ASO Titer (Antistreptolysin O titer)

Sample
- The test is done on the serum of the patient (clotted blood 3 to 5 ml).
- A random sample can be taken.
- Avoid hemolysis.
Purpose of the test (Indications)
- To diagnose, exposure to Streptococcal infection.
- To diagnose poststreptococcal diseases like:
- Glomerulonephritis.
- Rheumatic fever.
- Bacterial Endocarditis.
- Scarlet fever.
- A significant level was seen in glomerulonephritis and rheumatic fever.
Precaution
- Avoid the drug-like steroid and antibiotics, decrease the ASO titer.
- Increased beta-lipoprotein inhibit streptolysin O and give false high ASO titer.
Principle
- Beta-hemolytic group A streptococci produce many enzymes that can contribute to their pathogenicity.
- Streptolysin O:
- O stand for the oxygen-labile as it is inactivated by oxygen.
- 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.
- This enzyme is antigenic.
- Patients develop an anti-streptolysin antibody (ASO-antibody).
- Streptolysin S:
- The “S” stands for oxygen stable.
- This can cause beta hemolysis.
- This is not antigenic.
- Streptolysin O:
- ASO is measured by latex agglutination or haemagglutination procedure.
- This test will measure the antibodies against Streptolysin O which is antigenic and is produced by the Group A beta-hemolytic streptococci.
- RBCs are added as indicator system.
- The ASO antibody may be found in the blood weeks or months after the infection has gone away.
Pathophysiology
- Streptolysin O is an extracellular toxin produced by many of the streptococci groups, particularly by group A β-hemolyticus streptococci.
- Streptolysin O has the ability to destroy (hemolysis) the red blood cells.
- The Streptolysin O enzyme is the antigenic and the immune system will produce antibodies as Antistreptolysin O antibody (ASO).
- ASO appears in the serum after one week to one month of streptococcal infection.
- This ASO is a neutralizing antibody.
- ASO has no value for the diagnosis of acute streptococcal infection.
- The serial rising titer of ASO over weeks and followed by a slow fall in titer is more significant for streptococcal infection.
- After 6 months only 30% of the patient show abnormal titer.
- The highest level of ASO is seen in glomerulonephritis and rheumatic fever.
- A fourfold rise in the titer is significant for acute and convalescent patients.
- The level of 500 to 5000 Todd units/mL is suggestive of acute post-streptococcal glomerulonephritis, acute rheumatic fever, or acute poststreptococcal endocarditis.
- 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:
- Streptococcal infections.
- Bacterial Endocarditis.
- Post-streptococcal glomerulonephritis (50 to 70 % of the patients shows no high titer).
- Rheumatic fever.
- Scarlet fever.
- Streptococcal pyoderma.
- ASO is often not raised in these diseases.
Important Factors
- An increased titer may be found in the healthy carrier.
- Antibiotic therapy may suppress the antibody response.
- Increased Lipoprotein level inhibit Streptolysin O and produce false high ASO titer.