Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell 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.

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

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 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.

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

Elevated levels may indicate:

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

• 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

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.