Emily C. Welch, PharmD

Study Location and Patients

This study was conducted at a university‐affiliated, urban teaching hospital: Barnes‐Jewish Hospital (1200 beds). During a 6‐year period (January 2002 to December 2007), all hospitalized patients with a positive blood culture for Gram‐negative bacteria, with antimicrobial susceptibility testing performed for the blood isolate(s), were eligible for this investigation. This study was approved by the Washington University School of Medicine Human Studies Committee.

Study Design and Data Collection

A retrospective cohort study design was employed. Two investigators (J.A.D., R.M.R.) identified potential study patients by the presence of a positive blood culture for Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) combined with primary or secondary International Classification of Diseases (ICD‐9‐CM) codes indicative of acute organ dysfunction, at least two criteria from the systemic inflammatory response syndrome (SIRS),10 and initial antibiotic treatment with either cefepime, piperacillin‐tazobactam, or a carbapenem (imipenem or meropenem). These antimicrobials represent the primary agents employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital during the study period, and had to be administered within 12 hours of having the subsequently positive blood cultures drawn. Based on the initial study database construction, 3 investigators (E.C.W., J.K., M.P.) merged patient‐specific data from the automated hospital medical records, microbiology database, and pharmacy database of Barnes‐Jewish Hospital to complete the clinical database under the auspices of the definitions described below.

The baseline characteristics collected by the study investigators included: age, gender, race, the presence of congestive heart failure, chronic obstructive pulmonary disease, diabetes mellitus, chronic liver disease, underlying malignancy, and end‐stage renal disease requiring renal replacement therapy. All cause hospital mortality was evaluated as the primary outcome variable. Secondary outcomes included acquired organ dysfunction and hospital length of stay. The Acute Physiology and Chronic Health Evaluation (APACHE) II11 and Charlson co‐morbidity scores were also calculated during the 24 hours after the positive blood cultures were drawn. This was done because we included patients with community‐acquired infections who only had clinical data available after blood cultures were drawn.

Definitions

All definitions were selected prospectively as part of the original study design. Cases of Gram‐negative bacteremia were classified into mutually exclusive groups comprised of either community‐acquired or healthcare‐associated infection. Patients with healthcare‐associated bacteremia were categorized as community‐onset or hospital‐onset, as previously described.12 In brief, patients with healthcare‐associated community‐onset bacteremia had the positive culture obtained within the first 48 hours of hospital admission in combination with one or more of the following risk factors: (1) residence in a nursing home, rehabilitation hospital, or other long‐term nursing facility; (2) previous hospitalization within the immediately preceding 12 months; (3) receiving outpatient hemodialysis, peritoneal dialysis, wound care, or infusion therapy necessitating regular visits to a hospital‐based clinic; and (4) having an immune‐compromised state. Patients were classified as having healthcare‐associated hospital‐onset bacteremia when the culture was obtained 48 hours or more after admission. Community‐acquired bacteremia occurred in patients without healthcare risk factors and a positive blood culture within the first 48 hours of admission. Prior antibiotic exposure was defined as having occurred within the previous 30 days from the onset of severe sepsis.

To be included in the analysis, patients had to meet criteria for severe sepsis based on discharge ICD‐9‐CM codes for acute organ dysfunction, as previously described.13 The organs of interest included the heart, lungs, kidneys, bone marrow (hematologic), brain, and liver. Patients were classified as having septic shock if vasopressors (norepinephrine, dopamine, epinephrine, phenylephrine, or vasopressin) were initiated within 24 hours of the blood culture collection date and time. Empiric antimicrobial treatment was classified as being appropriate if the initially prescribed antibiotic regimen was active against the identified pathogen(s) based on in vitro susceptibility testing and administered within 12 hours following blood culture collection. Appropriate antimicrobial treatment also had to be prescribed for at least 24 hours. However, the total duration of antimicrobial therapy was at the discretion of the treating physicians. The Charlson co‐morbidity score was calculated using ICD‐9‐CM codes abstracted from the index hospitalization employing MS‐DRG Grouper version 26.

Antimicrobial Monitoring

From January 2002 through the present, Barnes‐Jewish Hospital utilized an antibiotic control program to help guide antimicrobial therapy. During this time, the use of cefepime and gentamicin was unrestricted. However, initiation of intravenous ciprofloxacin, imipenem/cilastatin, meropenem, or piperacillin/tazobactam was restricted and required preauthorization from either a clinical pharmacist or infectious diseases physician. Each intensive care unit (ICU) had a clinical pharmacist who reviewed all antibiotic orders to insure that dosing and interval of antibiotic administration was adequate for individual patients based on body size, renal function, and the resuscitation status of the patient. After daytime hours, the on‐call clinical pharmacist reviewed and approved the antibiotic orders. The initial antibiotic dosages for the antibiotics employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital were as follows: cefepime, 1 to 2 grams every eight hours; pipercillin‐tazobactam, 4.5 grams every six hours; imipenem, 0.5 grams every six hours; meropenem, 1 gram every eight hours; ciprofloxacin, 400 mg every eight hours; gentamicin, 5 mg/kg once daily.

Starting in June 2005, a sepsis order set was implemented in the emergency department, general medical wards, and the intensive care units with the intent of standardizing empiric antibiotic selection for patients with sepsis based on the infection type (ie, community‐acquired pneumonia, healthcare‐associated pneumonia, intra‐abdominal infection, etc) and the hospital's antibiogram.14, 15 However, antimicrobial selection, dosing, and de‐escalation of therapy were still optimized by clinical pharmacists in these clinical areas.

Antimicrobial Susceptibility Testing

The microbiology laboratory performed antimicrobial susceptibility testing of the Gram‐negative blood isolates using the disk diffusion method according to guidelines and breakpoints established by the Clinical Laboratory and Standards Institute (CLSI) and published during the inclusive years of the study.16, 17 Zone diameters obtained by disk diffusion testing were converted to minimum inhibitory concentrations (MICs in mg/L) by linear regression analysis for each antimicrobial agent using the BIOMIC V³ antimicrobial susceptibility system (Giles Scientific, Inc., Santa Barbara, CA). Linear regression algorithms contained in the software of this system were determined by comparative studies correlating microbroth dilution‐determined MIC values with zone sizes obtained by disk diffusion testing.18

Data Analysis

Continuous variables were reported as mean the standard deviation, or median and quartiles. The Student's t test was used when comparing normally distributed data, and the MannWhitney U test was employed to analyze nonnormally distributed data. Categorical data were expressed as frequency distributions and the Chi‐squared test was used to determine if differences existed between groups. We performed multiple logistic regression analysis to identify clinical risk factors that were associated with hospital mortality (SPSS, Inc., Chicago, IL). All risk factors from Table 1, as well as the individual pathogens examined, were included in the corresponding multivariable analysis with the exception of acquired organ dysfunction (considered a secondary outcome). All tests were two‐tailed, and a P value <0.05 was determined to represent statistical significance.

Patient Characteristics

Included in the study were 535 consecutive patients with severe sepsis attributed to Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae bacteremia, of whom 233 (43.6%) died during their hospitalization. The mean age was 58.9 16.0 years (range, 18 to 96 years) with 288 (53.8%) males and 247 (46.2%) females. The infection sources included community‐acquired (n = 46, 8.6%), healthcare‐associated community‐onset (n = 187, 35.0%), and healthcare‐associated hospital‐onset (n = 302, 56.4%). Hospital nonsurvivors were statistically more likely to have a healthcare‐associated hospital‐onset infection, congestive heart failure, chronic obstructive pulmonary disease, chronic kidney disease, ICU admission, need for mechanical ventilation and/or vasopressors, administration of drotrecogin alfa (activated), prior antibiotic administration, the lungs as the source of infection, acquired dysfunction of the cardiovascular, respiratory, renal, hepatic, and neurologic organ systems, and greater APACHE II and Charlson co‐morbidity scores compared to hospital survivors (Table 1). Hospital nonsurvivors were also statistically less likely to have a healthcare‐associated community‐onset infection and a urinary source of infection compared to hospital survivors (Table 1).

Microbiology

Among the 547 Gram‐negative bacteria isolated from blood, the most common were Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) (70.2%) followed by Pseudomonas aeruginosa (20.8%) and Acinetobacter species (9.0%) (Table 2). Nine patients had two different Enterobacteriaceae species isolated from their blood cultures, and three patients had an Enterobacteriaceae species and Pseudomonas aeruginosa isolated from their blood cultures. Hospital nonsurvivors were statistically more likely to be infected with Pseudomonas aeruginosa and less likely to be infected with Enterobacteriaceae. The pathogen‐specific hospital mortality rate was significantly greater for Pseudomonas aeruginosa and Acinetobacter species compared to Enterobacteriaceae (P < 0.001 and P = 0.008, respectively).

Antimicrobial Treatment and Resistance

Among the study patients, 358 (66.9%) received cefepime, 102 (19.1%) received piperacillin‐tazobactam, and 75 (14.0%) received a carbapenem (meropenem or imipenem) as their initial antibiotic treatment. There were 169 (31.6%) patients who received initial combination therapy with either an aminoglycoside (n = 99, 58.6%) or ciprofloxacin (n = 70, 41.4%). Eighty‐two (15.3%) patients were infected with a pathogen that was resistant to the initial antibiotic treatment regimen [cefepime (n = 41; 50.0%), piperacillin‐tazobactam (n = 25; 30.5%), or imipenem/meropenem (n = 16; 19.5%), plus either an aminoglycoside or ciprofloxacin (n = 28; 34.1%)], and were classified as receiving inappropriate initial antibiotic therapy. Among the 453 (84.7%) patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no relationship identified between minimum inhibitory concentration values and hospital mortality.

Patients infected with a pathogen resistant to the initial antibiotic regimen had significantly greater risk of hospital mortality (63.4% vs 40.0%; P < 0.001) (Figure 1). For the 82 individuals infected with a pathogen that was resistant to the initial antibiotic regimen, no difference in hospital mortality was observed among those prescribed initial combination treatment with an aminoglycoside (n = 17) (64.7% vs 61.1%; P = 0.790) or ciprofloxacin (n = 11) (72.7% vs 61.1%; P = 0.733) compared to monotherapy (n = 54). Similarly, among the patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no difference in hospital mortality among those whose bloodstream isolate was only susceptible to the prescribed aminoglycoside (n = 12) compared to patients with isolates that were susceptible to the prescribed beta‐lactam antibiotic (n = 441) (41.7% vs 39.9%; P = 0.902).

Figure 1

Logistic regression analysis identified infection with a pathogen resistant to the initial antibiotic regimen [adjusted odds ratio (AOR), 2.28; 95% confidence interval (CI), 1.69‐3.08; P = 0.006], increasing APACHE II scores (1‐point increments) (AOR, 1.13; 95% CI, 1.10‐1.15; P < 0.001), the need for vasopressors (AOR, 2.57; 95% CI, 2.15‐3.53; P < 0.001), the need for mechanical ventilation (AOR, 2.54; 95% CI, 2.19‐3.47; P < 0.001), healthcare‐associated hospital‐onset infection (AOR, 1.67; 95% CI, 1.32‐2.10; P =0.027), and infection with Pseudomonas aeruginosa (AOR, 2.21; 95% CI, 1.74‐2.86; P =0.002) as independent risk factors for hospital mortality (Hosmer‐Lemeshow goodness‐of‐fit test = 0.305). The model explained between 29.7% (Cox and Snell R square) and 39.8% (Nagelkerke R squared) of the variance in hospital mortality, and correctly classified 75.3% of cases.

Secondary Outcomes

Two or more acquired organ system derangements occurred significantly more often among patients with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates (84.1% vs 68.0%; P = 0.003). Hospital length of stay was significantly longer for patients infected with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates [39.9 50.6 days (median 27 days; quartiles 12 days and 45.5 days) vs 21.6 22.0 days (median 15 days; quartiles 7 days and 30 days); P < 0.001].

Study Location and Patients

This study was conducted at a university‐affiliated, urban teaching hospital: Barnes‐Jewish Hospital (1200 beds). During a 6‐year period (January 2002 to December 2007), all hospitalized patients with a positive blood culture for Gram‐negative bacteria, with antimicrobial susceptibility testing performed for the blood isolate(s), were eligible for this investigation. This study was approved by the Washington University School of Medicine Human Studies Committee.

Study Design and Data Collection

A retrospective cohort study design was employed. Two investigators (J.A.D., R.M.R.) identified potential study patients by the presence of a positive blood culture for Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) combined with primary or secondary International Classification of Diseases (ICD‐9‐CM) codes indicative of acute organ dysfunction, at least two criteria from the systemic inflammatory response syndrome (SIRS),10 and initial antibiotic treatment with either cefepime, piperacillin‐tazobactam, or a carbapenem (imipenem or meropenem). These antimicrobials represent the primary agents employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital during the study period, and had to be administered within 12 hours of having the subsequently positive blood cultures drawn. Based on the initial study database construction, 3 investigators (E.C.W., J.K., M.P.) merged patient‐specific data from the automated hospital medical records, microbiology database, and pharmacy database of Barnes‐Jewish Hospital to complete the clinical database under the auspices of the definitions described below.

The baseline characteristics collected by the study investigators included: age, gender, race, the presence of congestive heart failure, chronic obstructive pulmonary disease, diabetes mellitus, chronic liver disease, underlying malignancy, and end‐stage renal disease requiring renal replacement therapy. All cause hospital mortality was evaluated as the primary outcome variable. Secondary outcomes included acquired organ dysfunction and hospital length of stay. The Acute Physiology and Chronic Health Evaluation (APACHE) II11 and Charlson co‐morbidity scores were also calculated during the 24 hours after the positive blood cultures were drawn. This was done because we included patients with community‐acquired infections who only had clinical data available after blood cultures were drawn.

Definitions

All definitions were selected prospectively as part of the original study design. Cases of Gram‐negative bacteremia were classified into mutually exclusive groups comprised of either community‐acquired or healthcare‐associated infection. Patients with healthcare‐associated bacteremia were categorized as community‐onset or hospital‐onset, as previously described.12 In brief, patients with healthcare‐associated community‐onset bacteremia had the positive culture obtained within the first 48 hours of hospital admission in combination with one or more of the following risk factors: (1) residence in a nursing home, rehabilitation hospital, or other long‐term nursing facility; (2) previous hospitalization within the immediately preceding 12 months; (3) receiving outpatient hemodialysis, peritoneal dialysis, wound care, or infusion therapy necessitating regular visits to a hospital‐based clinic; and (4) having an immune‐compromised state. Patients were classified as having healthcare‐associated hospital‐onset bacteremia when the culture was obtained 48 hours or more after admission. Community‐acquired bacteremia occurred in patients without healthcare risk factors and a positive blood culture within the first 48 hours of admission. Prior antibiotic exposure was defined as having occurred within the previous 30 days from the onset of severe sepsis.

To be included in the analysis, patients had to meet criteria for severe sepsis based on discharge ICD‐9‐CM codes for acute organ dysfunction, as previously described.13 The organs of interest included the heart, lungs, kidneys, bone marrow (hematologic), brain, and liver. Patients were classified as having septic shock if vasopressors (norepinephrine, dopamine, epinephrine, phenylephrine, or vasopressin) were initiated within 24 hours of the blood culture collection date and time. Empiric antimicrobial treatment was classified as being appropriate if the initially prescribed antibiotic regimen was active against the identified pathogen(s) based on in vitro susceptibility testing and administered within 12 hours following blood culture collection. Appropriate antimicrobial treatment also had to be prescribed for at least 24 hours. However, the total duration of antimicrobial therapy was at the discretion of the treating physicians. The Charlson co‐morbidity score was calculated using ICD‐9‐CM codes abstracted from the index hospitalization employing MS‐DRG Grouper version 26.

Antimicrobial Monitoring

From January 2002 through the present, Barnes‐Jewish Hospital utilized an antibiotic control program to help guide antimicrobial therapy. During this time, the use of cefepime and gentamicin was unrestricted. However, initiation of intravenous ciprofloxacin, imipenem/cilastatin, meropenem, or piperacillin/tazobactam was restricted and required preauthorization from either a clinical pharmacist or infectious diseases physician. Each intensive care unit (ICU) had a clinical pharmacist who reviewed all antibiotic orders to insure that dosing and interval of antibiotic administration was adequate for individual patients based on body size, renal function, and the resuscitation status of the patient. After daytime hours, the on‐call clinical pharmacist reviewed and approved the antibiotic orders. The initial antibiotic dosages for the antibiotics employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital were as follows: cefepime, 1 to 2 grams every eight hours; pipercillin‐tazobactam, 4.5 grams every six hours; imipenem, 0.5 grams every six hours; meropenem, 1 gram every eight hours; ciprofloxacin, 400 mg every eight hours; gentamicin, 5 mg/kg once daily.

Starting in June 2005, a sepsis order set was implemented in the emergency department, general medical wards, and the intensive care units with the intent of standardizing empiric antibiotic selection for patients with sepsis based on the infection type (ie, community‐acquired pneumonia, healthcare‐associated pneumonia, intra‐abdominal infection, etc) and the hospital's antibiogram.14, 15 However, antimicrobial selection, dosing, and de‐escalation of therapy were still optimized by clinical pharmacists in these clinical areas.

Antimicrobial Susceptibility Testing

The microbiology laboratory performed antimicrobial susceptibility testing of the Gram‐negative blood isolates using the disk diffusion method according to guidelines and breakpoints established by the Clinical Laboratory and Standards Institute (CLSI) and published during the inclusive years of the study.16, 17 Zone diameters obtained by disk diffusion testing were converted to minimum inhibitory concentrations (MICs in mg/L) by linear regression analysis for each antimicrobial agent using the BIOMIC V³ antimicrobial susceptibility system (Giles Scientific, Inc., Santa Barbara, CA). Linear regression algorithms contained in the software of this system were determined by comparative studies correlating microbroth dilution‐determined MIC values with zone sizes obtained by disk diffusion testing.18

Data Analysis

Continuous variables were reported as mean the standard deviation, or median and quartiles. The Student's t test was used when comparing normally distributed data, and the MannWhitney U test was employed to analyze nonnormally distributed data. Categorical data were expressed as frequency distributions and the Chi‐squared test was used to determine if differences existed between groups. We performed multiple logistic regression analysis to identify clinical risk factors that were associated with hospital mortality (SPSS, Inc., Chicago, IL). All risk factors from Table 1, as well as the individual pathogens examined, were included in the corresponding multivariable analysis with the exception of acquired organ dysfunction (considered a secondary outcome). All tests were two‐tailed, and a P value <0.05 was determined to represent statistical significance.

Patient Characteristics

Included in the study were 535 consecutive patients with severe sepsis attributed to Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae bacteremia, of whom 233 (43.6%) died during their hospitalization. The mean age was 58.9 16.0 years (range, 18 to 96 years) with 288 (53.8%) males and 247 (46.2%) females. The infection sources included community‐acquired (n = 46, 8.6%), healthcare‐associated community‐onset (n = 187, 35.0%), and healthcare‐associated hospital‐onset (n = 302, 56.4%). Hospital nonsurvivors were statistically more likely to have a healthcare‐associated hospital‐onset infection, congestive heart failure, chronic obstructive pulmonary disease, chronic kidney disease, ICU admission, need for mechanical ventilation and/or vasopressors, administration of drotrecogin alfa (activated), prior antibiotic administration, the lungs as the source of infection, acquired dysfunction of the cardiovascular, respiratory, renal, hepatic, and neurologic organ systems, and greater APACHE II and Charlson co‐morbidity scores compared to hospital survivors (Table 1). Hospital nonsurvivors were also statistically less likely to have a healthcare‐associated community‐onset infection and a urinary source of infection compared to hospital survivors (Table 1).

Microbiology

Among the 547 Gram‐negative bacteria isolated from blood, the most common were Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) (70.2%) followed by Pseudomonas aeruginosa (20.8%) and Acinetobacter species (9.0%) (Table 2). Nine patients had two different Enterobacteriaceae species isolated from their blood cultures, and three patients had an Enterobacteriaceae species and Pseudomonas aeruginosa isolated from their blood cultures. Hospital nonsurvivors were statistically more likely to be infected with Pseudomonas aeruginosa and less likely to be infected with Enterobacteriaceae. The pathogen‐specific hospital mortality rate was significantly greater for Pseudomonas aeruginosa and Acinetobacter species compared to Enterobacteriaceae (P < 0.001 and P = 0.008, respectively).

Antimicrobial Treatment and Resistance

Among the study patients, 358 (66.9%) received cefepime, 102 (19.1%) received piperacillin‐tazobactam, and 75 (14.0%) received a carbapenem (meropenem or imipenem) as their initial antibiotic treatment. There were 169 (31.6%) patients who received initial combination therapy with either an aminoglycoside (n = 99, 58.6%) or ciprofloxacin (n = 70, 41.4%). Eighty‐two (15.3%) patients were infected with a pathogen that was resistant to the initial antibiotic treatment regimen [cefepime (n = 41; 50.0%), piperacillin‐tazobactam (n = 25; 30.5%), or imipenem/meropenem (n = 16; 19.5%), plus either an aminoglycoside or ciprofloxacin (n = 28; 34.1%)], and were classified as receiving inappropriate initial antibiotic therapy. Among the 453 (84.7%) patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no relationship identified between minimum inhibitory concentration values and hospital mortality.

Patients infected with a pathogen resistant to the initial antibiotic regimen had significantly greater risk of hospital mortality (63.4% vs 40.0%; P < 0.001) (Figure 1). For the 82 individuals infected with a pathogen that was resistant to the initial antibiotic regimen, no difference in hospital mortality was observed among those prescribed initial combination treatment with an aminoglycoside (n = 17) (64.7% vs 61.1%; P = 0.790) or ciprofloxacin (n = 11) (72.7% vs 61.1%; P = 0.733) compared to monotherapy (n = 54). Similarly, among the patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no difference in hospital mortality among those whose bloodstream isolate was only susceptible to the prescribed aminoglycoside (n = 12) compared to patients with isolates that were susceptible to the prescribed beta‐lactam antibiotic (n = 441) (41.7% vs 39.9%; P = 0.902).

Figure 1

Logistic regression analysis identified infection with a pathogen resistant to the initial antibiotic regimen [adjusted odds ratio (AOR), 2.28; 95% confidence interval (CI), 1.69‐3.08; P = 0.006], increasing APACHE II scores (1‐point increments) (AOR, 1.13; 95% CI, 1.10‐1.15; P < 0.001), the need for vasopressors (AOR, 2.57; 95% CI, 2.15‐3.53; P < 0.001), the need for mechanical ventilation (AOR, 2.54; 95% CI, 2.19‐3.47; P < 0.001), healthcare‐associated hospital‐onset infection (AOR, 1.67; 95% CI, 1.32‐2.10; P =0.027), and infection with Pseudomonas aeruginosa (AOR, 2.21; 95% CI, 1.74‐2.86; P =0.002) as independent risk factors for hospital mortality (Hosmer‐Lemeshow goodness‐of‐fit test = 0.305). The model explained between 29.7% (Cox and Snell R square) and 39.8% (Nagelkerke R squared) of the variance in hospital mortality, and correctly classified 75.3% of cases.

Secondary Outcomes

Two or more acquired organ system derangements occurred significantly more often among patients with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates (84.1% vs 68.0%; P = 0.003). Hospital length of stay was significantly longer for patients infected with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates [39.9 50.6 days (median 27 days; quartiles 12 days and 45.5 days) vs 21.6 22.0 days (median 15 days; quartiles 7 days and 30 days); P < 0.001].

Study Location and Patients

This study was conducted at a university‐affiliated, urban teaching hospital: Barnes‐Jewish Hospital (1200 beds). During a 6‐year period (January 2002 to December 2007), all hospitalized patients with a positive blood culture for Gram‐negative bacteria, with antimicrobial susceptibility testing performed for the blood isolate(s), were eligible for this investigation. This study was approved by the Washington University School of Medicine Human Studies Committee.

Study Design and Data Collection

A retrospective cohort study design was employed. Two investigators (J.A.D., R.M.R.) identified potential study patients by the presence of a positive blood culture for Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) combined with primary or secondary International Classification of Diseases (ICD‐9‐CM) codes indicative of acute organ dysfunction, at least two criteria from the systemic inflammatory response syndrome (SIRS),10 and initial antibiotic treatment with either cefepime, piperacillin‐tazobactam, or a carbapenem (imipenem or meropenem). These antimicrobials represent the primary agents employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital during the study period, and had to be administered within 12 hours of having the subsequently positive blood cultures drawn. Based on the initial study database construction, 3 investigators (E.C.W., J.K., M.P.) merged patient‐specific data from the automated hospital medical records, microbiology database, and pharmacy database of Barnes‐Jewish Hospital to complete the clinical database under the auspices of the definitions described below.

The baseline characteristics collected by the study investigators included: age, gender, race, the presence of congestive heart failure, chronic obstructive pulmonary disease, diabetes mellitus, chronic liver disease, underlying malignancy, and end‐stage renal disease requiring renal replacement therapy. All cause hospital mortality was evaluated as the primary outcome variable. Secondary outcomes included acquired organ dysfunction and hospital length of stay. The Acute Physiology and Chronic Health Evaluation (APACHE) II11 and Charlson co‐morbidity scores were also calculated during the 24 hours after the positive blood cultures were drawn. This was done because we included patients with community‐acquired infections who only had clinical data available after blood cultures were drawn.

Definitions

All definitions were selected prospectively as part of the original study design. Cases of Gram‐negative bacteremia were classified into mutually exclusive groups comprised of either community‐acquired or healthcare‐associated infection. Patients with healthcare‐associated bacteremia were categorized as community‐onset or hospital‐onset, as previously described.12 In brief, patients with healthcare‐associated community‐onset bacteremia had the positive culture obtained within the first 48 hours of hospital admission in combination with one or more of the following risk factors: (1) residence in a nursing home, rehabilitation hospital, or other long‐term nursing facility; (2) previous hospitalization within the immediately preceding 12 months; (3) receiving outpatient hemodialysis, peritoneal dialysis, wound care, or infusion therapy necessitating regular visits to a hospital‐based clinic; and (4) having an immune‐compromised state. Patients were classified as having healthcare‐associated hospital‐onset bacteremia when the culture was obtained 48 hours or more after admission. Community‐acquired bacteremia occurred in patients without healthcare risk factors and a positive blood culture within the first 48 hours of admission. Prior antibiotic exposure was defined as having occurred within the previous 30 days from the onset of severe sepsis.

To be included in the analysis, patients had to meet criteria for severe sepsis based on discharge ICD‐9‐CM codes for acute organ dysfunction, as previously described.13 The organs of interest included the heart, lungs, kidneys, bone marrow (hematologic), brain, and liver. Patients were classified as having septic shock if vasopressors (norepinephrine, dopamine, epinephrine, phenylephrine, or vasopressin) were initiated within 24 hours of the blood culture collection date and time. Empiric antimicrobial treatment was classified as being appropriate if the initially prescribed antibiotic regimen was active against the identified pathogen(s) based on in vitro susceptibility testing and administered within 12 hours following blood culture collection. Appropriate antimicrobial treatment also had to be prescribed for at least 24 hours. However, the total duration of antimicrobial therapy was at the discretion of the treating physicians. The Charlson co‐morbidity score was calculated using ICD‐9‐CM codes abstracted from the index hospitalization employing MS‐DRG Grouper version 26.

Antimicrobial Monitoring

From January 2002 through the present, Barnes‐Jewish Hospital utilized an antibiotic control program to help guide antimicrobial therapy. During this time, the use of cefepime and gentamicin was unrestricted. However, initiation of intravenous ciprofloxacin, imipenem/cilastatin, meropenem, or piperacillin/tazobactam was restricted and required preauthorization from either a clinical pharmacist or infectious diseases physician. Each intensive care unit (ICU) had a clinical pharmacist who reviewed all antibiotic orders to insure that dosing and interval of antibiotic administration was adequate for individual patients based on body size, renal function, and the resuscitation status of the patient. After daytime hours, the on‐call clinical pharmacist reviewed and approved the antibiotic orders. The initial antibiotic dosages for the antibiotics employed for the treatment of Gram‐negative infections at Barnes‐Jewish Hospital were as follows: cefepime, 1 to 2 grams every eight hours; pipercillin‐tazobactam, 4.5 grams every six hours; imipenem, 0.5 grams every six hours; meropenem, 1 gram every eight hours; ciprofloxacin, 400 mg every eight hours; gentamicin, 5 mg/kg once daily.

Starting in June 2005, a sepsis order set was implemented in the emergency department, general medical wards, and the intensive care units with the intent of standardizing empiric antibiotic selection for patients with sepsis based on the infection type (ie, community‐acquired pneumonia, healthcare‐associated pneumonia, intra‐abdominal infection, etc) and the hospital's antibiogram.14, 15 However, antimicrobial selection, dosing, and de‐escalation of therapy were still optimized by clinical pharmacists in these clinical areas.

Antimicrobial Susceptibility Testing

The microbiology laboratory performed antimicrobial susceptibility testing of the Gram‐negative blood isolates using the disk diffusion method according to guidelines and breakpoints established by the Clinical Laboratory and Standards Institute (CLSI) and published during the inclusive years of the study.16, 17 Zone diameters obtained by disk diffusion testing were converted to minimum inhibitory concentrations (MICs in mg/L) by linear regression analysis for each antimicrobial agent using the BIOMIC V³ antimicrobial susceptibility system (Giles Scientific, Inc., Santa Barbara, CA). Linear regression algorithms contained in the software of this system were determined by comparative studies correlating microbroth dilution‐determined MIC values with zone sizes obtained by disk diffusion testing.18

Data Analysis

Continuous variables were reported as mean the standard deviation, or median and quartiles. The Student's t test was used when comparing normally distributed data, and the MannWhitney U test was employed to analyze nonnormally distributed data. Categorical data were expressed as frequency distributions and the Chi‐squared test was used to determine if differences existed between groups. We performed multiple logistic regression analysis to identify clinical risk factors that were associated with hospital mortality (SPSS, Inc., Chicago, IL). All risk factors from Table 1, as well as the individual pathogens examined, were included in the corresponding multivariable analysis with the exception of acquired organ dysfunction (considered a secondary outcome). All tests were two‐tailed, and a P value <0.05 was determined to represent statistical significance.

Patient Characteristics

Included in the study were 535 consecutive patients with severe sepsis attributed to Pseudomonas aeruginosa, Acinetobacter species, or Enterobacteriaceae bacteremia, of whom 233 (43.6%) died during their hospitalization. The mean age was 58.9 16.0 years (range, 18 to 96 years) with 288 (53.8%) males and 247 (46.2%) females. The infection sources included community‐acquired (n = 46, 8.6%), healthcare‐associated community‐onset (n = 187, 35.0%), and healthcare‐associated hospital‐onset (n = 302, 56.4%). Hospital nonsurvivors were statistically more likely to have a healthcare‐associated hospital‐onset infection, congestive heart failure, chronic obstructive pulmonary disease, chronic kidney disease, ICU admission, need for mechanical ventilation and/or vasopressors, administration of drotrecogin alfa (activated), prior antibiotic administration, the lungs as the source of infection, acquired dysfunction of the cardiovascular, respiratory, renal, hepatic, and neurologic organ systems, and greater APACHE II and Charlson co‐morbidity scores compared to hospital survivors (Table 1). Hospital nonsurvivors were also statistically less likely to have a healthcare‐associated community‐onset infection and a urinary source of infection compared to hospital survivors (Table 1).

Microbiology

Among the 547 Gram‐negative bacteria isolated from blood, the most common were Enterobacteriaceae (Escherichia coli, Klebsiella species, Enterobacter species) (70.2%) followed by Pseudomonas aeruginosa (20.8%) and Acinetobacter species (9.0%) (Table 2). Nine patients had two different Enterobacteriaceae species isolated from their blood cultures, and three patients had an Enterobacteriaceae species and Pseudomonas aeruginosa isolated from their blood cultures. Hospital nonsurvivors were statistically more likely to be infected with Pseudomonas aeruginosa and less likely to be infected with Enterobacteriaceae. The pathogen‐specific hospital mortality rate was significantly greater for Pseudomonas aeruginosa and Acinetobacter species compared to Enterobacteriaceae (P < 0.001 and P = 0.008, respectively).

Antimicrobial Treatment and Resistance

Among the study patients, 358 (66.9%) received cefepime, 102 (19.1%) received piperacillin‐tazobactam, and 75 (14.0%) received a carbapenem (meropenem or imipenem) as their initial antibiotic treatment. There were 169 (31.6%) patients who received initial combination therapy with either an aminoglycoside (n = 99, 58.6%) or ciprofloxacin (n = 70, 41.4%). Eighty‐two (15.3%) patients were infected with a pathogen that was resistant to the initial antibiotic treatment regimen [cefepime (n = 41; 50.0%), piperacillin‐tazobactam (n = 25; 30.5%), or imipenem/meropenem (n = 16; 19.5%), plus either an aminoglycoside or ciprofloxacin (n = 28; 34.1%)], and were classified as receiving inappropriate initial antibiotic therapy. Among the 453 (84.7%) patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no relationship identified between minimum inhibitory concentration values and hospital mortality.

Patients infected with a pathogen resistant to the initial antibiotic regimen had significantly greater risk of hospital mortality (63.4% vs 40.0%; P < 0.001) (Figure 1). For the 82 individuals infected with a pathogen that was resistant to the initial antibiotic regimen, no difference in hospital mortality was observed among those prescribed initial combination treatment with an aminoglycoside (n = 17) (64.7% vs 61.1%; P = 0.790) or ciprofloxacin (n = 11) (72.7% vs 61.1%; P = 0.733) compared to monotherapy (n = 54). Similarly, among the patients infected with a pathogen that was susceptible to the initial antibiotic regimen, there was no difference in hospital mortality among those whose bloodstream isolate was only susceptible to the prescribed aminoglycoside (n = 12) compared to patients with isolates that were susceptible to the prescribed beta‐lactam antibiotic (n = 441) (41.7% vs 39.9%; P = 0.902).

Figure 1

Logistic regression analysis identified infection with a pathogen resistant to the initial antibiotic regimen [adjusted odds ratio (AOR), 2.28; 95% confidence interval (CI), 1.69‐3.08; P = 0.006], increasing APACHE II scores (1‐point increments) (AOR, 1.13; 95% CI, 1.10‐1.15; P < 0.001), the need for vasopressors (AOR, 2.57; 95% CI, 2.15‐3.53; P < 0.001), the need for mechanical ventilation (AOR, 2.54; 95% CI, 2.19‐3.47; P < 0.001), healthcare‐associated hospital‐onset infection (AOR, 1.67; 95% CI, 1.32‐2.10; P =0.027), and infection with Pseudomonas aeruginosa (AOR, 2.21; 95% CI, 1.74‐2.86; P =0.002) as independent risk factors for hospital mortality (Hosmer‐Lemeshow goodness‐of‐fit test = 0.305). The model explained between 29.7% (Cox and Snell R square) and 39.8% (Nagelkerke R squared) of the variance in hospital mortality, and correctly classified 75.3% of cases.

Secondary Outcomes

Two or more acquired organ system derangements occurred significantly more often among patients with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates (84.1% vs 68.0%; P = 0.003). Hospital length of stay was significantly longer for patients infected with a pathogen resistant to the initial antibiotic regimen compared to those infected with susceptible isolates [39.9 50.6 days (median 27 days; quartiles 12 days and 45.5 days) vs 21.6 22.0 days (median 15 days; quartiles 7 days and 30 days); P < 0.001].