How to Interpret Antistreptolysin Latex Results

The antistreptolysin O (ASO) latex test indicates the presence of antibodies targeted against the toxin streptolysin O produced by streptococcal bacteria. Physicians request an ASO latex test when a patient has recently had a strep infection and is presenting with symptoms of rheumatic fever or glomerulonephritis, kidney infection. The ASO latex test will identify levels at or above 200 IU/mL, which is considered the upper limit for normal levels. Interpreting the antistreptolysin O latex test will reveal the presences of a possibly high level of antistreptolysin O but does not indicate the exact blood titer. Additional tests are necessary for a specific diagnosis.

Things You'll Need

• Positive ASO control
• Negative ASO control
• Test sample

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Instructions

• 1

  Compare unknown sample with the positive control. The positive control contains at least 200 IU/mL, the minimum level of ASO necessary to trigger a positive reaction. The ASO present in the positive control will cause the latex particles to clump or agglutinate, forming a single white dot in the center of the test area. If both the positive control and the unknown sample have a white dot, the unknown sample contains a minimum of 200 IU/mL, indicating the need for further testing.

• 2

  Compare unknown sample with the negative control. The negative control contains less than 200 IU/mL of ASO. The exact amount varies depending on the manufacture of the ASO kit. The latex particles in the negative control will remain separate, and the test sample will have a milky appearance. If both the negative control and the unknown sample are milky, the unknown sample contains less than of 200 IU/mL.

• 3

  Consider factors for false negative or positive. Children between six months and two years may give false negative reactions because their immune systems are not mature enough to produce the level of antibody necessary to trigger a positive result. In addition, early stages of a streptococcal infection do not produce high enough levels of ASO to trigger a positive. False positives may occur in persons whose normal levels are greater than 200 IU/mL and for any Group A streptococcal infections, not just rheumatic fever and glomerulonephritis.