Leukocytes in urine

  Our daughter was diagnosed with mild hydronephrosis last summer.  
  The urologist is monitoring her for now with monthly urine checks
  with our pediatrician's office.  Every month her urine has had
  leukocytes (spelling?) so they culture it overnight.  The culture comes
  back fine.  Why does she always have leukocytes and is this
  normal?  She goes back to the urologist this summer for another
  ultrasound and checkup.

[hfhs M.D.-AK]

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Dear Jill,
Leukocytes are simply white blood cells.  They are often a sign of infection and that is why your pediatrician continues to culture the urine.  Pyuria is defined as 8 or more leukocytes per cubic millimeter, this translates into 2-5 leukocytes per high power field on a slide.(microscopic evaluation)
The assessment of pyuria however is imperfect.  The leukocyte count on a routine urinalysis can be affected by factors that alter the concentration of the urine.  Furthermore, error may be introduced by variation in the amount of fluid that is eventually placed on the slide.
A second test that assesses the leukocyte esterase content on a rapid dipstick test is also used to predict pyuria.  This dipstick test is 94% specific  with  2-4% false positives.  Substitution of this test for the gold standard microscopic evaluation may prove to be cost effective, yet , other information on the microscopic evaluation may be lost.
When in doubt a culture is sent, this seems to be the case at your pediatrician