The Full form of ASO is Antistreptolysin O. ASO titer is a blood test done to measure antibodies against Streptolysin O, a substance produced by group A streptococcus bacteria. Antibodies are proteins our bodies produce when they detect harmful substances, such as bacteria, virus. You will need the test when you have symptoms of a previous infection by group A streptococcus. Some illnesses caused by this bacteria are bacterial Endocarditis, an infection of the inner lining of your heart, a kidney problem called Glomerulonephritis, Rheumatic fever, which can affect the heart, bones, or joints, or bones, scarlet fever and strep throat. The ASO antibody may be found in the blood weeks or months after the strep infection has gone away. A negative test result means that you do not have strep infection. Your doctor may do the test again in 2 to 4 weeks. At times, a test that was negative in the first time may be positive (meaning it finds ASO antibodies) when done again. Normal value ranges may vary slightly. Talk to your doctor about the meaning of your test results. A positive or abnormal test result means you recently had a strep infection, even if you had no symptoms.