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Background
Antistreptolysin O Titer (ASO) test is a blood test which is used to check the infection which is caused by Streptococci. Streptococci bacteria are a gram-positive bacteria. Streptococci contains many immunological groups like groups A, B, C, D, F, G, and H. Groups A, G, and C produce the streptolysin O enzyme. This enzyme is an oxygen-labile hemolytic exotoxin. The main function of the streptolysin O is to cause hemolysis of the RBC (beta-hemolysis).
When the infection occurs in the body by the groups A, G, and C, then the antibodies ASO are produced against the streptolysin O toxin. ASO titer test measures the antibodies in the serum of the blood. After the streptococcal infection occurred, the antibodies level increased within 1 to 3 weeks. It reaches to peak level within 3 to 5 weeks. After 4 to 12 months, this level insignificantly decreases. A positive test can determine the current infection, and this will help to diagnose the complication of post-streptococcal infection. Increasing the titer value is an indicator of the infection and it is better than the single test. It is important to repeat the test after the 10 days of the first test.
Many people are infected to these bacteria and not having any symptoms. The ASO presence is not an indicator of the disease in that cases but a toter value greater than the 166 Todd units indicate an increased and positive test result of ASO.
Indications/Applications
A positive test result of ASO confirms the post infection of streptococcus. It is necessary to diagnose the post streptococcal infection like pediatric autoimmune neuropsychiatric diseases associated with streptococcus (PANDAS), poststreptococcal glomerulonephritis, and rheumatic fever (RA). About 20 % of patients who are having acute rheumatic fever have a negative ASO titer result.
The ASO test is performed in the conditions like:
Bacterial endocarditis
Glomerulonephritis
Scarlet fever
Rheumatic fever
Strep throat
Clinical Significance:
By suppressing the streptococcal antibodies responses, antibiotics may lead to the falsely negative result. It increased the beta lipoprotein level. Tuberculosis and liver disease may also lead to falsely positive results. So, it is suggested to repeat the test in 10 days.
Only ASO titer test can detect 80% of the streptococcal infections while with the other test like anti-DNase B (anti-deoxyribonuclease B) can detect 95% of the infections.
Reference Range
The ASO titer test measures the antistreptolysin O antibodies level in the plasma of the blood.
The normal result of the ASO titer in adults and elders is less than or equal to 160 Todd units/mL.
In newborns, the normal titer value is similar to the mother’s value. In children with age of 6 months to 2 years have normal value is less than or equal to 50 Todd units/mL, with age of 2 to 4 years, it is less than or equal to 160 Todd units/mL, and age of 5 to 12 years, it is 170 to 330 Todd units/mL.
Interpretation
If the normal value is less than 166 Todd units/mL. in adults, it indicates a negative test result.
A positive test may indicate current or recent infection of streptococcal groups like A, G, and C. This infection is associated with the conditions like toxic shock syndrome, scarlet fever, upper respiratory infections. It also supports to detect the complications of post-streptococcal infection like rheumatic fever, glomerulonephritis.
It is important to keep in mind that only 80 to 85% of streptococcal infection caused by group A give the positive result. It is not necessary to exclude the diagnosis if the result is negative.
Collection And Panels
Sample type: Blood
Sample collection method: Routine venipuncture
Sample collection container: Red top tube
Sample volume: 7 mL
Considerations: After collecting the sample, keep the tube in a biohazard bag, then transfer the tube to the laboratory. Avoid tube movement because it can cause hemolysis of RBC. It is recommended to repeat the test after 10 days of the initial ASO test.
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