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.