Question
How do you know when a positive urine culture for bacteria is indicative of infection? When and how should asymptomatic bacteriuria be managed?
| Response from Michael J. Postelnick, BSPharm, Lecturer, Department of Family and Community Medicine, Northwestern University School of Medicine |
A common dilemma faced by the clinician, primarily in institutional settings, is whether to treat bacteria that grow on a urine culture with antimicrobial therapy. Many different laboratory and clinical factors -- bacterial colony count, presence or absence of white blood cells in the urine, presence of a Foley catheter, abnormal urinary anatomy, sex, and type of organism isolated -- should be considered when determining whether antimicrobial therapy is warranted. The Infectious Diseases Society of America has published guidelines that examine many of these issues.
This discussion will highlight some of the most common issues encountered in these situations and attempt to help clinicians steward the use and choice of antimicrobial agents toward situations where they are indicated and likely to be active against commonly isolated urinary pathogens.
The prime driver of whether a urine culture should be sent off to the laboratory should be patient signs and symptoms indicative of a urinary tract infection. There are only 2 clear indications for urine cultures in asymptomatic patients: pregnancy and presurgical evaluation for transurethral resection of the prostate. Among patients undergoing solid-organ transplant, the utility of culture in those without urinary tract symptoms is unclear.
Other patients who in the past may have been considered high risk with possible benefit from routine culturing of urine include nonpregnant premenopausal women, elderly persons, diabetic patients, spinal cord patients, and patients with indwelling catheters. However, available evidence discourages such practice.[1]
Pyuria as an indicator for treatment of asymptomatic patients who may have positive urine cultures is also not supported.[1] A colony count of bacteria is a way to determine the reliability of the culture: 105colonies in a noncatheterized specimen in both women and men and 102 colonies in a catheterized specimen, when a single bacterial species is isolated, provides evidence of true bacteriuria.
The specific populations of patients with asymptomatic bacteriuria for whom treatment is indicated require special consideration when antimicrobial therapy is chosen. For the pregnant patient, the usual agents, such as trimethoprim/sulfamethoxazole and fluoroquinolones, are not indicated owing to concerns about adverse fetal affects. Nitrofurantoin and beta-lactam antimicrobials, such as amoxicillin/clavulanate and first- and second-generation cephalosporins, usually provide safe and effective therapy.
For patients about to undergo transurethral prostate resection, a history of fluoroquinolone use, which is widespread in this patient population, often selects out fluoroquinolone-resistant pathogens. This, along with the growing incidence of multidrug-resistant gram-negative pathogens, makes antimicrobial choice challenging. Local susceptibility patterns and individual culture and susceptibility results should always be used to guide antimicrobial choice in this patient population.
We have encountered situations where patients who received fluoroquinolone prophylaxis before undergoing prostate biopsy (a somewhat less invasive procedure) developed gram-negative sepsis due to fluoroquinolone resistant organisms. This has prompted us to develop a policy of routine culture-driven therapy in these procedures as opposed to exclusive use of fluoroquinolones.
In conclusion, routine treatment of asymptomatic bacteriuria in all but a few selected patient populations is not supported by evidence and is likely to contribute to the growing burden of antimicrobial resistance.
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