The acidity of urine helps to prevent infection in the urinary tract. As urine sits, however, the pH increases which makes the patient more susceptible to bacteria growth. Therefore, the postvoid residual must be evacuated. Although accurate intake and output measurement is important, this isn’t the rationale for performing an invasive procedure. An overfull bladder can lead to hydronephrosis, but postvoid residual volume indicates urine that is left over after voiding, not necessarily an overfull bladder. The mucosa of the bladder is protected from the acidic urine.