Can a Silent Kidney Infection or Genetic Predisposition Underlie Recurrent UTIs?

, University of Calgary and Alberta Children's Hospital


Medscape General Medicine. 1996;1(1) 

In This Article

Laboratory Diagnosis of UTI

A urinalysis documenting the presence of pyuria (ie, WBCs in the urine) and bacteriuria suggests a clinical diagnosis of UTI. A microscopic full urinalysis should also be done whenever a urine culture is ordered to accurately confirm pyuria and bacteriuria as well as to check the urine osmolality. The laboratory cut-off values for pyuria and bacteriuria in unspun and spun urine specimens are shown in Table III. Many physicians do not perform direct microscopy on urine in their offices, but instead use urine dipsticks that incorporate the Griess test for detecting bacteriuria and the leukocyte esterase (LE) test for detecting pyuria. The Griess/LE urine dipstick is a rapid method with good sensitivity and specificity for confirming infection in this population.[23]

A specimen for culture is generally collected as a midstream urine (MSU) sample. Patients must be instructed on the proper technique for collecting a clean-catch MSU sample in order to minimize the contamination of the sample by the normal periurethral bacterial skin flora. Urine may either be directly inoculated onto agar using a dipped-paddle culture method or immediately planted to agar plates using quantitative calibrated loops (ie, 0.01ul and 0.001ul volumes). Most microbiology laboratories will fully identify a single uropathogen in significant numbers (ie, colony count >105 CFU/mL [>108 CFU/L]). However, it has been well documentedthat women with uncomplicated UTIs may have bacteriuria at lower colony counts of 103 or 104 CFU/mL.[23,24] If an appropriate clinical history (ie, "patient has signs of uncomplicated cystitis") is provided, the laboratory will proceed to fully identify a potential uropathogen and provide an antibiotic susceptibility result when the colony count is 103 CFU/mL or higher.[25] If 2 or more organisms are cultured in significant numbers from urine in this population, most laboratories will not identify the organisms; instead the lab will report mixed growth.

A urine osmolarity is also needed to interpret the colony count from the culture accurately. Because many women drink an increased amount of fluid to alleviate some of the symptoms of a UTI, their urine may be diluted compared with normal urine. Dilution in some cases may be great enough to lower the "true" urine bacterial colony count so as to not be significant (ie, the colony count will not reach 105 CFU/mL).