Antistreptolysin O (ASO) Titer

Overview

Antistreptolysin O (ASO) Titer is a serological test used to detect antibodies produced against Streptolysin O, a toxin released by Group A β-hemolytic Streptococcus (GAS) bacteria. These bacteria produce exotoxins such as Streptolysin O and Streptolysin S, which act as potent antigens capable of inducing an immune response.

Following a streptococcal infection, the body produces specific antibodies known as Antistreptolysin O (ASO). These antibodies appear 1–3 weeks after infection, peak at 3–6 weeks, and gradually decline over several months. ASO serves as a retrospective marker of streptococcal infection, aiding in diagnosing post-streptococcal complications such as rheumatic fever, glomerulonephritis, reactive arthritis, and guttate psoriasis.

Two primary methods are used for ASO detection:

1. Slide Agglutination Method – A qualitative or semi-quantitative test for quick results.
2. Turbidometric Method – A quantitative assay that measures ASO concentration based on turbidity changes.

Symptoms

ASO itself does not cause symptoms; instead, elevated ASO titers reflect recent or ongoing streptococcal infection. The underlying infection or immune reaction may produce the following symptoms:

1. Fever
2. Persistent high temperature following sore throat or skin infection.
3. Joint Pain and Swelling
4. Polyarthritis affecting multiple joints, often migrating from one joint to another (ankles, knees, wrists, or elbows).
5. Skin Changes
6. Development of small, painless nodules under the skin or rash-like eruptions.
7. Chorea (Sydenham’s Chorea)
8. Rapid, involuntary, jerky movements caused by autoimmune involvement of the nervous system.
9. Cardiac Symptoms
10. Shortness of breath, chest discomfort, or palpitations due to rheumatic heart disease.
11. Renal Symptoms
12. Edema, reduced urine output, or dark-colored urine indicating post-streptococcal glomerulonephritis.

These symptoms often prompt testing for ASO titers, especially when rheumatic fever or glomerulonephritis is suspected after a recent sore throat or skin infection.

Causes

Elevated ASO titers result from the immune response to streptococcal infection, especially those caused by Group A Streptococcus (Streptococcus pyogenes).

1. Infectious Causes:
   1. Streptococcal pharyngitis (strep throat)
   2. Scarlet feverTonsillitis
   3. Skin infections (impetigo, erysipelas)
2. Post-Streptococcal Immune Reactions:
   1. Acute rheumatic fever
   2. Post-streptococcal glomerulonephritis
   3. Reactive arthritis
   4. Guttate psoriasis

Due to antigenic cross-reactivity between streptococcal antigens and human tissues, antibodies may attack the body’s own cells, leading to non-suppurative complications.

Risk Factors

Certain groups are at greater risk of developing high ASO titers or streptococcal complications:

1. Children and Adolescents (5–15 years): Higher susceptibility to streptococcal throat infections.
2. Individuals with Recent Throat or Skin Infections: Recurrent pharyngitis or tonsillitis increases risk.
3. Genetic Predisposition: Some individuals are more prone to autoimmune reactions post-infection.
4. Poor Hygiene or Crowded Living Conditions: Favors spread of Group A Streptococcus.
5. Untreated or Partially Treated Infections: Increases risk of post-streptococcal sequelae.
6. Weakened Immune System: Greater risk of recurrent infections and exaggerated antibody response.

Prevention

Prevention focuses on early diagnosis, prompt treatment, and proper monitoring of streptococcal infections to prevent complications associated with elevated ASO titers.

1. Prompt Treatment:
   1. Early antibiotic therapy for strep throat or skin infections prevents antibody formation and post-infectious complications.
2. Testing and Monitoring:
   1. Specimen: 3 mL blood collected in a plain (red-capped) tube.
   2. Storage: Stable at room temperature for 24 hours; 2–8°C for 14 days.
   3. Interpretation:
      1. <200 IU/mL → Normal (Adults)
      2. <100 IU/mL → Normal (Children)
      3. 200 IU/mL → Suggests recent streptococcal infection
3. Follow-Up Testing:
   1. Rising ASO titers suggest an active or recent infection.
   2. Falling titers indicate resolving infection.
4. Avoiding False Negatives:
   1. Dilution testing may be required in cases of the prozone effect (excess antibodies masking results).
5. Lifestyle and Hygiene:
   1. Regular handwashing and avoiding contact with infected individuals.
   2. Maintaining good oral hygiene and completing prescribed antibiotic courses.
6. Laboratory Techniques:
   1. Agglutination: Positive result indicated by visible clumping.
   2. Turbidometry: Increased turbidity proportional to ASO concentration (75–600 IU/mL).