Explanation

The diagnostic test typically used for rheumatic fever is ASO (Anti-streptolysin O) titer.

Rheumatic fever is an autoimmune inflammatory disease

That can develop after a group A streptococcal infection, such as strep throat. 

The ASO titer measures the antibodies produced in response to a recent streptococcal infection.

Elevated ASO titers indicate recent streptococcal infection and are a key diagnostic marker for rheumatic fever.