the patient has had a recent Group A Streptococcus infection. Normal ranges vary depending on the testing laboratory and the patient’s age, with children often having higher normal limits than adults.
A negative or low ASO titer typically means there has been no recent GAS infection, or that the infection occurred long ago and antibody levels have returned to baseline. A small percentage of people who develop post-streptococcal complications, particularly after a skin infection, may not have an elevated ASO titer. In these situations, other antibody tests, such as the anti-DNase B titer, may be used to increase diagnostic sensitivity.
An elevated ASO titer indicates a recent strep exposure, usually within the past few months. This positive result supports a diagnosis of a post-streptococcal complication like ARF or PSGN if the patient is exhibiting corresponding clinical symptoms. An elevated ASO level alone does not confirm a diagnosis, as some healthy individuals may have persistently high titers without active disease. The results must always be interpreted by a healthcare provider alongside a thorough review of the patient’s symptoms and other inflammatory markers.