Antistreptolysin O (ASO) is a specific type of antibody the human body produces in response to a particular bacterial threat. This antibody is measured through a blood test, known as an ASO titer, which helps determine if a person has had a recent infection caused by Group A Streptococcus (GAS) bacteria. The test is not typically used to diagnose an active case of strep throat, but rather to confirm a past infection that may have led to subsequent complications. Understanding this antibody and what the test signifies is important for interpreting results.
Understanding Streptolysin O
Group A Streptococcus (S. pyogenes) is a common bacterium responsible for illnesses such as strep throat and scarlet fever. The bacteria produce several toxic substances to evade the immune system, including a protein called Streptolysin O (SLO). SLO is a hemolytic toxin, meaning its function is to break open red blood cells, allowing the bacteria to access necessary nutrients. It is also characterized as an oxygen-labile toxin, meaning its activity is diminished in the presence of oxygen.
When the body encounters the SLO toxin, the immune system mounts a defense by creating specific antibodies against it. Antistreptolysin O (ASO) is the antibody designed to neutralize the effects of SLO by binding to the protein and blocking its ability to cause cellular damage. The production of ASO is a direct immunological reaction, making its presence in the bloodstream a reliable marker of a recent encounter with the toxin-producing bacteria. Because the antibody is produced in response to the toxin rather than the bacteria itself, the ASO titer is an indirect measure of infection. High levels of ASO indicate that the immune system was recently stimulated by Streptolysin O.
The ASO Titer Test and What It Measures
The ASO titer test is a serological blood test that quantifies the amount of ASO antibodies circulating in the serum. A blood sample is drawn, typically from a vein in the arm, and then analyzed in a laboratory to determine the concentration of the antibody. The result is reported as a “titer,” which reflects the highest dilution of the patient’s serum that still exhibits the ability to neutralize the Streptolysin O toxin.
A very low or “negative” ASO titer indicates that the individual has not had a recent GAS infection. Conversely, an “elevated” or “positive” titer suggests recent exposure to the bacteria. While normal ranges vary between laboratories and age groups, a titer exceeding a certain threshold (often around 200 International Units per milliliter (IU/mL) in adults) is considered presumptive evidence of a recent infection.
The timing of the antibody response defines the ASO test’s utility. ASO antibodies begin to rise one to three weeks after the initial strep infection, reaching their peak level about three to five weeks following the acute illness. This delayed peak means the test is not suitable for diagnosing a current case of strep throat. Instead, it is useful for confirming a past infection that may have gone unnoticed or untreated.
After peaking, ASO levels gradually decline but can remain detectable for several months, or even up to a year. Because a single elevated result can persist long after the infection has resolved, a rising titer is more informative than a single measurement. Clinicians often order two separate ASO tests, taken 10 to 14 days apart, to determine if the antibody level is actively increasing. This provides stronger evidence of a recent infection, confirming exposure without predicting complications or severity.
Post-Streptococcal Conditions Confirmed by ASO
The primary reason for performing the ASO titer test is to investigate symptoms suggestive of immune-mediated conditions that follow an untreated GAS infection. These conditions, known as non-suppurative sequelae, are caused by the immune system’s reaction rather than the bacteria themselves. The ASO test provides the laboratory evidence necessary to link these inflammatory diseases back to a recent strep exposure.
Acute rheumatic fever is an inflammatory disorder that can affect the heart, joints, brain, and skin. The inflammation results from an autoimmune process where antibodies, initially created to fight the strep bacteria, mistakenly attack similar proteins in the body’s own tissues, particularly the heart valves. A highly elevated or rising ASO titer provides strong support that a recent GAS infection triggered this immune response.
Another complication is post-streptococcal glomerulonephritis, a disorder causing inflammation in the small filtering units of the kidneys. This condition typically appears one to three weeks after a strep throat infection or three to six weeks after a strep skin infection (impetigo). A positive ASO test, especially when accompanied by other laboratory findings like low complement levels, helps establish the recent GAS infection as the underlying cause of the kidney damage.
An elevated ASO titer is significant evidence that helps confirm a diagnosis of these post-streptococcal conditions when clinical symptoms are present. Up to 20% of patients who develop acute rheumatic fever may not have an elevated ASO titer, necessitating additional antibody testing. The ASO test supports the diagnosis by confirming the preceding infection, highlighting the importance of timely strep treatment to prevent these delayed consequences.