UTI is most commonly caused by bacteria, particularly gram-negative pathogens that normally inhabit the gastrointestinal (GI) tract, such as Escherichia coli (E. coli) and Klebsiella (Table 41-1). Acute pyelonephritis refers to involvement of the renal parenchyma, which can occur due to reflux of infected urine from the bladder, ureteral obstruction or hematogenous seeding during periods of bacteremia (e.g., staphylococcus), or fungemia (e.g., candida). There are many conditions and anatomic defects that put patients at risk for recurrent UTI and pyelonephritis (Table 41-2). Such patients, many of whom are children, are at risk of developing parenchymal scarring, renal failure, and hypertension later in life.

urinary tract infection. The term lower UTI is used to describe infection of structures distal to the ureters (bladder, urethra), whereas upper UTI refers to infection of the ureters and/or kidneys. The signs and symptoms of UTI often point toward the area and degree of involvement. Isolated bacteriuria is often asymptomatic, while symptoms of external urinary dysuria suggest urethritis. Increased urinary frequency, internal dysuria, suprapubic pain and cloudy urine or hematuria are suggestive of cystitis. Although costovertebral tenderness and flank pain have classically been associated with upper UTI (i.e., pyelonephritis) they are in fact nonspecific findings that commonly accompany lower UTI. When fever, rigors, and other signs of sepsis are also present, all of the symptoms mentioned become much more specific for acute pyelonephritis. Common laboratory findings include pyuria, hematuria, white blood cell (WBC) casts, bacteriuria, leukocytosis, and other signs of acute infection. Elevations in creatinine levels and other signs of renal impairment and hypertension are suggestive of chronic renal pathology (including chronic pyelonephritis), but are unlikely to be caused by acute infection. Children and neonates pose a diagnostic challenge in that they typically present with nonspecific symptoms such as lethargy, irritability, diarrhea, and fever.

“Complicated” cases are those at high risk of failing treatment or developing serious complications, usually due to an underlying condition such as urinary tract obstruction. Generally speaking, an “uncomplicated” UTI is one in a young, healthy, nonpregnant woman and a “complicated” UTI is one occurring in anyone else. It is important to note that most episodes of pyelonephritis represent uncomplicated “upper” UTIs and will respond well to treatment.

Acute pyelonephritis is typically a clinical diagnosis confirmed by imaging studies, whereas chronic UTI is a radiologic diagnosis based on the demonstration of clubbed calyces associated with focal or diffuse renal scarring. It develops as a result of repeated subacute infection of the renal parenchyma as seen in children with vesicoureteral reflux (VUR). Over the course of multiple infections, the renal parenchyma becomes increasingly scarred, putting the patient at risk for poor renal function and the development of hypertension.

Differential diagnosis includes both upper and lower UTI, urethritis, vaginitis, and nephrolithiasis.