Urinary tract infection (UTI) is a general term that describes infection anywhere in the urinary system. UTI can be further subclassified by the exact location of the infection, presence or absence of symptoms, and complications. In everyday practice, these classifications are subject to significant overlap and cause clinical confusion.

Definitions

• Bacteriuria – Presence of bacteria in the urine.
• Asymptomatic bacteriuria – Presence of >10⁵ colony-forming units (CFU)/mL urine in the absence of symptoms.
• Cystitis – Syndrome involving constellation of dysuria, frequency, urgency, and occasionally suprapubic tenderness; can be present in the setting of lower urinary tract inflammation in the absence of infection or urethritis.
• Acute pyelonephritis – Clinical syndrome characterized by flank pain, flank tenderness, and fever often associated with cystitis symptoms in the presence of significant bacteriuria. Triad of pyelonephritis (flank pain, fever, and nausea/vomiting) can have noninfectious etiologies like renal calculi and renal infarction.
• Uncomplicated UTI – UTI in a structurally and neurologically normal urinary tract.
• Complicated UTI – UTI in the presence of anatomical or functional abnormalities. In general, UTIs in men, pregnant individuals, children, and patients in a health care-associated setting may be considered complicated.
• Urosepsis – Sepsis caused by a UTI.

Incidence and Prevalence

• The prevalence of asymptomatic bacteruria increases in the female population once adulthood is reached, with a female-to-male ratio approaching 30:1.
• After the age of 65 the female-to-male ratio steadily declines while the prevalence rises consistently in both sexes; asymptomatic bacteriuria is much more common than symptomatic UTI.
• Prevalence of asymptomatic bacteriuria in pregnant individuals is estimated to be 1.9%-9.5%.

Risk Factors
Both sexes:

• History of previous UTIs
• History of urologic instrumentation and surgery
• History of urethral catheterization
• Urinary tract obstruction
• Neurogenic bladder
• Renal transplantation
• Functional or mental impairment (in elderly)

In women:

• Diabetes mellitus
• Low socioeconomic status
• Sexual intercourse
• Lack of urination after intercourse
• Spermicidal contraceptive jellies
• Diaphragm use
• Pregnancy
• Sickle cell trait in pregnancy
• Estrogen deficiency
• Bladder prolapse

In men:

• Insertive anal intercourse
• Condom catheter drainage

Etiology

• The most common infecting organisms are Escherichia coli (70%), Klebsiella pneumoniae (7%), and Enterococcus spp. (6%) in community-acquired infections. Staphylococcus saprophyticus accounts for 5%-15% of UTIs in young sexually active women in the United States.
• Patients admitted to hospitals and long-term care facilities have a higher number of Proteus, Enterobacter, Pseudomonas, Staphylococcus, and Enterococcus isolates compared with outpatients. Anaerobic organisms are rarely the cause of UTIs.
• Uncommon organisms include Mycobacterium tuberculosis.

Pathogenesis

• Ascending route – In the majority of the UTIs, pathogens gain access to the urinary tract by ascending up the urethra to the bladder. Further ascent of pathogens is the cause of the majority of renal parenchymal infections. Urethral massage and sexual intercourse facilitate the first step of this process in females. Indwelling catheters facilitate ascent, and spermicides promote vaginal colonization with pathogens.
• Hematogenous route – Associated with abscess formation in the kidneys. Usually results from bacteremia with virulent pathogens such as Staphylococcus aureus, Salmonella, or possibly Candida. Infection of the kidney with gram-negative bacilli through a hematogenous route is rare in humans, although ascending pyelonephritis can lead to bacteremia in 20%-30% of cases.

Signs and Symptoms
Strong diagnostic indicators of UTI include self diagnosis, hematuria, urinary frequency, costovertebral angle tenderness on physical exam, back pain, and fever. Poor diagnostic indicators include vaginal discharge and vaginal irritation.