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.