Final answer:
The antistreptolysin O (ASO) titer indicates a relatively recent streptococcal infection by measuring antibodies against the toxin streptolysin O released by Streptococcus pyogenes, useful in diagnosing post-infection complications.
Step-by-step explanation:
An antistreptolysin O (ASO) titer indicates a relatively recent streptococcal infection. The ASO titer measures antibodies in the blood produced in response to a toxin called streptolysin O, which is released by the bacteria Streptococcus pyogenes. These antibodies can remain elevated in the blood for several weeks to months following a streptococcal infection, therefore helping to identify if an infection has occurred in a relatively recent timeframe, rather than indicating a current acute infection or a chronic one. It does not indicate carditis of acute rheumatic fever within the past 24 hours or a response to a streptococcal infection within the past 48 hours specifically. The ASO test is particularly useful for diagnosing complications such as rheumatic fever or glomerulonephritis that can develop after streptococcal pharyngitis.