Serum or Plasma Immunoassay for Anti-Streptolysin O Antibodies (ASO)

When and Why Is This Test Done?

Your healthcare provider may order an ASO test if you’ve recently had symptoms like sore throat, fever, or skin rashes that could suggest strep throat or another streptococcal infection. It’s also used to check for recurrent infections or to monitor treatment effectiveness. The test is typically done on serum or plasma, collected from a blood draw.

How Is the Test Performed?

A small amount of blood is drawn from a vein in your arm. The blood is then processed to separate the serum or plasma, which is tested for ASO antibodies using an immunoassay method. This process detects the presence and concentration of antibodies without needing to culture the bacteria.

What Do the Results Mean?

Elevated ASO levels (above normal) may indicate recent or ongoing Group A Streptococcus infection. However, results can vary, and they’re often interpreted alongside clinical symptoms and other tests. False positives can occur in some conditions, so results should be reviewed by your healthcare provider.

What Are Common Causes of Abnormal Results?

High ASO levels are most commonly associated with untreated or recent Group A Streptococcus infections. Rarely, they can be elevated in other conditions like rheumatic fever, certain autoimmune diseases, or after antibiotic treatment for strep infections. Other infections or illnesses may also cause temporary fluctuations.