A 3-month-old with irritability and fever had normal CBC results but shows positive leukocyte esterase in urinalysis. What should the clinician suspect?

The presence of positive leukocyte esterase in the urinalysis indicates that there is a significant likelihood of a urinary tract infection (UTI). Leukocyte esterase is an enzyme that is released by white blood cells, and its presence suggests that there is inflammation or infection, commonly occurring in the urinary tract. In infants, UTI can present with nonspecific symptoms such as irritability and fever, which aligns with the clinical scenario described.

Given that the complete blood count (CBC) results are normal, it decreases the suspicion for systemic infections like gastroenteritis or respiratory infections, which often present with more pronounced abnormalities in blood work, such as leukocytosis or other indications of infection. Viral illnesses typically do not lead to leukocyte esterase positivity either. Therefore, the clinical signs combined with the urinalysis findings strongly direct suspicion toward a urinary tract infection.