It may never show up
Utis is a clinical diagnosis - that means for a physician, we rely mainly on the patient's symptoms (pain on urination
, hesitancy, frequency, flank/supra pubic pain) to make a dx.
In many cases, we can grow and isolate the offending organism in the lab in <24hrs.. In other cases, it is much harder to do this (eg chlamydia, ureaplasma, mycoplasma) - sometimes resulting in a negative culture.