Health Encyclopedia

Antistreptolysin O titer

Definition

Antistreptolysin O (ASO) titer is a blood test to measure antibodies against streptolysin O, a substance produced by Group A Streptococcus bacteria.

Alternative Names

ASO titer; ASLO

How the test is performed

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The blood is then sent to the laboratory for analysis.

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 used to detect prior infection by Group A Streptococcus, the bacteria responsible for diseases such as:

The ASO antibody may be found in the blood weeks or months after the strep infection has gone away.

Normal Values

The ASO level is usually less than 160 Todd units per milliliter.

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

Abnormal results may be due to:

Active streptococcal infection
Bacterial endocarditis
Post-streptococcal glomerulonephritis
Rheumatic fever
Scarlet fever

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)

References

Gerber MA. Group A Streptococcus. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 182.

Antistreptolysin O Titer. In: Ferri FF, ed. Ferri's Clinical Advisor 2008: Instant Diagnosis and Treatment. 1st ed. Philadelphia, Pa: Mosby Elsevier; 2008.

Review Date:8/12/2008
Reviewed By:Linda Vorvick, MD, Seattle Site Coordinator, Lecturer, Pathophysiology, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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