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The antistreptolysin O (ASO) titer test is a blood test that checks for a strep infection. When a person comes in contact with harmful bacteria, the body produces antibodies to defend itself against these bacteria. Body produces antibodies specific to the bacteria they fight. The ASO titer test measures antibodies produced by the body in response to a toxin known as streptolysin O. Streptolysin O is a toxin produced by group A Streptococcus (GAS) bacteria. The body makes the antistreptolysin O antibodies when you have a strep infection caused by GAS bacteria.
Sample Collection and Handling Only fresh serum specimens should be used. Plasma must not be used since fibrinogen may cause non-specific agglutination of the latex. It is preferable to test samples on the same day as collected. Serum samples may be stored at 2-8°C for up to 48 hours prior to testing. If longer storage is necessary, sera should be stored frozen at -20°C. Materials used in the ASO Test ASO antigen: A stabilized buffered suspension of polystyrene latex particles coated with Streptolysin O and 0.1% sodium azide as a preservative. Shake well prior to use.
Deliver one free-falling drop of positive and negative control into its identified circle. Mix the ASO latex reagent by gently shaking. Add one free-falling drop of a reagent to each control and sample. Using the flattened end of the appropriate plastic pipette as a stirrer (step 2), thoroughly mix each sample with reagent within the full area of the circle. Discard the disposable pipette. Slowly rock the slide for exactly two (2) minutes and observe for agglutination under a high-intensity light. Record results. Re-wash glass slide for future use.
Test Result A test sample is considered to contain ASO antibodies in excess of 200 IU/ml when agglutination (clumping) is observed when compared to the result of the negative control.
ASO titer should be interpreted with considerable caution, especially in the absence of a convincing history of rheumatic fever or glomerulonephritis. Limitations False-negative ASO results can occur for patients with hyperlipidemia. False-positive ASO results may occur due to cross-reactivity in patients with myeloma, hypergammaglobulinemia, liver disease, and autoimmune disease with elevated rheumatoid factor.
