Sterile pyuria is a condition in which white blood cells are present in the urine without bacteria growing on the culture. In this condition, white blood cells are present in the absence of infection or proven infection. It might be only a passing thing or it can be a sign of kidney disease, tuberculosis, or lupus. There are a variety of different drugs that can also cause this to occur.

Your pediatrician is right not to ignore this. It is something that should be evaluated to look for the cause.