The antistreptolysin O (ASO) titer test is a blood test that checks for a strep infection. When you come into contact with harmful bacteria, your body produces antibodies to defend itself against these bacteria. Your body produces antibodies specific to the bacteria they fight.
The ASO titer test measures antibodies produced by your body in response to a toxin known as streptolysin O. Streptolysin O is a toxin produced by group A Streptococcus (GAS) bacteria. Your body makes the antistreptolysin O antibodies when you have a strep infection caused by GAS bacteria.
Usually, when you have a strep infection like strep throat, you receive antibiotics that kill the strep bacteria. But some people don’t have any symptoms during a strep infection and may not know they need treatment. When this happens, an untreated infection can lead to future complications. These complications are known as post-streptococcal complications.
The ASO titer test can help your doctor determine if you recently had a strep infection by measuring the presence of antistreptolysin antibodies in your blood.
Your doctor will order the ASO titer test if you have symptoms of post-streptococcal complications. Some of the common complications of strep include:
- bacterial endocarditis
- rheumatic fever
The antistreptolysin antibodies peak in your system between three to eight weeks after a strep infection. Levels can remain high for several months. Your doctor can determine if your symptoms are due to a post-streptococcal complication by checking your antibody levels.
Your doctor will tell you if you need to do any special preparation for the test. For instance, you may need to refrain from eating or drinking anything for six hours before the test.
Your doctor may recommend that you stop taking certain medications before this test. Corticosteroids and certain antibiotics may reduce ASO antibody levels. This may make it difficult for your doctor to confirm your diagnosis.
Tell your doctor about all the medications you’re taking. Be sure to include both prescription and over-the-counter drugs. Don’t stop taking your medication unless told to do so.
You’ll need to give a blood sample for an ASO titer test. A nurse or lab technician will take a blood sample from a vein in your inner arm or hand. They’ll use a needle to enter your vein and draw your blood into a tube. Then they’ll send this tube to a lab for analysis. Your doctor will share your results with you.
The risks of the ASO test are minimal. You may experience some pain during or after the blood draw.
Other risks of a blood draw include:
- difficulty obtaining a sample, resulting in multiple needle sticks
- excessive bleeding at the needle site
- fainting as a result of drawing blood
- accumulation of blood under the skin, known as a hematoma
- infection at the puncture site
Generally, an ASO test value below 200 is considered normal. In children under the age of 5, the test value should be less than 100. Results will vary by laboratory.
If your results show that you have an elevated ASO value, you may have a post-streptococcal complication.
If your test is negative and your doctor still thinks you might have a post-streptococcal complication, they may order a second type of antibody test for a follow-up.
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. If both tests are negative, your symptoms are not due to a Streptococcus infection, although your doctor may order a different antibody test to be sure.
If the results of your tests show that your ASO antibodies are increasing, it’s likely that your infection is recent. Declining antibody levels suggest that your infection is getting better.