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.

Precaution

1. Avoid the drug-like steroid and antibiotics, decrease the ASO titer.
2. Increased beta-lipoprotein inhibit streptolysin O and give false high ASO titer.

Principle

1. Beta-hemolytic group A streptococci produce many enzymes that can contribute to their pathogenicity.
   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.
2. ASO is measured by latex agglutination or haemagglutination procedure.
3. This test will measure the antibodies against Streptolysin O which is antigenic and is produced by the Group A beta-hemolytic streptococci.
4. RBCs are added as indicator system.

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

Pathophysiology

1. 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.
2. The Streptolysin O enzyme is the antigenic and the immune system will produce antibodies as Antistreptolysin O antibody (ASO).

3. ASO appears in the serum after one week to one month of streptococcal infection.
   1. This ASO is a neutralizing antibody.
4. ASO has no value for the diagnosis of acute streptococcal infection.
5. The serial rising titer of ASO over weeks and 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.
6. 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 level of 500 to 5000 Todd units/mL is suggestive of acute post-streptococcal glomerulonephritis, acute rheumatic fever, or acute poststreptococcal endocarditis.
   3. 15% of the active Rheumatic fever patients show test negative.

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 above 200 then it is a significant level.
• Greater than 500 Todd unite indicate Acute rheumatic fever or acute post-streptococcal glomerulonephritis.
• Serial assay of ASO should be advised for rising titer.

ASO is positive in:

1. Streptococcal infections.
2. Bacterial Endocarditis.
3. Post-streptococcal glomerulonephritis (50 to 70 % of the patients shows 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 level inhibit Streptolysin O and produce false high ASO titer.

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