An ASO (Anti-Streptolysin O) test measures the level of antibodies produced by the body in response to an infection caused by Streptococcus bacteria. These bacteria can lead to throat infections, scarlet fever, or skin infections. When left untreated, they may cause complications such as rheumatic fever or post-streptococcal glomerulonephritis. Elevated ASO levels help doctors confirm a recent streptococcal infection, especially when symptoms like joint pain, fever, rash, or swelling appear.

For the ASO test, a small blood sample is collected from a vein, and no fasting or special preparation is required. This test is often used along with CRP and ESR to understand the severity of inflammation or infection. ASO levels typically rise 1–3 weeks after infection, so repeat testing may sometimes be recommended to track changes.