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Kass criterion

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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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

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