Antistreptolysin O (ASO) titer is a blood test to measure antibodies against streptolysin O, a substance produced by group A Streptococcus bacteria. Antibodies are proteins our bodies produce when they detect harmful substances, such as bacteria.
ASO titer; ASLO
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
You should not eat for 6 hours before the test.
How the Test will Feel
When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing where the needle was inserted.
Why the Test is Performed
This test is done if you have symptoms of a previous infection by group A Streptococcus. Some illnesses caused by this bacteria are:
Bacterial endocarditis, an infection of the inner lining of your heart
The ASO antibody may be found in the blood weeks or months after the strep infection has gone away.
A negative test result means you likely have not had a recent strep infection. Your doctor may repeat the test in 2 to 4 weeks. Sometimes, a test that was negative the first time will be positive (meaning it detects ASO antibodies) when it is done again..
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
An abnormal or positive test result means you recently had a strep infection, even if you had no symptoms.
The ASO test may stay positive (sometimes called detectable) for 2 to 4 months after you were first infected.
Veins and arteries vary in size from person to person, and from one side of the body to the other. Because of this, it may be harder to get a blood sample from some people than it is from others.
Other risks associated with having blood drawn are slight, but may include:
Excessive bleeding where the needle is inserted
Fainting or feeling light-headed
Hematoma (blood buildup under the skin)
Infection (a slight risk any time the skin is broken)
Bisno AL. Nonsuppurative poststreptococcal sequelae: Rheumatic fever and glomerulonephritis. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2009:chap 199.
Bisno AL, Stevens DL. Streptococcus pyogenes. In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2009:chap 198.
Comeau D, Heaton K, Gordon A. Rheumatology and musculoskeletal problems. In: Rakel RE, ed. Textbook of Family Medicine. 8th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 32.
Laboratory tests and interpretation of results. In: Alvero R, Borkan JM, Ferri FF, et al, eds. Ferri's Clinical Advisor, 2014. 1st ed. Philadelphia, PA: Elsevier Mosby; 2014:section IV.
Linda J. Vorvick, MD, Medical Director and Director of Didactic Curriculum, MEDEX Northwest Division of Physician Assistant Studies, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.