Urinary Tract Infection : Pathophysiology, Diagnosis, and Treatment
DEFINITION of URINARY TRACT INFECTIONS :
- Infections of the urinary tract represent a wide variety of clinical syndromes including urethritis, cystitis, prostatitis, and pyelonephritis.
- A urinary tract infection (UTI) is defined as the presence of microorganisms in the urine that cannot be accounted for by contamination. The organisms have the potential to invade the tissues of the urinary tract and adjacent structures.
- Lower tract infections include cystitis (bladder), urethritis (urethra), prostatitis (prostate gland), and epididymitis. Upper tract infections involve the kidney and are referred to as pyelonephritis.
- Uncomplicated UTIs are not associated with structural or neurologic abnormalities that may interfere with the normal flow of urine or the voiding mechanism. Complicated UTIs are the result of a predisposing lesion of the urinary tract such as a congenital abnormality or distortion of the urinary tract, a stone, indwelling catheter, prostatic hypertrophy, obstruction, or neurologic deficit that interferes with the normal flow of urine and urinary tract defenses.
- Recurrent UTIs are characterized by multiple symptomatic episodes with asymptomatic periods occurring between these episodes. These infections are either due to reinfection or to relapse.
- Reinfections are caused by a new organism and account for the majority of recurrent UTIs.
- Relapse represents the development of repeated infections caused by the same initial organism.
PATHOPHYSIOLOGY of URINARY TRACT INFECTION :
- The bacteria causing UTIs usually originate from bowel flora of the host.
- UTIs can be acquired via three possible routes: the ascending, hematogenous, or lymphatic pathways.
- In females, the short length of the urethra and proximity to the perirectal area make colonization of the urethra likely. Bacteria are then believed to enter the bladder from the urethra. Once in the bladder, the organisms multiply quickly and can ascend the ureters to the kidney.
- Three factors determine the development of urinary tract infection: the size of the inoculum, virulence of the microorganism, and competency of the natural host defense mechanisms.
- Patients who are unable to void urine completely are at greater risk of developing urinary tract infections and frequently have recurrent infections.
- An important virulence factor of bacteria is their ability to adhere to urinary epithelial cells by fimbriae. Other virulence factors include hemolysin, a cytotoxic protein produced by bacteria that lyses a wide range of cells including erythrocytes, polymorphonuclear leukocytes, and monocytes; and aerobactin, which facilitates the binding and uptake of iron by Escherichia coli.
CLINICAL PRESENTATION of URINARY TRACT INFECTION :
- Symptoms alone are unreliable for the diagnosis of bacterial UTIs. The key to the diagnosis of UTI is the ability to demonstrate significant numbers of microorganisms present in an appropriate urine specimen to distinguish contamination from infection.
- A standard urinalysis should be obtained in the initial assessment of a patient. Microscopic examination of the urine should be performed by preparation of a Gram stain of unspun or centrifuged urine. The presence of at least one organism per oil-immersion field in a properly collected uncentrifuged specimen correlates with more than 100,000 bacteria/mL of urine.
- Criteria for defining significant bacteriuria are listed in Table 48-2.
- The presence of pyuria (more than 10 WBCs/mm3 in a symptomatic patient correlates with significant bacteriuria.
- The nitrite test can be used to detect the presence of nitrate-reducing bacteria in the urine (such as E. coli). The leukocyte esterase test is a rapid dipstick test to detect pyuria.
- The most reliable method of diagnosing UTIs is by quantitative urine culture. Patients with infection usually have more than 105 bacteria/mL of urine, although as many as one-third of women with symptomatic infection have less than 105 bacteria/mL.
- A method to detect upper UTI is the antibody-coated bacteria (ACB) test, an immunofluorescent method that detects bacteria coated with immunoglobulin in freshly voided urine.
TREATMENT Of URINARY TRACT INFECTION:
- The management of a patient with a UTI includes initial evaluation, selection of an antibacterial agent and duration of therapy, and follow-up evaluation.
- The initial selection of an antimicrobial agent for the treatment of UTI is primarily based on the severity of the presenting signs and symptoms, the site of infection, and whether the infection is determined to be complicated or uncomplicated.
- The ability to eradicate bacteria from the urinary tract is directly related to the sensitivity of the organism and the achievable concentration of the antimicrobial agent in the urine.
- Acute Uncomplicated Cystitis
- These infections are predominantly caused by E. coli, and antimicrobial therapy should be directed against this organism initially. Other causes include S. saprophyticus and occasionally K. pneumoniae and Proteus mirabilis.
- Because the causative organisms and their susceptibilities are generally known, a cost-effective approach to management is recommended that includes a urinalysis and initiation of empiric therapy without a urine culture.
- Short-course therapy (3-day therapy) with trimethoprim-sulfamethoxazole or a fluoroquinolone (e.g., ciprofloxacin, levofloxacin, norfloxacin, or gatifloxacin) is superior to single-dose therapy for uncomplicated infection and should be the treatment of choice. Amoxicillin or sulfonamides are not recommended because of the high incidence of resistant E. coli. Follow-up urine cultures are not necessary in patients who respond.
Complicated Urinary Tract Infections
- The presentation of high-grade fever (greater than 38.3Â°C) and severe flank pain should be treated as acute pyelonephritis, and aggressive management is warranted. Severely ill patients with pyelonephritis should be hospitalized and intravenous drugs administered initially.
- At the time of presentation, a Gram stain of the urine should be performed, along with urinalysis, culture, and sensitivities.
- In the mild to moderately symptomatic patient for whom oral therapy is considered, an effective agent should be administered for at least a 2-week period, although use of highly active agents for 7 to 10 days may be sufficient. Oral antibiotics that have shown efficacy in this setting include trimethoprim-sulfamethoxazole or fluoroquinolones. If a Gram stain reveals gram-positive cocci, S. faecalis should be considered and treatment directed against this pathogen (ampicillin).
- In the seriously ill patient, the traditional initial therapy has included an intravenous fluoroquinolone, an aminoglycoside with or without ampicillin, or an extended-spectrum cephalosporin with or without an aminoglycoside.
- If the patient has been hospitalized in the last 6 months, has a urinary catheter, or is in a nursing home, the possibility of P. aeruginosa and enterococci infection, as well as multiply resistant organisms, should be considered. In this setting, ceftazidime, ticarcillin-clavulanic acid, piperacillin, aztreonam, meropenem, or imipenem, in combination with an aminoglycoside, is recommended. If the patient responds to initial combination therapy, the aminoglycoside may be discontinued after 3 days.
- Follow-up urine cultures should be obtained 2 weeks after the completion of therapy to ensure a satisfactory response and to detect possible relapse.
Urinary Tract Infections in Males
- A urine culture should be obtained before treatment, because the cause of infection in men is not as predictable as in women.
- If gram-negative bacteria are presumed, trimethoprim-sulfamethoxazole or a fluoroquinolone is a preferred agent. Initial therapy is for 10 to 14 days. For recurrent infections in males, cure rates are much higher with a 6-week regimen of trimethoprim-sulfamethoxazole.
Urinary Tract Infection in Pregnancy
In patients with significant bacteriuria, symptomatic or asymptomatic, treatment is recommended in order to avoid possible complications during the pregnancy. Therapy should consist of an agent with a relatively low adverse-effect potential (a sulfonamide, cephalexin, amoxicillin, amoxicillin/clavulanate, nitrofurantoin) administered for 7 days.
Tetracyclines should be avoided because of teratogenic effects, and sulfonamides should not be administered during the third trimester because of the possible development of kernicterus and hyperbilirubinemia. Also, the quinolones should not be given because of their potential to inhibit cartilage and bone development in the newborn.
When bacteriuria occurs in the asymptomatic, short-term catheterized patient (less than 30 days), the use of systemic antibiotic therapy should be withheld and the catheter removed as soon as possible. If the patient becomes symptomatic, the catheter should again be removed and treatment as described for complicated infections should be started.
The use of prophylactic systemic antibiotics in patients with short-term catheterization reduces the incidence of infection over the first 4 to 7 days. In long-term catheterized patients, however, antibiotics only postpone the development of bacteriuria and lead to emergence of resistant organisms.