What lab finding is most indicative of rheumatic fever?

Elevated anti-streptolysin O titers are most indicative of rheumatic fever because they reflect an immune response to a previous infection with Group A beta-hemolytic Streptococcus (GAS). This elevation occurs due to the production of antibodies against streptolysin O, a toxin produced by the bacteria. In the context of rheumatic fever, which can develop as a complication of untreated strep throat, elevated titers indicate that the patient's immune system has been activated in response to the streptococcal infection, leading to the characteristic inflammatory manifestations associated with the disease, such as carditis, polyarthritis, and chorea.

Although elevated C-reactive protein levels and an increased white blood cell count are associated with inflammation and infection, they are not specific indicators for rheumatic fever itself. A positive throat culture confirms the presence of the bacteria but does not necessarily demonstrate the sequelae of rheumatic fever, which occurs after the initial infection has resolved. Therefore, elevated anti-streptolysin O titers are the most specific lab finding associated with the immunological response leading to rheumatic fever.