From the Guidelines
Anti-streptolysin O (ASO) titers are useful in diagnosing rheumatic fever and post-streptococcal glomerulonephritis (PSGN) because they provide evidence of a recent Group A Streptococcus infection, which is the triggering event for both conditions. ASO titers measure antibodies produced against streptolysin O, a toxin released by Group A Streptococcus bacteria. In rheumatic fever and PSGN, symptoms typically appear 2-3 weeks after the initial streptococcal infection, when the throat culture may already be negative. Rising or elevated ASO titers (typically >200 units in adults or >300 units in children) indicate recent streptococcal exposure, supporting the diagnosis when clinical features are present, as stated in the American Heart Association's guidelines for the diagnosis and treatment of acute streptococcal pharyngitis 1.
The test is particularly valuable because many streptococcal infections are asymptomatic or mild, and patients may not recall having a sore throat. However, ASO titers should be interpreted alongside clinical findings, as 20% of rheumatic fever patients may have normal titers, and other streptococcal antibody tests (anti-DNase B, anti-hyaluronidase) may be needed for comprehensive evaluation. Serial measurements showing rising titers provide stronger evidence than a single elevated result. The revised Jones criteria for the diagnosis of acute rheumatic fever also support the use of ASO titers in diagnosis, particularly in patients with a history of ARF or RHD, where a presumptive diagnosis can be made with documented group A streptococcal infection and 2 major or 1 major and 2 minor or 3 minor manifestations 1.
Some key points to consider when interpreting ASO titers include:
- Normal levels of ASO titers are higher among school-age children than among adults 1
- The antistreptolysin O test is usually obtained first, and if it is not elevated, an antideoxyribonuclease B test may be performed 1
- Elevated titers for both tests may persist for several months after even uncomplicated GAS infections 1
- Newer tests using latex agglutination or nephelometric assays have not been well standardized against the traditional neutralization assays 1
From the Research
Anti-ASO Titers in Diagnosing Rheumatic Fever and PSGN
- Anti-ASO titers are useful in diagnosing rheumatic fever and post-streptococcal glomerulonephritis (PSGN) because they indicate a recent streptococcal infection, which is a precursor to these conditions 2, 3.
- The ASO test measures the levels of antibodies against streptolysin O, a toxin produced by group A streptococcus, and elevated titers can confirm a recent infection 4, 5.
- In the diagnosis of acute rheumatic fever, anti-ASO titers are one of the criteria used to fulfill the Jones criteria, which are a set of guidelines used to diagnose the condition 3.
- While anti-ASO titers can be useful in diagnosing PSGN, they are not always elevated in this condition, and other tests such as anti-DNase B may also be useful 6.
- The combination of ASO and ADNaseB is the most sensitive and specific combination for identifying post-streptococcal disease, including PSGN 6.
- It is essential to interpret anti-ASO titers in the context of clinical presentation and other laboratory results, as elevated titers can also be found in other conditions and in healthy individuals 4, 5.