A positive reading on the Leukocytes test pad is an indication of white blood cells (WBC) in the urinary tract. White blood cells normally enter the urinary tract because of a previous infection and they are standing guard to ward off further infection.
Bacteria produces Nitrite, so there are usually traces of Nitrite found as well if the infection is still present. It is possible to have positive results on the Leukocyte value and negative on Nitrite, simply because there isn’t enough bacteria present in the urinary tract to product Nitrite.
A positive value on either Leukocyte or Nitrite should be followed up with a visit to the vet to culture the sample and determine what type of bacteria is present so that it can be treated.