Test essence and diagnostic titers
An increase in the level of specific antibodies to streptococcus is observed as early as the initial stages of acute rheumatism. Among serological methods, determination of antistreptolysin O (ASO) is the most widespread.
For a single test, the following threshold values are considered diagnostically significant:
- In adults: $\ge$ 250 Todd units.
- In children over 5 years old: $\ge$ 333 Todd units.
It is important to remember that a high titer indicates the presence of or recent contact with an infection; however, it does not reflect the activity of the rheumatic process itself.
Clinical significance and limitations
The main feature of ASO is that this analysis serves as a tool for retrospective diagnostics. The antibody concentration begins to rise only on days 7–9 after the introduction of streptococcal infection. For this reason, the test is not used for the acute diagnosis of tonsillopharyngitis.
In addition, a positive result may reflect not only a current problem, but also a previously postponed disease or a chronic focus of infection.
Indications for ordering the test
Routine determination of ASO in all patients with acute tonsillopharyngitis is not recommended (level of recommendation strength C, level of evidence validity 5).
Direct indications for prescribing the test are suspicions of severe immunopathological complications:
- Acute rheumatic fever (ARF).
- Poststreptococcal glomerulonephritis.
- PANDAS syndrome.
Differential diagnosis of streptococcal infection
To confirm acute Streptococcus pyogenes (GAS) infection, other methods are used:
- Rapid diagnostics: immunochromatographic antigen detection (streptatest) allows obtaining a result in a matter of minutes at the patient's bedside.
- Culture study: nasopharyngeal or throat swab takes 2–3 days, but is a less sensitive method compared to antibody serology, although it is mandatory for confirmation in children with a negative or absent rapid test.
Sampling for culture is carried out strictly before the start of antibiotic therapy, on an empty stomach or 2 hours after a meal.
ASO is an "echo from the past": antibodies rise only by the end of the first week (days 7–9) and indicate that streptococcus has already visited, rather than how violently it is raging right now.
Class and exam questions
Can ASO be used for emergency diagnosis of tonsillitis?
No, antistreptolysin-O determination is not used for the acute diagnosis of tonsillopharyngitis, since antibodies rise only on days 7–9 of the illness.
Does a high ASO titer reflect the degree of rheumatism activity?
No. An elevated titer indicates the presence of an infection or a past disease, but it does not reflect the degree of activity of the rheumatic process.
What are the diagnostic thresholds for ASO in a single test?
For adults, a level of 250 Todd units and above is considered diagnostically significant, and for children over 5 years old — 333 Todd units and above.
What else to study on this topic
- Kissel–Jones–Nesterov criteria for rheumatic process activity
- Differential diagnosis of acute tonsillopharyngitis in children
- Rules of the pre-analytical phase when sampling for GAS
- Cure assessment criteria after acute tonsillopharyngitis
- The role of complete blood count and acute-phase reactants in inflammation
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