Clinical outcome of empiric antimicrobial therapy of bacteremia due to extended-spectrum beta-lactamase producing Escherichia coli and Klebsiella pneumoniae

Study population

Prior to a recent restructuring as Alberta Health Services, The Calgary Health Region (CHR) administered virtually all medical and surgical care to the residents of the cities of Calgary and Airdrie and a large surrounding area (population 1.2 million) in the Province of Alberta, Canada. All patients with bacteremias caused by EK-ESBL organisms from January 1, 2000 to December 31, 2007 in the CHR were included.

Study protocol

An active, surveillance cohort design was utilized. As previously described, surveillance for EK-ESBL bacteremias was conducted by Calgary Laboratory Services [12]. International Statistical Classification of Disease and Health Related Problems (ICD) codes were translated into Charlson Comorbidity Scores using standardized algorithms [13–15] and an online macro [16]. All antimicrobial therapies prescribed were determined using the regional pharmacy database [17]. EK-ESBL infections were defined by the isolation of these organisms from one or more sets of aseptically obtained blood culture bottles. The presence or absence of ESBL-producing E. coli or K. pneumoniae cultures obtained from non-blood sites within ± 48 hours of the index incident blood culture draw was also assessed. Repeated isolation from blood within 365 days was considered to be the same incident infection and thereafter as new episodes.

Empiric therapy was defined as that therapy initiated prior to the availability of blood culture results and sensitivities. Time to first therapy was defined as the time from blood culture draw to receipt of the first dose of antibiotics. Adequate therapy was defined by the receipt of a standard parenteral dose of an antimicrobial to which the organism was fully susceptible in vitro based on Clinical and Laboratory Standards Institute (CLSI) breakpoints or a standard oral dose of an antimicrobial with high bioavailability by this route, to which the organism was also fully susceptible. ESBL-producing organisms were assumed to be resistant to cefazolin, ceftriaxone, ceftazidime, aztreonam, and cefipime. If the patient was given adequate antibiotics before or at the time of the blood culture draw then the time to first adequate therapy was recorded as zero hours. Definitions of nosocomial, healthcare associated and community-acquired infections have been previously described [12].

Laboratory Methods

Clinical isolates were cultured, speciated, and tested for antimicrobial susceptibility by standard techniques [18]. At Calgary Laboratory Services, all Enterobacteriaceae isolates are routinely screened for ESBL production. Minimum inhibitory concentrations (MICs) were determined by Vitek 2™ (Vitek AMS; bioMérieux Vitek Systems Inc., Hazelwood, MO). The quality control strains used for this part of the study were E. coli ATCC 25922, E. coli ATCC 35218 and Pseudomonas aeruginosa ATCC 27853. The presence of ESBLs was detected in clinical isolates of E. coli by using the CLSI criteria for ESBL screening and disk confirmation tests [18]. Disks for ESBL confirmation tests were obtained from Oxoid Inc. (Nepean, Ontario, Canada). K. pneumoniae ATCC 700603 and E. coli ATCC 25922 were used as positive and negative controls, respectively. Isoelectric focusing (IEF) which included cefotaxime hydrolysis and inhibitor profiles in polyacrylamide gels was performed on freeze-thaw extracts as previously described [19]. Polymerase chain reaction (PCR) amplification for bla_CTX-Ms, bla _OXAs , bla _TEMs , bla _SHV , was carried out on the isolates with a GeneAmp 9700 ThermoCycler instrument (Applied Biosystems, Norwalk, Con) using PCR conditions and primers as previously described [19, 20]. Automated sequencing was performed on the PCR products with the ABI Prism 3100 Genetic analyzer (Applied Bio-systems, Norwalk, Con) as previously described [21, 22].

Statistical analysis

Analysis was performed using Stata version 10.0 (Stata Corp, College Station, TX). Non-normally distributed data were described using medians with interquartile ranges (IQR) and compared using the Mann-Whitney test. Categorical data were compared using the Fisher's Exact or chi² tests. P-values less than 0.05 were considered significant and those between 0.05 and 0.1 were considered to represent a trend.

Population Characteristics

Microbiological Characteristics

Of the 79 bacteremic episodes 72 (91%) were E. coli and 7 (9%) were K. pneumoniae. One Klebsiella pneumoniae bacteremia was polymicrobial with Pseudomonas aeruginosa. One Escherichia coli bacteremia was polymicrobial with a viridans group streptococcus. Neither of these second organisms affected adequacy of antimicrobial therapy as they were sensitive to all major classes of antibiotics. Thirty-nine bacteremias (49%) were primary; 38 (48%) were from a urinary source; 2 (3%) resulted from pneumonias and 1 (1%) patient had a biliary source. One patient had two sources documented (one urinary and the other biliary). Although typing was not performed for K. pneumoniae isolates, E. coli ESBL's were typed as 1 CTX-M-2, 1 CTX-M-3, 27 CTX-M-14, 35 CTX-M-15, 5 SHV-2, 1 TEM-52 and 1 was unknown.

Description of therapy

Outcomes

Overall 63 cases (80%) survived to discharge. The case-fatality rate associated with an episode of E. coli bacteremia was 22% (16/72) whereas this was 0% (0/7) for K. pneumoniae. Those who received adequate therapy by 8 hours (6/22; 27% vs. 6/53; 11%; p = 0.088) and 24 hours (7/28; 25% vs. 5/42; 12%; p = 0.096) trended towards a higher likelihood of death. However, there was no significant difference in mortality when comparing those patients who were adequately treated to those inadequately treated by 48 hours (9/42; 21% vs. 7/37; 19%; p = 1.0). Statistical trends toward a worse clinical outcome was associated with the following factors: age-adjusted Charlson score greater than the median (12/45; 27% vs. 4/34; 12% mortality; p = 0.087), female gender (10/35; 28.5% vs. 6/38; 13.6% mortality; p = 0.087); and having been treated empirically with a beta-lactam/beta-lactamase inhibitor combination (6/16; 38% vs. 10/53; 18% mortality; p = 0.063). There was no association between ESBL type and mortality. Of 10 patients empirically treated with a carbapenem none died vs. 16/53 (30%) patients having died who were not empirically treated with a carbapenem (p = 0.089). Of note empiric cephalosporin therapy was not associated with a worse outcome (5/30; 16% vs. 11/49; 22%, p = 0.375). The case-fatality rate was highest for patients with nosocomial infections (38%; 9/24), followed by those with healthcare associated infections (15%; 5/33) and community-acquired infections (9%; 2/22 patients); p = 0.046.