Antistreptolysin O titer

Antistreptolysin O titer (ASO titer) is a blood test used to measure antibodies against streptolysin O, a substance produced by Group A Streptococcus (GAS) bacteria. This test is important in diagnosing previous infections caused by GAS, such as rheumatic fever and glomerulonephritis, which are post-streptococcal complications. The presence of high levels of antistreptolysin O antibodies indicates a recent or past infection by these bacteria.

Overview

Streptolysin O is a hemolytic enzyme produced by GAS that can cause damage to red blood cells and tissues. The body's immune system responds to this enzyme by producing antistreptolysin O antibodies. The ASO titer test quantifies these antibodies to assess if a significant immune response has occurred, suggesting an infection by GAS.

Indications

The ASO titer test is indicated in patients who present with symptoms suggestive of post-streptococcal complications such as rheumatic fever, which can affect the heart, joints, skin, and brain, and post-streptococcal glomerulonephritis, a condition affecting the kidneys. It is also used to monitor the effectiveness of treatment for these conditions.

Procedure

The ASO titer test is performed on a sample of the patient's blood. The test involves the quantification of ASO antibodies through techniques such as nephelometry or enzyme-linked immunosorbent assay (ELISA). Results are typically reported in Todd units, with levels above the normal range indicating a recent or past infection by GAS.

Interpretation

Elevated ASO titers suggest a recent or past infection with Group A Streptococcus. However, it is important to note that ASO titers can also be elevated in individuals without any symptoms of a GAS infection. Therefore, the results should be interpreted in conjunction with clinical findings and other diagnostic tests.

Limitations

The ASO titer test has its limitations. It may not detect all cases of GAS infections, as some individuals may not produce a significant antibody response. Additionally, elevated ASO levels can persist for several months after an infection, making it difficult to determine the timing of the infection based solely on this test.

Conclusion

The Antistreptolysin O titer test is a valuable tool in diagnosing past or recent infections caused by Group A Streptococcus, particularly in the context of post-streptococcal complications. However, its interpretation requires careful consideration of the patient's clinical history, symptoms, and other diagnostic findings.