Kass criterion

Last reviewed 01/2018

The Kass criterion describes the likelihood of a urinary tract infection from a mid stream urine specimen. The lower limit of significance is 10 power 8 bacteria/litre in a properly collected specimen, preferably on 2 occasions, and the presence of more than 100 white cells per high power field.

This result should be applied with caution, as:

  • different bacteria multiply at different rates
  • frequency of urine can interfere with the result
  • in some women bacteria may come from a urethral lesion where bacteria may not get the opportunity to multiply
  • in men, counts may be low due to prostatic antibacterial factor
  • in some infections, for example proteus, low levels of bacteria can still predispose to stone formation