What lab finding in the urinalysis of the 26-year-old pregnant woman indicates a urinary tract infection?

A positive nitrite count in a urinalysis is a significant indicator of a urinary tract infection (UTI). This lab finding occurs because certain bacteria, particularly E. coli, which are common culprits in UTIs, possess the enzyme nitrate reductase, allowing them to convert urinary nitrates to nitrites. Therefore, when nitrites are detected in urine, it suggests the presence of these bacteria, pointing strongly towards a UTI.

In this context, the other findings do not serve as reliable indicators for a UTI. A negative glucose finding suggests that glucose is not present in the urine, which does not help in diagnosing a UTI. Trace leukocyte esterase can indicate the presence of white blood cells, but it is not specific to infections and may occur in other conditions. Finally, 1+ proteinuria may suggest kidney issues or other conditions but is not specific for UTIs either. Thus, the presence of positive nitrites is a clear and specific marker for a urinary tract infection in this scenario.