Antistreptolysin O titer
Antistreptolysin O (ASO) titer is a blood test to measure antibodies against streptolysin O, a substance produced by group A Streptococcus bacteria.
ASO titer; ASLO
How the test is performed
A blood sample is needed. For information on how this is done, see: Venipuncture
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.
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
- Glomerulonephritis, a kidney problem
- Rheumatic fever , which can affect the heart, joints, or bones
- Scarlet fever
- Strep throat
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 have likely not had a recent strep infection. The healthcare provider may repeat the test again in 2 - 4 weeks. Sometimes a test that was first negative will come back positive.
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 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 afterward you are first infected.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
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.
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.
Nussenbaum B, Bradford CR. Pharyngitis in adults. In: Flint PW, Haughey BH, Lund LJ, et al, eds. Cummings Otolaryngology: Head & Neck Surgery. 5th ed. Philadelphia, Pa: Mosby Elsevier; 2010:chap 13.
Comeau D, Heaton K, Gordon A. Rheumatology and musculoskeletal problems. In: Rakel RE, ed. Textbook of Family Medicine. 8th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 32.
Last reviewed 5/15/2012 by 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. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.
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