Feline urologic syndrome (fus).-postrenal azotemia results from either an obstruction (ie, foreign body, fus, neoplasia) in the urinary outflow tract or interruption (ie, rupture, laceration) of the urinary outflow tract leading to the escape of urine into the peritoneal cavity. early in the course of the disease, lab findings usually demonstrate an increase in both the blood urea nitrogen (bun) and creatinine values. urine specific gravity is usually normal. postrenal azotemia, however, can progress to intrinsic renal disease due to increased pressure in the urinary system or due to a sustained decrease in renal blood flow. the prognosis of postrenal azotemia is good if the underlying cause can be treated or corrected early in the course of disease.