Diagnostic caveats for difficult bacterial urinary tract infections

Diagnostic caveats for difficult bacterial urinary tract infections

Jul 01, 2005

The objective of part one of this two-part series is to summarize diagnostic caveats derived from our experience with medical management of urinary tract infections (UTI) during the past 35 years.

This discussion is based on the premise that as veterinarians we should offer the quality of medical care that we would choose if we were the patients. When the diagnosis of the underlying causes of urinary tract infections becomes the rule rather than the exception, therapeutic failures will become the exception rather than the rule. Therapeutic caveats will follow in next month's Diagnote.

Why is it important to recognize that bacterial UTI is not a primary diagnostic entity?

Although the urinary tract communicates with an external environment loaded with bacteria and other potentially pathogenic agents, most of it is normally sterile, and all of it is normally resistant to infection. Resistance to urinary tract infection (UTI) is dependent on the interaction of several host defense mechanisms. The pathogenesis of UTIs is related to a balance between the virulence of uropathic infectious agents (analogous to seeds) and the functional status of host defense mechanisms (analogous to soil Table 1). Growth of bacteria (seeds) usually will not occur unless abnormalities of host defenses (suitable soil) is present. Therefore, in context of diagnosis, prognosis and therapy, a bacterial urinary tract infection (UTI) may be viewed as a secondary (or complicating) rather than a primary (or definitive) diagnostic entity.

Caveat: In addition to focusing on antimicrobial treatment of bacterial pathogens (which are usually secondary causes of urinary tract disease), it is also important to consider detection and treatment of abnormalities in host defenses that allow bacteria to colonize and invade tissues of the urinary tract (Tables 1 and 2). If UTIs are managed inappropriately, one or more sequela may occur (Table 3). Early detection followed by proper treatment and follow-up evaluation will minimize the occurrence and severity of these sequela.

Classifying it What diagnostic classification will facilitate treatment of difficult UTIs?

Table 1 Natural and acquired urinary tract defenses against bacterial infection
Classification of UTIs based on the presence or absence of detectable abnormalities in host defense mechanisms allows differentiation of uncomplicated (or simple) urinary tract infections from complicated urinary tract infections (Tables 1 and 2).

An uncomplicated (or simple) UTI is defined as an infection in which an underlying structural, neurologic, immunologic or functional abnormality can not be identified. However, most bacteria survive and multiply only when host defenses are compromised. Many simple UTIs encompass transient and potentially reversible defects in the patient's innate defense mechanisms, even though the underlying cause may escape detection. Others occur when normal host defenses are overwhelmed by virulent uropathogens. For example, nosocomial UTI could occur as a result of improper transurethral catheterization in a hospital intensive care unit harboring resistant uropathogens. Uncomplicated UTIs are usually associated with a better prognosis for recovery.

Table 2 Checklist of some predisposing causes of complicated urinary tract infections
Complicated UTIs occur as a result of bacterial invasion of the urinary system secondary to an identifiable disease that interferes with one or more defense mechanisms (Table 2). In general, the underlying cause must be removed or corrected if secondary bacterial infection is to be completely eradicated and prevented from recurring. Failure or inability to do so is a common cause of recurrent UTI (relapse or reinfection).

Caveat: Differentiation of uncomplicated from complicated UTIs requires appropriate diagnostic evaluation, which may include transrectal palpation of the genitourinary tract, ultrasonography, survey and contrast radiography, cystoscopy, and aspiration, punch or surgical biopsy.

Preventing relapses Why is it important to differentiate recurrent UTIs as relapses or re-infections?