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Antistreptolysin O Titer 

  • Author: Tarek Hammad, MD; Chief Editor: Eric B Staros, MD  more...
 
Updated: Nov 21, 2014
 

Reference Range

The antistreptolysin O titer measures the level of antistreptolysin O antibodies in the blood plasma.

An antistreptolysin titer greater than 166 Todd units (or >200 IU) is considered a positive test.[1] This upper limit of normal may vary from lab to lab and by age, with it being higher in school-aged children.[2]

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Interpretation

The normal value for adults is less than 166 Todd units, which indicates a negative test.

A positive test can indicate recent or current group A, C, and G streptococcal infection (eg, upper airway infections, scarlet fever, toxic shock syndrome) and may support the diagnosis of post-streptococcal infection complication (eg, glomerulonephritis and rheumatic fever).

Be aware that the test is positive in only 80-85% of group A streptococcal infections,[3] so a negative test does not necessarily exclude the diagnosis.

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Collection and Panels

Collect 7 mL of blood in a red-topped tube, place the tube in a biohazard bag, and transfer to the laboratory.[3] Avoid tube agitation because it may cause RBC hemolysis.[1]

Antibiotics may give false-negative results by inhibiting streptococcal antibody response, while increased beta-lipoprotein levels produce falsely positive results.[3]

Repeating the test 10 days after the first test is recommended.[3]

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Background

Description

Streptococci are gram-positive bacteria; they have several immunologic groups named by letters A-H and K-O. These organisms produce enzymes; group C, G, and A produce the same enzyme, streptolysin O, an oxygen-labile hemolytic toxin that mainly cause hemolysis of the red blood cells.

When the body is infected with one of the above groups (C, G, or A), it produces antibodies to the streptolysin O toxin, called antistreptolysin O or ASO. ASO titer is a test that measures these antibodies in the blood serum. The antibodies level starts to rise in 1-3 weeks after streptococcal infection, peaks in 3-5 weeks, and then goes back to insignificant level over 6-12 months, so a positive test can indicate current but more recent group A, C, and G streptococcal infection and may support the diagnosis of poststreptococcal infection complication. Rising titers over time are more indicative of infection than a single test,[1] which is why repeating the test is recommended 10 days after the initial test.

One more point to keep in mind is that too many people are exposed to these bacteria without being symptomatic, thus the presence of ASO by itself does not indicate the disease, but a titer of more than 166 Todd units in general is considered a definite elevation and positive ASO test in adults.

Indications/Applications

Positive ASO test confirms past infection; thus it’s useful to support the diagnosis of the poststreptococcal illness when it’s suspected, like poststreptococcal glomerulonephritis, pediatric autoimmune neuropsychiatric disorders associated with streptococcus (PANDAS), and rheumatic fever. ASO titers may be negative in up to 20% of patients who develop acute rheumatic fever.[4, 5]

Considerations

Increased titer can occur in healthy carrier.[3]

Antibiotics may give false negative results by inhibiting streptococcal antibody response, while increased Beta-lipoprotein levels, liver disease, and tuberculosis may give false positive results. Repeating test in 10 days is recommended.

Consider also performing another test like anti-DNase B (anti-deoxyribonuclease B), which, when combined with the ASO titer, can detect up to 95% of streptococcal infections compared to 80% with the ASO titer test alone.[3]

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Contributor Information and Disclosures
Author

Tarek Hammad, MD Resident Physician, Department of Internal Medicine, St Louis University School of Medicine

Disclosure: Nothing to disclose.

Chief Editor

Eric B Staros, MD Associate Professor of Pathology, St Louis University School of Medicine; Director of Clinical Laboratories, Director of Cytopathology, Department of Pathology, St Louis University Hospital

Eric B Staros, MD is a member of the following medical societies: American Medical Association, American Society for Clinical Pathology, College of American Pathologists, Association for Molecular Pathology

Disclosure: Nothing to disclose.

References
  1. DELMAR'S Guide to Laboratory and Diagnostic Tests. 2nd Edition. 2010.

  2. J. MH Danchin, JB Carlin,W Devenish, TM Nolan and JR Carapetis. Paediatr. New normal ranges of antistreptolysin O and antideoxyribonuclease B titres for Australian children. Child Health. 2005. 41:583–586.

  3. Manual of Laboratory & Diagnostic Tests.

  4. Hilário MO, Terreri MT. Rheumatic fever and post-streptococcal arthritis. Best Pract Res Clin Rheumatol. 2002 Jul. 16(3):481-94. [Medline].

  5. Datta G. Syncope in a child. Ann Pediatr Cardiol. 2013 Jan. 6(1):93-4. [Medline]. [Full Text].

 
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