What laboratory finding should be expected in a preschooler diagnosed with rheumatic fever?

In the context of rheumatic fever, one of the key laboratory findings that is expected is a positive antistreptolysin titer. This test measures the level of antibodies against streptolysin O, which is produced by Group A Streptococcus bacteria. The presence of these antibodies indicates that the child had a recent streptococcal infection, which is a significant factor in the diagnosis of rheumatic fever. Rheumatic fever often follows an untreated streptococcal throat infection, and the positive antistreptolysin titer supports the association between the previous infection and the development of rheumatic fever.

The other laboratory findings that could be present in rheumatic fever, such as changes in white blood cell count or hemoglobin levels, may occur but are not as definitive for the diagnosis as the antistreptolysin titer. Elevated white blood counts can be indicative of various infections or inflammatory conditions, while low hemoglobin levels or decreased platelet counts could be associated with other unrelated hematologic issues or diseases. Therefore, the positive antistreptolysin titer is the most specific and relevant laboratory finding in confirming a diagnosis of rheumatic fever in a preschooler.