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In outpatient clinics, you’re sure to often encounter children like this: high fever, swollen and sore tonsils, throat that looks like streptococcal infection, and a dense red rash all over the body. After completing a routine blood test, adding an antistreptolysin O (ASO) test almost becomes a default operation.
What exactly does antistreptolysin O reflect? What factors can cause ASO to be inexplicably high or simply normal enough to cause misjudgment? Why sometimes does it not only fail to help but also mislead the diagnostic direction?
First, the conclusion: after reading the following, you will understand everything:
1 | ASO testing is not recommended for diagnosing uncomplicated acute pharyngitis |
2 | A fourfold increase in ASO in two samples collected 10–14 days apart indicates a prior streptococcal infection |
3 | The definition of elevated ASO should be determined by comparison with age-matched controls from the same geographic area |
4 | Negative ASO cannot exclude acute rheumatic fever (ARF) or post-streptococcal glomerulonephritis (PSGN) |
5 | Determination of anti-DNase B (the second streptococcal serological marker) along with ASO is recommended to improve sensitivity |
What is antistreptolysin O (ASO)?
In 1932, British pathologist and bacteriologist E.W. Todd conducted pioneering work. While studying streptococcal infections, he observed and described a key phenomenon: the serum of patients or animals previously infected with streptococci contains a substance capable of neutralizing the hemolytic activity of streptolysin O. He proved this substance was an antibody and named it antistreptolysin O [1]. Simply put, after a streptococcal infection, the bacteria produce a toxin called “streptolysin O,” and the human immune system immediately produces corresponding antibodies to combat this toxin. These antibodies are what we refer to clinically as antistreptolysin O, abbreviated as ASO.
Todd’s foundational discovery quickly became an extremely important clinical diagnostic tool—the ASO titer test. It is especially necessary for repeated testing of titers in the diagnosis and treatment of non-purulent complications after streptococcal infection such as rheumatic fever, acute post-streptococcal glomerulonephritis, arthritis, etc.
Effects of streptococcal infection on the body
Pyogenic streptococci are classified by hemolysis reaction into:
(1) Alpha (a) hemolysis (incomplete hemolysis);
(2) Beta (β) hemolysis (complete hemolysis);
(3) Gamma (γ) hemolysis (no hemolysis).
Pyogenic streptococci are usually β-hemolytic. According to group-specific antigens, they are classified into 18-20 groups (clusters or serogroups), with group A being the main pathogenic group for humans, and groups B, C, D, F, G and others also existing.
Pyogenic streptococci refer to group A streptococci, combined with hemolytic characteristics often called group A β-hemolytic streptococci, commonly abbreviated as “group A streptococci” (GAS) [2]. Group A streptococci (GAS; pyogenic streptococci) are Gram-positive β-hemolytic bacteria that can cause acute pyogenic diseases and non-purulent complications in humans, as shown in Table 1.
Table 1 Pyogenic and non-pyogenic manifestations after GAS infection [1]
In recent years, immune-related diseases implicated in pathogenesis include: narcolepsy, ankylosing spondylitis with uveitis, Henoch-Schönlein purpura, psoriasis, type 2 diabetes, among which elevated ASO levels have been reported.
Factors influencing ASO test results
Streptolysin O as an antigen triggers specific B cells to produce antistreptolysin O (ASO) antibodies. Studies on the natural history of antibody response after GAS infection show the antibody titer begins to increase within 1 week after infection, peaks at 3–6 weeks post-infection, then gradually decreases, returning to insignificant levels after 6–12 months—this is the “classic” immune response.
Clinically, the reference value of ASO is generally 0–200 IU/mL. Many factors influence this value, broadly as follows [1]:
(1) Child’s age
(2) Infection site
(3) Geographic location
(4) Season of the year
(5) Use of antibiotics or steroids
(6) Laboratory false positives and false negatives
ASO titers in children’s serum increase with age. One study [3] showed average titers of 21 IU/mL for infants under 3 years old, 211 IU/mL for children aged 7–8 years; titers in 4-year-olds had no significant difference from 5- to 6-year-olds but were significantly lower than all older age groups. In individuals aged 20 years or older, ASO titers decrease as age increases.
Many authors have studied ASO in different populations: Minnesota, USA, 333 IU/mL; another US state, 15–240 IU/mL; South Korea, 9–326 IU/mL; Mumbai, 16–305 IU/mL; different regions of India, 17–239 IU/mL; Tanzania, 10–200 IU/mL; Sweden, 18–200 IU/mL. Most of these values exceed the laboratory-defined normal level of 200 IU/mL. Therefore, significant differences in antibody titers may exist among different populations at different geographic locations, which can be attributed to local climate or socioeconomic conditions [4].
Observations show that compared to patients with pharyngeal infections, patients with impetigo usually exhibit a stronger anti-DNase B response and a weaker ASO response. This is because free cholesterol in the skin binds to streptolysin O molecules, thus reducing antigen amounts and resulting in weaker antibody titers.
One study [4] showed ASO values are significantly higher in winter and autumn, a seasonal variation also confirmed by a study conducted in Tehran. As streptococcal pharyngitis is more common in winter, higher antibody titers during that season are not surprising.
It goes without saying that the natural course of the ASO response can be altered by administration of medications, especially penicillins or cephalosporins, which reduce the overall number of responders and affect antibody responses.
Laboratory testing may have false positives and false negatives, as shown in Table 2.
Table 2 Causes of false positives and false negatives in ASO results [5]
Assigning a “normal” value to ASO is very difficult; no single value or range applies universally to all children worldwide.
Interpretation of ASO in certain diseases
Since it takes 1–2 weeks after GAS acute infection for antibodies to be produced and detected, serological testing is not used to diagnose acute GAS infection but is often used as an auxiliary diagnosis for rheumatic fever and glomerulonephritis. The most commonly used diagnostic antibodies for streptococcal infection are antistreptolysin O (ASO) and anti-DNase B (DNaseB).
Group C streptococci (GCS) and group G streptococci (GGS) can also produce streptolysin O, so elevated ASO titers are not a specific indicator for GAS infection. DNaseB is a nuclease produced by GAS that is critical for bacterial immune evasion. Anti-DNase B antibody titers generally appear 2 weeks post-infection, persist longer than ASO antibodies, may not peak even after 6–8 weeks, and DNaseB is a GAS-specific component not present in GCS or GGS.
About 80%–85% of rheumatic fever patients show elevated ASO titers, so combined testing of ASO and anti-DNase B antibodies helps confirm diagnosis. If both increase, this indicates GAS infection; if only ASO increases while anti-DNase B remains unchanged, it could indicate GAS or GCS or GGS infection.
Acute post-streptococcal glomerulonephritis (APSGN) is the most common acute glomerulonephritis in children. Atypical APSGN patients are more likely to have high ASO titers, high urine specific gravity, severe hematuria, and pyuria at onset.
ASO measurement is not helpful for diagnosing streptococcal pharyngitis. For diagnosis of streptococcal pharyngitis, throat swab culture is still recommended as it identifies the streptococcal group, distinguishing whether ASO is produced by GAS or by groups C or G streptococci. Given the delayed rise of ASO titers after GAS pharyngeal infection, the World Health Organization states, “For cases of uncomplicated streptococcal upper respiratory infections, streptococcal antibody serology is not necessary.”
Accurate interpretation of ASO titers is crucial in diagnosing post-streptococcal sequelae to avoid over-diagnosis or inappropriate/overuse of antibiotics. Elevated ASO indicates previous acute exposure to a streptococcal bacterial immune memory response and does not indicate active disease, as stated in rheumatic fever diagnostic criteria: recent history of streptococcal infection. Therefore, ASO titer interpretation must be combined with clinical evaluation.
ASO titers start to rise about 1 week after infection, peak in 3 to 6 weeks, and titer magnitude is unrelated to disease severity. The decrease rate is also unrelated to disease course; thus, ASO has limited value for later follow-up.
Elevated ASO indicates past GAS infection, but the timing remains unclear. Misdiagnosis is easy in such cases. To optimize pre-GAS infection diagnosis, it is recommended to perform at least two consecutive ASO tests, along with testing the second anti-streptococcal antibody, anti-DNase B. The interval between the two ASO tests is 10–14 days. WHO recommends that only a fourfold rise in titer between two serum samples should be considered evidence of recent GAS infection.
In summary, although the diagnostic performance of ASO titer testing is ambiguous, it remains one of the auxiliary tests satisfying rheumatic fever diagnostic criteria. Despite limited evidence, ASO will continue to play a clinical diagnostic role.
Special note | This article is for reference by medical and healthcare professionals only
Author | Zhang Guangcheng
Cover image | Zcool Hailuo
First published | DXY Pediatrics Time
References (scroll to view)
[1] E S Sen, A V Ramanan. How to use antistreptolysin O titre. ARCHIVES OF DISEASE IN CHILDHOOD-EDUCATION AND PRACTICE EDITION, 2014, 99 (5): 231-238.
[2] Yu Dingle, Lu Qinghua, You Yuanhai, et al. Expert consensus on diagnosis, treatment and prevention of group A streptococcus-associated diseases in Chinese children. Chinese Journal of Practical Pediatrics, 2022, 37(21): 1604-1618.
[3] J Renneberg 1, M Söderström, K Prellner, et al. Age-related variations in anti-streptococcal antibody levels. Eur J Clin Microbiol Infect Dis. 1989 Sep;8(9):792-795.
[4] Kotby, A.A. Habeeb, N.M. El Elarab, S.E. Antistreptolysin O titer in health and disease: Levels and significance (Article). Pediatric Reports, 2012, 4(1): e8.
[5] I Geerts, N De Vos, J Frans, et al. The clinical-diagnostic role of antistreptolysin O antibodies. Acta Clinica Belgica, 2011, 66 (6): 410-415.