An antistreptolysin-O (ASO) test is a blood test that measures the amount of ASO in the blood. ASO tests are generally used to diagnose and monitor the treatment of a strep infection. A series of tests every 10 to 14 days for six weeks is often required to confirm diagnoses and monitor the progress of an infection.

ASO is an antibody generated in response to infection by a specific type of bacteria called Group A streptococci. A serious strep infection will cause ASO levels to rise significantly. Therefore, a physician may order an ASO test to determine whether the person has a condition (e.g., endocarditis or rheumatic fever) related to a serious strep infection.

The antistreptolysin-O (ASO) test is a blood test that measures levels of ASO in the blood. ASO is an antibody produced to fight streptolysin-O, an enzyme produced by a specific type of bacteria called Group A streptococci (“strep”). This bacteria is related to infections such as endocarditis and rheumatic fever. 

High levels of ASO may remain in the blood for months following the onset of infection, even after the bacteria causing the infection have been eliminated.

Several strep-related diseases can cause the release of ASO. The test is used to help diagnose or to monitor the treatment of any of the following:

• Endocarditis. A relatively uncommon but potentially dangerous inflammation, usually caused by infection (e.g., strep infection) in vulnerable areas of the endocardium, or the innermost layer of the heart’s chambers and valves. The ASO test is not used as a primary tool for diagnosis, but may be used as in a supporting role.
  
  
• Rheumatic fever. A disease that may begin with a streptococcal infection that was not properly treated with antibiotics. It can cause serious damage to the heart, joints and kidneys. About 80 percent of patients with rheumatic fever have elevated ASO titers. However, this is not enough to diagnose rheumatic fever alone and physicians will often run additional antibody tests, as well as follow established criteria to confirm the diagnosis.
  
  
• Glomerulonephritis. A type of serious kidney disease that can follow a streptococcal infection. The glomeruli (small, round filters in the kidneys) become inflamed and cannot effectively filter waste products from the bloodstream.
  
  
• Scarlet fever. A contagious childhood disease caused by streptococci and marked by a bright red rash that affects the face, throat, skin and tonsils.

ASO is always present in the body to ward off minor strep infections. Therefore, even healthy patients will have some ASO detectable in their blood. Normal levels of ASO in the blood are 120 Todd units per milliliter for adults and preschoolers, and 170 Todd units per milliliter for school-aged children, although these results may vary somewhat depending on the lab.

Factors that may interfere with the accuracy of ASO test include:

• Skin infections. A streptococcal skin infection may cause a false-negative ASO test result. 
  
  
• Diet. High levels of certain fats in the blood can affect test results. Patients are generally asked to fast for six hours prior to testing.

In addition, taking some types of the following medications may affect ASO test results:

• Antibiotics. Medications that harm or kill microorganisms and are commonly used to treat infections.
  
  
• Corticosteroids. Anti-inflammatory medications containing steroids that are similar to the naturally occurring hormone cortisone.

Elevated ASO levels always indicate some form of infection from Group A streptococci. High ASO levels are found only after a prolonged or recurrent infection. An ASO level of 250 Todd units/ml is consistent with a prior streptococcal infection such as rheumatic fever, while a level of 500 to 5,000 Todd units/ml suggests acute rheumatic fever or glomerulonephritis.

Up to 20 percent of patients with an infection caused by Group A Streptococci will have normal ASO levels (a false negative result). Therefore, repeated tests may be done to confirm initial results. 

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about the antistreptolysin-O (ASO) test:

1. Am I putting myself at any sort of risk by taking the ASO test?
   
   
2. What will the ASO test tell you about my health?
   
   
3. Do you think a strep throat infection could affect my heart?
   
   
4. Are there alternate tests available that might be more appropriate or provide more information?
   
   
5. What type of results would you expect to see from this test? What type of results would you consider abnormal?
   
   
6. Do I have any medical conditions that might affect the test results?
   
   
7. Do I need to make any advanced preparations in order to take this test? Should I discontinue the use of any medications?
   
   
8. Will I need to take any additional tests in order for you to better understand my condition?
   
   
9. How will I find out the results of the test?
   
   
10. Who can I talk to if I have questions about the test results?
    
    
11. I previously had this type of test performed at a different lab, are the results comparable?
    
    
12. Is it safe to take the ASO test if I am pregnant?