Definition

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.

Alternative Names

ASO titer; ASLO

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

DO 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. After the test, you may have some throbbing at the site.

Why the Test is Performed

You will need the test if you have symptoms of a previous infection by group A streptococcus. Some illnesses caused by these bacteria are:

• Bacterial endocarditis, an infection of the inner lining of your heart
• Guttate psoriasis
• A kidney problem called glomerulonephritis
• 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.

Normal Results

A negative test result means that you do not have strep infection. Your health care provider may do the test again in 2 to 4 weeks. At times, a test that was negative the first time may be positive (meaning it finds ASO antibodies) when done again.

Normal value ranges may vary slightly. Talk to your provider about the meaning of your 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.

Risks

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 lightheaded
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

References

Allen CT, Nussenbaum B, Merati AL. Acute and chronic laryngopharyngitis. In: Flint PW, Francis HW, Haughey BH, et al, eds. Cummings Otolaryngology: Head & Neck Surgery. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 61.

Bryant AE, Stevens DL. Streptococcus pyogenes. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 197.

Comeau D, Corey D. Rheumatology and musculoskeletal problems. In: Rakel RE, Rakel DP, eds. Textbook of Family Medicine. 9th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 32.

Stevens DL, Bryant AE, Hagman MM. Nonpneumococcal streptococcal infections and rheumatic fever. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 269.