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Antistreptolysin O, commonly called ASO, is an antibody found in human blood produced upon an infection by Group A Streptococcus bacteria. 

In an infected individual, the Group A Streptococci produced Streptolysin O acts as a protein antigen and causes the patient’s immune system to mount a defensive response with Antistreptolysin O antibodies. A rise in ASO titer level begins about 1 week after infection and peaks 2-3 weeks later. In the absence of complications or re-infection, the ASO titer will usually fall to pre-infection levels within 6-12 months. Approximately 80-85% of the patients who demonstrate a Group A Streptococcal infection will also demonstrate an elevated ASO titer.

Group A Streptococcus has caused more widespread diseases than any other group of bacteria. Upon initial infection by Group A Streptococcus, a patient may present with a sore throat and general malaise. However, there also exists a correlation between the initial illness and development of post-streptococcal syndromes. Of these, acute rheumatic fever and acute glomerulonephritis are the most debilitating. To determine if a streptococcal infection was the root cause, an Antistreptolysin O test is performed.

ASO titer determination can help distinguish beta-hemolytic Group A Streptococcal rheumatic fever from acute rheumatic diseases. A normal ASO reference range for adults is <100 Todd units or a 1:99 dilution. The majority of physicians will order a differential diagnosis panel to be run on a sample consisting of ASO titer, C-Reactive Protein (CRP) and Rheumatoid Factor (Rf). Eighty percent of Group A Streptococcal infected patients also have elevated CRP levels greater than 1.2 mg/dL.

False positive ASO titers can be caused by increased levels of serum beta-lipoprotein produced in liver disease and by contamination of the serum with Bacillus cereus and Pseudomonas. ASO titers are elevated in 85% of patients with rheumatic fever but may not be elevated in cases involving skin or renal sequelae. A marked rise in titer or a persistently elevated titer indicates that a Streptococcus infection or post-streptococcal sequelae are present. Both clinical and laboratory findings should be correlated in reaching a diagnosis.

Group A Streptococcus – A common but virulent streptococcus that damages the tissue it infects and produces toxins that trigger a form of shock that affects the vital organs.
• Rheumatic Fever – A severe infectious disease occurring chiefly in children, characterized by fever and painful inflammation of the joints and frequently resulting in permanent damage to the valves of the heart.
Glomerulonephritis – A form of nephritis characterized by inflammation of the renal glomeruli.

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