ASO Titer (Antistreptolysin O titer)

Sample

1. The test is done on the serum of the patient (clotted blood 3 to 5 ml).
2. A random sample can be taken.
3. Avoid hemolysis.

Purpose of the test (Indications)

1. To diagnose exposure to Streptococcal infection.
2. To diagnose poststreptococcal diseases like:
   1. Glomerulonephritis.
   2. Rheumatic fever.
   3. Bacterial Endocarditis.
   4. Scarlet fever.
3. A significant level was seen in glomerulonephritis and rheumatic fever.

Precautions

1. Avoid the drug-like steroid and antibiotics, decrease the ASO titer.
2. Increased beta-lipoprotein inhibits streptolysin O and gives a false high ASO titer.
3. Early injection of penicillin prevents the rise of titer.

Principle

1. Lancefield group A streptococci are also known as streptococcus pyogenes.
2. Group A β-streptococci produce various infections, the most common is acute pharyngitis.
   1. It can produce scarlet fever, erysipelas, vesicular cellulitis, and puerperal fever.
   2. There are few reports showing conditions like staphylococcal toxic shock syndrome.
3. Beta-hemolytic group A streptococci produce many enzymes that can contribute to their pathogenicity. Two of these are:
   1. Streptolysin O:
      1. O stand for the oxygen-labile as it is inactivated by oxygen.
      2. This enzyme destroys the red blood cells and white cells. This is why it is called β-hemolytic group A streptococci and has the ability for beta-hemolytic ability.
      3. This enzyme is antigenic.
      4. Patients develop an anti-streptolysin antibody (ASO-antibody).
   2. Streptolysin S:
      1. The “S” stands for oxygen stable.
      2. This can cause beta hemolysis.
      3. This is not antigenic.
4. ASO is measured by latex agglutination or haemagglutination procedure.
5. This test will measure the antibodies against Streptolysin O, antigenically produced by the Group A beta-hemolytic streptococci.
6. RBCs are added as an indicator system.
   

   SO causes hemolysis of indicator system RBCs

7. The ASO antibody may be found in the blood weeks or months after the infection has gone away.

Pathophysiology

1. Group A beta streptococci can be isolated from throat or nasopharynx culture in 15% to 20% (range 11% to 60%) of the normal children.
   1. Taking two swabs instead of one will increase the positivity of the culture.
   2. Group A streptococci are always sensitive to penicillin, in vitro sensitivity testing.
2. Streptolysin O is an extracellular toxin produced by many of the streptococci groups, particularly by group A β-hemolyticus streptococci.
   1. Streptolysin O has the ability to destroy (hemolysis) the red blood cells.
3. The Streptolysin O enzyme is antigenic, and the immune system will produce antibodies as the Antistreptolysin O antibody (ASO).
   

   Formation of ASO-antibodies
4. ASO appears in the serum after one week to one month of streptococcal infection.
   1. This ASO is a neutralizing antibody.
5. ASO has no value for the diagnosis of acute streptococcal infection.
6. The serial rising titer of ASO over weeks followed by a slow fall in titer is more significant for streptococcal infection.
   1. After 6 months, only 30% of the patient show abnormal titer.
7. The highest level of ASO is seen in glomerulonephritis and rheumatic fever.
   1. A fourfold rise in the titer is significant for acute and convalescent patients.
   2. The 500 to 5000 Todd units/mL level suggests acute post-streptococcal glomerulonephritis, rheumatic fever, or acute poststreptococcal endocarditis.
   3. 15% of the active Rheumatic fever patients show test negative.

Procedure for ASO antibody titer:

1. The rising titer is more significant than a single test.
2. It is positive when it is >250 IU. Positive levels are from 400 to 5000 IU.
3. ASO, even in streptococcal infection, is raised in    70% to 80% of the patients.
4. The raised level of the ASO is not related to the severity of the disease, and the fall is also not related to the course of the disease.
5. 30% to 40% of patients show raised ASO in streptococcal pyoderma.
6. 50% shows raised ASO in glomerulonephritis patients, and 20% is seen in membranglomerulonephritis.

Interpretation of the ASO titer:

Clinical condition	ASO level in Todd units
Normal people	12 to 166
Active rheumatic fever	500 to 5000
Inactive rheumatic fever	12 to 250
Streptococcal upper respiratory infection	100 to 333
Acute post-streptococcal glomerulonephritis	500 to 5000
Rheumatoid arthritis	12 to 250
Collagen diseases	12 to 250

False-positive results are seen in:

1. Patients with tuberculosis.
2. Liver diseases like active viral hepatitis.
3. Bacterial contamination.
4. Latex agglutination test may give a false-positive result in the lipemic or contaminated samples.

NORMAL

Source 2

• Adult / elderly = ≥ 160 Todd unit/mL or <200 IU.
  • 6 months to 2 years = ≤50 Todd units/mL.
  • 2 to 4 years = ≤ 160 Todd units /mL
  • 5 to 12 years  = 170 to 330 Todd units/mL.
• If it is >200, then it is a significant level; it suggests recent or relatively recent group A streptococcal infection.
• Greater than 500 Todd unite indicate Acute rheumatic fever or acute post-streptococcal glomerulonephritis.
• Serial assay of ASO should be advised for rising titer.

Direct evidence of antistreptococcal antibodies for:

1. Streptococcal pharyngitis and tonsillitis.
2. Scarlet fever.
3. Erysipelas.

Indirect evidence for the diagnosis of:

1. Rheumatic fever.
2. Differential diagnosis of joint pains of rheumatoid arthritis and rheumatic fever.
3. Detection of the subclinical streptococcal infection.
4. Glomerulonephritis.

ASO antibody is positive in:

1. Streptococcal infections and ASO titer values.
   1. ASO raised titer indicates recent group A streptococcus pharyngitis within the last 2 months.
   2. Increased titer develops 7 to 14 days after the infection, rises rapidly to a peak in 4 to 6 weeks.
   3. Then it declines in next 4 to 6 months.
      

      ASO titer values in infection

2. Bacterial Endocarditis.
3. Post-streptococcal glomerulonephritis (50 to 70 % of the patients show no high titer).
4. Rheumatic fever.
5. Scarlet fever.
6. Streptococcal pyoderma.
   1. ASO is often not raised in these diseases.

Important Factors

1. An increased titer may be found in the healthy carrier.
2. Antibiotic therapy may suppress the antibody response.
3. Increased Lipoprotein levels inhibit Streptolysin O and produce false high ASO titer.

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