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Alternate Names

  • anti-streptolysin O


This test detects an antibody known as ASO, or anti-streptolysin O. This antibody appears when a person has certain bacteria in the blood. The bacteria are Group A streptococcus, which make an enzyme called streptolysin O. This enzyme destroys red blood cells.

Who is a candidate for the test?

This test can be used to:
  • confirm a recent or ongoing infection with Group A streptococcus
  • help diagnose rheumatic fever or a streptococcal infection in the kidneys
  • distinguish whether a person has rheumatic fever or arthritis from another cause

How is the test performed?

To do the test, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into the vein chosen and the tourniquet is removed. Blood flows from the vein through the needle. It is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

What is involved in preparation for the test?

A person should request specific instructions from his or her healthcare professional.

What do the test results mean?

Test results report the number of Todd units of antibodies per milliliter of blood. Normally, ASO antibody levels are less than 160 Todd units per milliliter.
Higher-than-normal levels of ASO antibodies may indicate:
  • scarlet fever, a streptococcal infection involving the throat and tongue that causes fever, rash and rapid pulse
  • other active or previous streptococcal infection
  • bacterial endocarditis, an inflammation in the lining of the heart
  • post-streptococcal glomerulonephritis, an infection in the kidney
  • rheumatic fever, a disease that may affect the heart and kidneys
  • scarlet fever, an infection involving the throat and tongue that causes fever, rash, and rapid pulse


Tabers Cyclopedic Medical Dictionary, F.A.Davis, 1993

Illustrated Guide to Diagnostic Tests, Springhouse, 1998

Mosby's Manual of Diagnostic and laboratory Tests, Kathleen Pagana and Timothy Pagana, 1998

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