Chapter 14 Urinary Tract Infections
Definition
Cystitis (bladder infection or inflammation) may occur with or without pyelonephritis (infection of the kidney[s]). Acute cystitis in a young woman without comorbidities is an uncomplicated urinary tract infection (UTI). The episode is considered a complicated UTI when the risk of treatment failure is higher (e.g., patients with pyelonephritis, diabetes mellitus, immunosuppression, urinary tract abnormality, male sex, older age, urinary tract instrumentation, or hospital-acquired infection).
Epidemiology
UTIs account for 8 million physician visits and over 100,000 hospitalizations per year. Women are affected by 0.5 UTIs per year in their 20s and roughly 0.1 infections per year in their 30s. Up to 60% of women report being affected by a UTI at some time in their lives. Among postmenopausal women, studies report an incidence of 0.07 cases per person-year. The incidence of UTIs in men is significantly lower (5–8 cases per 10,000 persons).
Escherichia coli is the most common cause of UTIs, accounting for 80% to 90% of cases. Staphylococcus saprophyticus is the next most common pathogen, accounting for 10% to 20% of cases. A small minority of cases are caused by Proteus, Klebsiella, or enterococci. Complicated UTIs are usually caused by Pseudomonas aeruginosa, resistant gram-negative organisms, and fungi. More than 90% of cases of uncomplicated cystitis are monomicrobial, whereas up to 30% of cases of complicated urinary tract infections may be polymicrobial.
UTIs are the most common nosocomial infection, with an incidence of 2 cases per 100 patients discharged from the hospital. Most cases of hospital-acquired UTIs are associated with urinary catheterization.
Renal cortical abscesses (renal carbuncle) are usually due to hematogenous seeding of bacteria. S. aureus is the causative organism in 90% of cases. Renal cortical abscesses tend to be unilateral and occur in the right kidney 63% of the time. Men are affected three times more often than women.
Renal corticomedullary abscesses, in contrast, are usually caused by an ascending UTI with gram-negative bacteria such as E. coli, Proteus, and Klebsiella. Men and women are affected equally. Severe infections may penetrate the renal capsule and cause a perinephric abscess. Patients with urinary tract abnormalities are at higher risk for the development of renal corticomedullary abscesses.
Perinephric abscesses develop from rupture of a renal abscess; therefore, both conditions share many of the same risk factors. Roughly 25% of perinephric abscesses occur in patients with diabetes.
Pathogenesis
Colonization of the vaginal introitus by pathogenic fecal flora, followed by introduction of the pathogens into the urinary system cause UTIs. Sexual intercourse and intercourse with spermicide-containing contraceptives, history of multiple UTIs, and presence of diabetes mellitus are the major risk factors for the development of UTIs.
Men experience far fewer cases of UTIs than do women because of the longer urethra, decreased colonization of the peri-urethral region, and secretion of antibacterial substances in prostatic fluid.
In cases of catheter-associated urinary tract infections, bacteria may gain access to the bladder via the catheter lumen or along the outside of the catheter.
Clinical Features and Diagnosis
History
The predominant symptom of cystitis is dysuria (pain when urinating), in association with increased urinary frequency, suprapubic pain, or hematuria. It is important to rule out other causes of dysuria, including urethritis caused by sexually transmitted diseases (STD), such as gonorrhea or chlamydia. Patients with urethral discharge, recent new sexual partner, or sexual partner with urethral symptoms should be investigated for urethritis caused by an STD. Vaginitis may also cause dysuria. Patients with dyspareunia, vaginal discharge, vaginal pruritus, or dysuria without associated increased urinary frequency should be worked up for vaginitis. Women with dysuria, increased urinary frequency, and no vaginal symptoms have a >90% likelihood of having acute cystitis.
Pyelonephritis or other upper UTIs should be suspected in patients with fevers, chills, nausea, vomiting, flank pain, and other systemic signs or symptoms. Fevers greater than 100° F (37.8° C) correlate well with the presence of pyelonephritis instead of cystitis.
Renal and perinephric abscesses may present with fevers, chills, and back, abdominal or flank pain. Patients with renal cortical abscesses may not report urinary symptoms because these abscesses rarely communicate with the urinary system.
Physical Examination
Patients with acute cystitis may have suprapubic tenderness. A pelvic examination (with testing for gonorrhea and chlamydia) should be performed if urethritis or vaginitis is suspected. Patients with upper UTI may have fever, tachycardia, and costovertebral angle tenderness.
Laboratory Tests
Urinalysis and Urine Dipsticks
More than 10 white blood cells per microliter in the urine are highly suggestive of a UTI. White blood cell casts suggest upper UTI. The presence of hematuria supports the diagnosis of UTI; hematuria is not found in urethritis or vaginitis.
Leukocyte esterase testing on urinary dipsticks has a sensitivity of up to 96%, and a specificity of up to 98% for the detection of white blood cells in the urine. The nitrite test detects the presence of Enterobacteriaceae, but may be falsely negative in patients with lower numbers of bacteria (<10,000 cfu/mL). The nitrite test is most accurate if applied to the first urine collected in the morning (prolonged exposure to bacteria is required to convert nitrates to nitrites). The nitrite test should not be used to rule out acute pyelonephritis because the test has lower sensitivity for this condition (35%–80%) and cannot detect non-nitrite-producing bacteria.
Urine Culture
Urine cultures should be reserved for patients likely to have complicated UTIs, patients failing initial anti-microbial therapy, patients with atypical symptoms, and patients with upper UTIs, including acute pyelonephritis. Although the standard cutoff for a positive urine culture is > 100,000 cfu/mL, in women with pyuria and UTI symptoms, a urine culture is positive if the bacterial count exceeds 100 cfu/mL.
Studies have shown that over one half of patients with typical UTI symptoms, pyuria, and negative cultures have Chlamydia trachomatis infection. The remainder of women do not have any identifiable organism but may still respond to standard therapy for acute cystitis.
Renal and Perinephric Abscesses
Patients with renal cortical abscesses may not have a positive urinalysis or urine culture unless the abscess communicates with the urinary tract. Blood cultures are rarely positive. Renal corticomedullary abscesses may be diagnosed by CT scanning or ultrasound. (CT scanning is the most sensitive diagnostic technique.) Perinephric abscesses may be visualized on CT or MRI scanning. MRI may provide better delineation of adjacent tissue and organ involvement.
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