The antistreptolysin O titer measures the level of antistreptolysin O antibodies in the blood plasma.
An antistreptolysin titer greater than 166 Todd units (or >200 IU) is considered a positive test.  This upper limit of normal may vary from lab to lab and by age, with it being higher in school-aged children. 
The normal value for adults is less than 166 Todd units, which indicates a negative test.
A positive test can indicate recent or current group A, C, and G streptococcal infection (eg, upper airway infections, scarlet fever, toxic shock syndrome) and may support the diagnosis of post-streptococcal infection complication (eg, glomerulonephritis and rheumatic fever).
Be aware that the test is positive in only 80-85% of group A streptococcal infections,  so a negative test does not necessarily exclude the diagnosis.
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Antibiotics may give false-negative results by inhibiting streptococcal antibody response, while increased beta-lipoprotein levels produce falsely positive results. 
Repeating the test 10 days after the first test is recommended. 
Streptococci are gram-positive bacteria; they have several immunologic groups named by letters A-H and K-O. These organisms produce enzymes; group C, G, and A produce the same enzyme, streptolysin O, an oxygen-labile hemolytic toxin that mainly cause hemolysis of the red blood cells.
When the body is infected with one of the above groups (C, G, or A), it produces antibodies to the streptolysin O toxin, called antistreptolysin O or ASO. ASO titer is a test that measures these antibodies in the blood serum. The antibodies level starts to rise in 1-3 weeks after streptococcal infection, peaks in 3-5 weeks, and then goes back to insignificant level over 6-12 months, so a positive test can indicate current but more recent group A, C, and G streptococcal infection and may support the diagnosis of poststreptococcal infection complication. Rising titers over time are more indicative of infection than a single test,  which is why repeating the test is recommended 10 days after the initial test.
One more point to keep in mind is that too many people are exposed to these bacteria without being symptomatic, thus the presence of ASO by itself does not indicate the disease, but a titer of more than 166 Todd units in general is considered a definite elevation and positive ASO test in adults.
Positive ASO test confirms past infection; thus it’s useful to support the diagnosis of the poststreptococcal illness when it’s suspected, like poststreptococcal glomerulonephritis, pediatric autoimmune neuropsychiatric disorders associated with streptococcus (PANDAS), and rheumatic fever. ASO titers may be negative in up to 20% of patients who develop acute rheumatic fever. [4, 5]
Increased titer can occur in healthy carrier. 
Antibiotics may give false negative results by inhibiting streptococcal antibody response, while increased Beta-lipoprotein levels, liver disease, and tuberculosis may give false positive results. Repeating test in 10 days is recommended.
Consider also performing another test like anti-DNase B (anti-deoxyribonuclease B), which, when combined with the ASO titer, can detect up to 95% of streptococcal infections compared to 80% with the ASO titer test alone.