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

Blood is 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.

The blood is then sent to the laboratory for analysis.

You should not eat for 6 hours before the test.

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.

This test is used to detect prior infection by Group A Streptococcus, the bacteria responsible for diseases such as strept throat, glomerulonephritis, rheumatic fever, bacterial endocarditis, and scarlet fever.

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

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

Abnormal results may be due to:

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

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)