Antistreptolysin O Titer (ASO) Test

Antistreptolysin O (ASO) titer is a blood test to measure antibodies against streptolysin O; a substance produced by group A Streptococcus bacteria. 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. When you are exposed to harmful bacteria, your body produces antibodies. These antibodies help protect your body against future infections from the same bacteria. They are specific to the bacteria they were produced against. These antibodies can be detected using blood tests. One such blood test is an antistreptolysin O titer (ASO) test. The ASO test measures antibodies against streptolysin O. These antibodies are made when you are exposed to group A Streptococcus (GAS). Symptomatic infections caused by GAS are treated with antibiotics. However, many people don’t have symptoms. Therefore, their infections may not be detected and treatment may not be provided. Complications—referred to as post-streptococcal complications—can arise if GASremains in your body. The ASO titer test is essential in detecting and treating GAS (group A Streptococcus).

Your doctor will order the ASO test if you have symptoms of poststreptococcal complications. Such complications include:

• bacterial endocarditis
• glomerulonephritis
• rheumatic fever
• scarlet fever
• strep throat

The antistreptolysin antibody remains in the body four to six weeks after you contract GAS bacteria. Your doctor can determine if symptoms are being caused by a post-streptococcal complication by checking antibody levels.

Generally, an ASO test value below 200 is considered normal. In preschool-aged children, the test value should be less than 100. However, specific results will vary by laboratory. If your results show that your ASO value is elevated, you may have a post-streptococcal complication. Your doctor may repeat the test within 10 to 14 days to confirm your results. The body produces ASO antibodies within a week after infection with GAS. If both tests are negative, your symptoms are probably not caused by a Streptococcus infection. If the results of your tests show that your ASO antibodies are increasing, it is likely that your infection is recent. Declining antibody levels suggest that your infection is probably getting better.