Services
News
Research
Education Materials
Protocols
.

Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.
Mount Sinai Hospital is a University of Toronto patient care, teaching, and research centre.

Frequently Asked Questions

» How do you tell when a positive blood culture is significant?

One can never assume that a blood culture is a contaminant without first carefully considering the patient and the organism. Common isolates that always or near always (>90%) represent true infection include S.aureus, E.coli and other Enterobacteriaceae, P.aeruginosa, S.pneumoniae, and Candida albicans. Other organisms such as Corynebacterium spp., Bacillus spp., and Propionibacterium acnes rarely (<5%) represent true bacteremia (1). Some organisms, particularly S.epidermidis, are more problematic in that they often represent contamination although in the appropriate patient and circumstance, they can be pathogenic.

S.epidermidis is a coagulase-negative staphylococcus (CNS) that is part of the normal skin flora and of lower virulence than S.aureus. The proportion of all bloodstream infections caused by CNS is increasing (2). It is the most common organism isolated accounting for approximately 40% of all positive blood cultures (1). Forty-one percent of the positive blood cultures from our institution to date this year have been secondary to CNS. It is also the most common contaminant, accounting for 75% of contaminated blood cultures (2). While most of these blood cultures represent contamination approximately 12-15% are clinically significant (1,3). Fortunately, nosocomial bacteremia secondary to CNS is associated with a significantly lower risk of death than other nosocomial bloodstream pathogens after adjusting for confounders (2). The attributable mortality is approximately 5% (1,2).

There are certain factors that help to determine whether CNS in a blood culture represents true bacteremia or a contaminant. CNS has certain adherence factors as well as the ability to produce an extracellular glycocalyx, both of which make it a common cause of true bacteremia in those patients with indwelling prosthetic material. It is therefore the most common causative organism in intravascular-catheter-related infections, early prosthetic valve endocarditis, and early prosthetic joint infection. Weinstein and colleagues (1) have found that intravascular catheters as the source of bacteremia has increased from 3-19% of cases in the last 20 years. This in large part explains the increase in CNS bacteremia. There is no difference in the positive predictive value of one versus two blood cultures positive for CNS in predicting true bacteremia (4). However, the number of positive blood cultures out of the total cultures obtained may be helpful.

There are certain issues regarding the acquisition of blood cultures that can decrease the percent of blood cultures that are contaminants. Skin antisepsis is paramount and should include 70% isopropyl alcohol, followed by an iodophor or iodine tincture. Blood culture teams consistently do better than house staff. Although somewhat controversial venipuncture results in less contamination compared with cultures obtained from intravenous catheters (3).

References:

  1. Clin Infect Dis 1997;24:584-602.
  2. Infect Cont and Hosp Epi 1998;19(8):581-9.
  3. Clin Inf Dis 1996;23:40-6.
  4. Clin Inf Dis 1996;22:14-20.

Response by: David Richardson, M.D.
Infectious Diseases Fellow
University of Toronto

» What is the significance of Candida in a urine specimen?

Candida in the urine frequently represents colonization particularly in association with indwelling urinary catheters. True infection is uncommon compared with colonization. The prevalence of candiduria has been estimated at 0.2%-6% among asymptomatic volunteers and at 6.5%-20% among hospitalized individuals (1). The presence of risk factors such as urinary catheters, anatomic or functional abnormalities of the genitourinary (GU) tract, exposure to antibacterial agents, immunosuppression and/or leukopenia, diabetes mellitus, and candida cultures positive from other sites all lend support to the clinical diagnosis of candidal urinary tract infection (2).

Fisher and colleagues (3) have suggested an algorithm for the evaluation of candiduria based on the sparse literature in this area. They recommend first verifying the presence of the organism by repeating a urine microscopy and culture. Of course, the urine should be carefully collected with a clean-voided midstream urine sample. If candida is absent with the second urine collection it means that the first positive sample represented either contamination or self-limited candiduria. The physician should then consider the patient and risk factors as outlined above and stratify them into one of three categories. The first category includes previously healthy individuals with asymptomatic candiduria. It is well established that these patients have favorable outcomes without treatment (3). These patients however should be evaluated at least for diabetes, renal insufficiency, and anatomic GU abnormalities. The second category includes those predisposed to candiduria because of their risk factors but in whom disseminated candidiasis is unlikely. The third category includes those predisposed to candiduria but in whom disseminated candidiasis is likely. The presence of disseminated candidiasis may be determined by examining the optic fundi and skin for candidal lesions, inspection of vascular-access sites, and obtaining blood for fungal cultures. Patients who are unstable or deteriorating and in whom disseminated candidiasis is likely should be treated empirically even if no direct evidence of dissemination is available. For all patients, predisposing conditions should be eliminated and/or treated. The authors suggest that all patients with candiduria who have not had their GU tract instrumented (ie. bladder catheterization) should have their GU tract investigated. Whether or not to treat with antifungal agents is fairly clear for patients who are asymptomatic and those who are critically ill, but for those inbetween it is not clear-cut. In general, all symptomatic patients should be treated, as should those with urinary tract obstruction. The type and duration of therapy should take into account the presence or absence of obstructing fungus balls, the anatomy of the collecting system, and the presumed site and extent of infection.

This algorithm has overall been supported by the literature. In 26 patients with confirmed candidemia and a urinary tract source, 23 (88%) had a urinary tract abnormality and 19 (73%) had urinary tract obstruction (1). In 1997, the results of an international conference for the development of a consensus on the management and prevention of severe candidal infections were published (4). They discussed several issues in relation to candiduria. There was consensus that patients without diabetes mellitus, GU abnormalities, or a renal transplant with asymptomatic candiduria should not receive treatment. Their rational was that candiduria is often caused by bladder catheterization or antibiotic use and that removing these factors usually leads to clearing of the candiduria. They recommended that all patients with candiduria and prosthetic materials in the urinary tract should have these removed if possible. Consensus was also reached that patients with candiduria who were to undergo a GU tract procedure be treated.

References:

  1. Clin Infect Dis 1993;17:662-6.
  2. Medicine North America 1995; June:519-29.
  3. Clin Infect Dis 1995;20:183-9.
  4. Clin Infect Dis 1997;25:43-59.

Response by: David Richardson, M.D.
Infectious Diseases Fellow
University of Toronto

This website has been made possible through an unrestricted educational grant from Pfizer Canada Inc.
Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.