ASO titer

Alternative names
Anti-streptolysin O titer

This is a blood test to measure anti-streptolysin O (ASO) antibodies.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

The blood is then sent to the laboratory for analysis.

How to prepare for the test
Fast for 6 hours before the test. Interfering factors such as high levels of beta lipoproteins can affect the test results.

For infants and children:
The preparation you can provide for this test depends on your child’s age and previous experiences. For specific information regarding how you can prepare your child, see the following topics:

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel 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 bacterium responsible for diseases such as glomerulonephritis, rheumatic fever, bacterial endocarditis, and scarlet fever.

ASO testing demonstrates the presence of antibodies generated by the body against the enzyme streptolysin O, which is produced by the bacteria and which causes destruction of red blood cells. The ASO antibody may be detected in the blood for weeks or months after the primary infection has been eradicated.

Normal Values

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

What abnormal results mean

Elevated levels may indicate:

  • active streptococcal infection  
  • bacterial endocarditis  
  • post-streptococcal glomerulonephritis  
  • rheumatic fever  
  • scarlet fever

What the risks are

  • excessive bleeding  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Special considerations

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.

Johns Hopkins patient information

Last revised: December 2, 2012
by Arthur A. Poghosian, M.D.

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