A urinary tract infection (UTI) refers to an infection anywhere in the urinary system in the presence of bacteriuria and symptoms. Cystitis is a lower tract infection of the bladder. Pyelonephritis is an upper tract infection of the kidney. An uncomplicated UTI occurs in patients without comorbidities and with an anatomically and functionally normal urinary tract. Complicated UTI occurs in patients with a functional or anatomic abnormality of their urinary tract or with the presence of serious comorbidities that place the patient at risk for serious adverse outcomes. These comorbidities include pregnancy, diabetes, immunocompromise, cancer, advanced age, and recent hospitalization or instrumentation. Anatomic factors that cause obstruction of urine flow resulting in complicated UTI include prostate enlargement, renal stones, obstructing tumors, and ureteral reflux, compression, or stricture.
UTI is one of the most common bacterial infections. In 2007, nearly 1.7 million UTIs were diagnosed in U.S. emergency departments (EDs), and 12% required hospital admission. Neonates, girls, and young women are at increased risk for infection. UTI is uncommon in young men; however, men older than 55 years have an increased risk due to incomplete bladder emptying from prostatic hypertrophy. UTI is the leading cause of sepsis in the elderly and the most common hospital-acquired infection.
The bacterial organisms that usually cause UTI are the enteric flora that colonize the perineum. Gram-negative aerobic organisms and Escherichia coli are the most common, causing more than 80% of infections. Staphylococcus saprophyticus has the ability to adhere to urinary tract tissue, even with normal urinary flow, and causes 10–15% of UTIs. Other less common causative bacteria include the gram-negative species Klebsiella, Proteus, Serratia, and Pseudomonas.