Empiric Antibiotic Selection Research Articles

Long-Term Care Acquired Urinary Tract Infections' Antibiotic Resistance Patterns and Empiric Therapy: A Pilot Study

Long-term care (LTC) acquired urinary tract infections (UTIs) are increasingly resistant to antibiotics, and the selection of appropriate empiric antibiotic therapy can be challenging for clinicians. The purpose of this study is to describe LTC-acquired UTI empiric antibiotic prescription patterns and UTI resistance patterns among older adults admitted to an acute psychiatric facility. This retrospective study found that ciprofloxacin was the agent most often used for empiric therapy (76% of cases). However, LTC-acquired UTIs in the sample were susceptible to ciprofloxacin in only 31% of cases. The study has implications for antibiotic stewardship with recommendations for empiric antibiotic selection for LTC-acquired UTIs given the prevalence of fluoroquinolone-resistant bacterial strains in the LTC setting.

Prevalence and antibacterial susceptibility pattern of Urinary Tract Infection Causing Human Pathogenic Bacteria

Toxigenic strains of Escherichia coli are common enteric The present study was conducted to detect common pathogens of urinary tract infection (UTI) and their susceptibility pattern to the commonly used antimicrobial agents in local scenario. Urinary tract infection is one of the common clinical conditions in the patients presenting to the clinics and hospitals. Despite the widespread availability of antibiotics, it remains the most common bacterial infection in the human being. Detection of common pathogens and their antimicrobial susceptibility pattern is mandatory for effective treatment. In the present study, 40 urine samples were collected from adult patients were analyzed for Multidrug Resistant (MDR) strain isolation and identified. The MDR strains were identified by the Kirby Bauer method following the definition of the National Committee of Clinical Laboratory Standards. Out of total 40 samples, 32 (80%) samples grew potential pathogens causing UTI. Escherichia coli were the predominant 10 (31.25%) isolates causing UTI, followed by Staphylococcus aureus -8 (25%), Pseudomonas aeruginosa- 5 (15.62%), Proteus mirabilis -5 (15.62%), Klebsiella pneumoniae -2 (6.25%) and Serratia marcescens – 2 (6.25%). The mean sensitivity of the antibiotics was Tetracyclin (76.66%), Penicillin (70.83%), Ciprofloxacin (60%). S. aureus showed 75% resistance to Methicillin, Oxacillin and Vancomycin. Uropathogens are sensitive to Norfloxacin, Co-trimoxazole and Ofloxacin. High prevalence of drug-resistant urinary tract pathogens, particularly to Tetracylin, Penicillin and Ampicillin among local patients suggests cautious use of antibiotic therapy for the treatment. Finally, we suggest that empirical antibiotic selection should be based on knowledge of the local prevalence of bacterial organisms and antibiotic sensitivities rather than on universal guidelines.

Empiric antibiotic selection strategies for healthcare‐associated pneumonia, intra‐abdominal infections, and catheter‐associated bacteremia

AbstractInitial selection and early deployment of appropriate/adequate empiric antimicrobial therapy is critical to minimize the significant morbidity and mortality associated with hospital‐ or healthcare‐associated infections (HAIs). Initial empiric therapy that inadequately covers the pathogen(s) causing a serious HAI has been associated with increased mortality, longer hospital stay, and elevated healthcare costs. Moreover, subsequent modification of initial inadequate therapy, later in the disease process when culture results become available, may not remedy the impact of the initial choice. Because of this, it is important that initial empiric therapy covers the most likely pathogens associated with infection in a particular patient, even if this initial regimen turns out to be unnecessarily broad, based on subsequent culture results. The current paradigm for management of serious HAIs is to initiate empiric therapy with a broad‐spectrum regimen covering likely pathogens, based on local surveillance and susceptibility data, and presence of risk factors for involvement of a resistant microorganism. Subsequent modification (de‐escalation) of the initial regimen becomes possible later, when culture results are available and clinical status can be better assessed, 2 to 4 days after initiation of empiric therapy. When possible, de‐escalation and other steps to modify antimicrobial exposure are important for minimizing risk of antimicrobial resistance development. This article examines the general process for selection of initial empiric antibiotic therapy for patients with HAIs, illustrated through 3 case studies dealing with healthcare‐associated pneumonia, complicated intra‐abdominal infection, and catheter‐associated bacteremia, respectively. Journal of Hospital Medicine 2012;7:S2–S12. © 2012 Society of Hospital Medicine

Pharmacodynamic evaluation of commonly prescribed oral antibiotics against respiratory bacterial pathogens

BackgroundUpper and lower respiratory tract infections (RTIs) account for a substantial portion of outpatient antibiotic utilization. However, the pharmacodynamic activity of commonly used oral antibiotic regimens has not been studied against clinically relevant pathogens. The objective of this study was to assess the probability of achieving the requisite pharmacodynamic exposure for oral antibacterial regimens commonly prescribed for RTIs in adults against bacterial isolates frequently involved in these processes (S. pneumoniae, H. influenzae, and M. catharralis).MethodsUsing a 5000-subject Monte Carlo simulation, the cumulative fractions of response (CFR), (i.e., probabilities of achieving requisite pharmacodynamic targets) for the most commonly prescribed oral antibiotic regimens, as determined by a structured survey of medical prescription patterns, were assessed against local respiratory bacterial isolates from adults in São Paulo collected during the same time period. Minimal inhibitory concentration (MIC) of 230 isolates of Streptococcus pneumoniae (103), Haemophilus influenzae (98), and Moraxella catharralis (29) from a previous local surveillance were used.ResultsThe most commonly prescribed antibiotic regimens were azithromycin 500 mg QD, amoxicillin 500 mg TID, and levofloxacin 500 mg QD, accounting for 58% of the prescriptions. Varied doses of these agents, plus gatifloxacin, amoxicillin-clavulanate, moxifloxacin, and cefaclor made up the remaining regimens. Utilizing aggressive pharmacodynamic exposure targets, the only regimens to achieve greater than 90% CFR against all three pathogens were amoxicillin/amoxicillin-clavulanate 500 mg TID (> 91%), gatifloxacin 400 mg QD (100%), and moxifloxacin 400 mg QD (100%). Considering S. pneumoniae isolates alone, azithromycin 1000 mg QD also achieved greater than 90% CFR (91.3%).ConclusionsThe only regimens to achieve high CFR against all three pathogen populations in both scenarios were gatifloxacin 400 mg QD, moxifloxacin 400 mg QD, and amoxicillin-clavulanate 500 mg TID. These data suggest the need for reconsideration of empiric antibiotic regimen selection among adult patients with RTIs in the São Paulo area. Additionally, this type of study could be used to optimize prescribing patterns in specific regions in light of emerging resistance.

Effect of an antibiotic algorithm on the adequacy of empiric antibiotic therapy given by a medical emergency team

IntroductionDelayed administration of effective antimicrobial therapy increases mortality in patients with septic shock. Empiric antibiotic selection in this setting can be inaccurate. The objective of this study was to determine whether an antibiotic algorithm (AA) tailored to institutional resistance patterns improves the adequacy of antimicrobial therapy. MethodsA retrospective review of our rapid response system database was performed. Patients with possible sepsis with positive microbiological culture results were enrolled. Pathogens identified by culture were used to determine adequacy of antibiotic selection before and after implementation of an AA. ResultsA total of 234 patients with septic shock were reviewed (before AA, n = 36; after AA, n = 198). Seventy-two patients had positive cultures and were enrolled (before AA, n = 13; after AA, n = 59). Significantly more patients received adequate coverage after AA implementation (54% vs 86%, P = .02). Before AA, inadequate Gram-negative coverage was the most common reason for failure. Reasons for failure in the after-AA group were nonadherence to the algorithm (n = 5) and multidrug-resistant pathogens (n = 3). The algorithm failed in patients with vancomycin-resistant enterococci (n = 3), multidrug-resistant Klebsiella pneumoniae (n = 1), and Candida albicans (n = 1). ConclusionsThe use of an AA significantly improves the adequacy of empiric antimicrobial therapy.

Diagnostic and Therapeutic Evaluation of Community-acquired Methicillin-resistant Staphylococcus Aureus (MRSA) Skin and Soft Tissue Infections in the Emergency Department

BackgroundCommunity-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) infections commonly present as skin and soft-tissue infections (SSTIs). Treatment often includes incision and drainage with or without adjunctive antibiotics. Emergency department (ED) pharmacists wished to provide specific data to emergency physicians to better inform antibiotic choices for patients with SSTIs. Study ObjectivesThe objectives of this study were to describe local susceptibility trends of CA-MRSA isolates obtained from patients with SSTIs and describe diagnostic and empiric therapeutic management of CA-MRSA SSTIs among ED health care providers at University of Utah Hospitals and Clinics. MethodsSusceptibility of all unique CA-MRSA SSTI isolates for 2008 were identified and compiled into an antibiogram. ED providers evaluated their diagnostic and treatment habits using a self-assessment questionnaire, which was verified against charted information documented in the electronic medical records for patients presenting to the ED with a CA-MRSA SSTI. ResultsThe ED antibiogram indicated that 57/58 (98%) CA-MRSA SSTI isolates were susceptible to sulfamethoxazole/trimethoprim (SMX/TMP); 50/58 (86%) isolates were susceptible to tetracycline, and 47/58 (81%) isolates were susceptible to clindamycin. Incision and drainage were performed in 23/25 (92%) patient cases, which was consistent with providers’ perceived habits (100%). SMX/TMP monotherapy was preferred among 23/35 (66%) providers, however, SMX/TMP combined with cephalexin was the antibiotic regimen prescribed in 9/22 (41%) patient cases. ConclusionsCephalexin was often added to cover for potential cellulitis due to Streptococcus spp., however, the surrounding erythema may simply be an extension of the CA-MRSA infection. Department-specific antibiograms are useful in guiding empiric antibiotic selection and may help providers judiciously prescribe antibiotics only when necessary.

Assessment of Risk Factors for Multi-Drug Resistant Organisms to Guide Empiric Antibiotic Selection in Long Term Care: A Dilemma

Assessment of Risk Factors for Multi-Drug Resistant Organisms to Guide Empiric Antibiotic Selection in Long Term Care: A Dilemma

Implementation of a care bundle for antimicrobial stewardship

The impact of an antibiotic-use care bundle on compliance with quality indicators was evaluated. Patients admitted to the internal medicine or surgery floor of a tertiary care center who were receiving an anti-pseudomonal beta-lactam, vancomycin, a fluoroquinolone, linezolid, an amino-glycoside, or any combination of these agents were included in the study. The study consisted of two phases: intervention (when a stewardship pharmacist was involved in patient care) and control (when no stewardship pharmacist was involved). The stewardship pharmacist completed interventions via prospective audit and suggested changes to empirical and definitive antimicrobials, monitored patients' cultures and antimicrobial therapy daily, and provided education on the institution's antibiogram. The primary outcome measured was compliance with the care bundle's quality indicators, which included documentation of treatment rationale, collection of appropriate culture specimens according to institutional and national guidelines, appropriate empirical selection of antibiotics according to institutional and national guidelines at initiation of antibiotic therapy and deescalation, and selection of appropriate agents for definitive therapy during antimicrobial therapy. A total of 160 patients and 442 antibiotic orders were evaluated. During the intervention phase, 168 interventions were made, with an acceptance rate of 91%. The rate of appropriate deescalation rose from 72% to 90% (p = 0.01). Compliance with all quality indicators rose from 16% to 43% (p < 0.001). Implementation of an antimicrobial stewardship program care bundle on two patient care units was associated with improved rates of compliance with quality indicators.

Bloodstream Infections in Children with Cancer between 2005 and 2008 in a Single Center

Purpose : In children on anticancer chemotherapy, bloodstream infections (BSIs) are a major cause of morbidity and mortality. We investigated febrile episodes and bloodstream infections in pediatric cancer patients to guide proper selection of empiric antibiotics for febrile pediatric hemato-oncologic patients. Methods : All febrile episodes treated in the division of hematology-oncology, the department of pediatrics, Hanyang University Hospital, between July 2005 and June 2008 were reviewed. Episodes with and without bloodstream infections were compared. Results : Forty cases (18.9%, 25 patients) of BSI occurred in 212 febrile episodes (63 patients). Thirty-seven cases (23.6%, 22 patients) of BSI occurred in 157 febrile episodes with neutropenia (54 patients). Microorganisms identified in BSI corresponded to 23 gram-positive bacteria (51.2%), 20 gram-negative bacteria (44.5%), and 2 fungi (4.4%). Rates of BSI between those who had received umbilical cord blood transplantation and those who had received transplantation from other source were significantly different (55.0% vs. 7.7%, P =0.001). No differences in mortality rate were observed among organisms in BSI patients. For febrile episodes the rate of BSI was higher among those with Chemoport than those with Hickman catheter (P =0.029) and gram-positive pathogens were more likely to be associated with Chemoport (P =0.001). Conclusion : The study showed the rate of BSI, distribution of pathogens with regard to neutropenia, transplantation, central venous catheters, and antimicrobial susceptibility of pathogens in order to help guide in the choice of optimal empiric antibiotics in pediatric febrile neutropenic hemato-oncologic patients.

Risk of acquiring drug-resistant (MDR) Gram-negative isolates with previous exposure to antibiotic

Prior antimicrobial therapy is one of the most important factors leading to the acquisition of MDR organisms. Formulating antibiotic policy and choice of empirical antibiotic selection will be helped by knowing the association of MDR Gram-negative organisms with a previous exposure of particular antibiotic.

Shortened Time to IdentifyStaphylococcusSpecies from Blood Cultures and Methicillin Resistance Testing Using CHROMAgar

The ability to rapidly differentiate coagulase-negative staphylococcus (CoNS) from Staphylococcus aureus and to determine methicillin resistance is important as it affects the decision to treat empiric antibiotic selection. The objective of this study was to evaluate CHROMagar S. aureus and CHROMagar MRSA (Becton Dickinson) for rapid identification of Staphylococcus spp. directly from blood cultures. Consecutive blood culture bottles (BacT Alert 3D SA and SN, bioMérieux) growing gram-positive cocci in clusters were evaluated. An aliquot was plated onto CHROMagar MRSA (C-MRSA) and CHROMagar S. aureus (C-SA) plates, which were read at 12 to 16 hours. C-SA correctly identified 147/147 S. aureus (100% sensitivity); 2 CoNS were misidentified as S. aureus (98% specificity). C-MRSA correctly identified 74/77 MRSA (96% sensitivity). None of the MSSA isolates grew on C-MRSA (100% specificity). In conclusion, CHROMagar is a rapid and sensitive method to distinguish MRSA, MSSA, and coagulase-negative Staphylococcus and may decrease time of reporting positive results.

Empiric antibiotic selection for infectious emergencies: Bacterial pneumonia, meningitis and sepsis

Appropriate early management of infectious emergencies is essential to preventing adverse outcomes. Clinicians in acute-care settings must be prepared to provide appropriate empiric antibiotics, as failure to do so has been associated with an increased risk of mortality. Empiric treatment of these infections requires knowledge of the disease state, most common pathogens, and patient-specific risk factors. Additionally, regional and institutional patterns of antibiotic resistance must be taken into consideration. At the same time, inappropriate use of broad-spectrum antibiotics should be avoided to prevent emergence of antimicrobial resistance. It is also essential that initial antibiotic therapy be de-escalated and redirected once the causative pathogen has been identified and antibiotic susceptibilities determined. Bacterial infection with pneumonia, meningitis and sepsis severe enough to warrant hospitalization are included in this review. The most common bacterial pathogens and empiric antibiotic recommendations for immunocompetent adults will be discussed. In addition, circumstances that may influence the selection of certain antibiotics are reviewed.

Decision support systems for antibiotic prescribing

To explore recent developments in computerized evidence-based guidelines and decision support systems that have been designed to improve the effectiveness and efficiency of antibiotic prescribing. The most frequently utilized decision support systems are electronic guidelines and protocols, especially for empirical selection of antibiotics. The majority of decision support systems result in improvement in clinical performance and, in at least half of the published trials, in patient outcomes. Despite the reported successes of individual applications, the safety of electronic prescribing systems in routine practice has been identified recently as an issue of potential concern. Bioinformatics-assisted prescribing may contribute to reducing the complexities of prescribing combinations of antimicrobials in the era of multidrug resistance. The reemerging interest in prescribing decision support reflects the recent change in emphasis from support for diagnostic decisions towards support for patient management and from systems targeting a broad range of clinical diagnoses to task specific and condition-specific decision aids.

The Impact of the Current Epidemiology of Pediatric Musculoskeletal Infection on Evaluation and Treatment Guidelines

Methicillin-resistant Staphylococcus aureus is thought to have led to an increase in the incidence of severe musculoskeletal infection in children. Our purpose was (1) to compare the current epidemiology of musculoskeletal infection with historical data at the same institution 20 years prior and (2) to evaluate the spectrum of the severity of this disease process within the current epidemiology. Children with musculoskeletal infection, treated between January 2002 and December 2004, were studied retrospectively. Diagnoses of osteomyelitis, septic arthritis, pyomyositis, and abscess were established for each child based on overall clinical impression, laboratory indices, culture results, radiology studies, and intraoperative findings. Comparison was made with the experience reported at the same institution in 1982. Children within each diagnostic category were compared with respect to mean values of C-reactive protein and erythrocyte sedimentation rate at admission, number of surgical procedures, intensive care unit admissions, identification of deep venous thrombosis, and length of hospitalization. Five hundred fifty-four children were studied (osteomyelitis, n = 212; septic arthritis, n = 118; pyomyositis, n = 20; and deep abscess, n = 204). The annualized per capita incidence of osteomyelitis increased 2.8-fold, whereas that of septic arthritis was unchanged when compared with historical data from 20 years prior. Methicillin-resistant Staphylococcus aureus was isolated as the causative organism in 30% of the children. We identified increasing severity of illness according to a hierarchy of tissue involvement (osteomyelitis > septic arthritis > pyomyositis > abscess) and according to the identification of contiguous infections within in each primary diagnostic category. The incidence of musculoskeletal infection appears to have increased within our community. We found that a more comprehensive diagnostic classification of this disease is useful in understanding the spectrum of the severity of illness and identifying those who require the greatest amount of resources. Magnetic resonance imaging is useful early in the diagnostic process to enable a more detailed disease classification and to expedite surgical decisions. The recognition of the incidence of methicillin-resistant Staphylococcus aureus within our community has also led to a change in empirical antibiotic selection.

Antimicrobial resistance patterns of urinary tract pathogens and rationale for empirical therapy in Turkish children for the years 2000–2006

In patients with suspected urinary tract infection (UTI), antibiotic treatment is usually started empirically, before urine culture results are available. Unfortunately, antibiotic resistance has become an increasingly pressing clinical issue in many countries. The objective of this study was to assess the changing susceptibility of urinary pathogens to commonly used antimicrobials in a six-year period to evaluate the options for empirical antibiotic therapy in children with community acquired UTI. A retrospective analysis of data from all pediatric urine samples processed at Fatih University Medical School microbiology laboratory was undertaken for a period of six years (January 2000-December 2006). A total of 767 urinary pathogens were isolated from 767 episodes of UTI in 698 patients. The most common causative agent was Escherichia coli (E. coli) followed by Klebsiella spp. and others. In 2000 almost 60% of the E. coli isolates were susceptible to ampicillin (AMP), more than 40% to Co-trimoxazole (SXT), more than 80% to gentamicin (GN), more than 90% to cefuroxime (CXM) and amikacin (AN), and more than 60% to piperacillin (PIP). By 2006 more than 70% were resistant to AMP and more than 50% were resistant to PIP. In 2000 CIP (2.7% resistant isolates) and CXM (3.4% resistant isolates) were the most active agents against Klebsiella spp.; and none of the isolates was found to be resistant to imipenem (IMP). In 2006 GN (2.7% resistant isolates), CIP (3.5% resistant isolates), CXM (2.7% resistant isolates), and AN (8.9% resistant isolates) were the most active agents against these species and still no resistance to IMP was found. For E. Coli the increase in resistance to AMP, CTX, IMP, and PIP was statistically significant (P < 0.05). For Klebsiella spp. the increase in resistance to AMP and CXM was statistically significant (P < 0.05). Empirical antibiotic selection should be based on knowledge of the local prevalence of bacterial organisms and antibiotic sensitivities, because resistance patterns may vary in different regions.

Surveillance culture utility and safety using low‐volume blind bronchoalveolar lavage in the diagnosis of ventilator‐associated pneumonia

Surveillance cultures may improve the prediction of ventilator-associated pneumonia (VAP) and empirical antibiotic selection. This study examined the utility and patient safety of blind, non-protected, low-volume mini-bronchial lavage (BM-BAL) surveillance cultures in predicting VAP. A prospective, cohort study was performed in a large general intensive care unit. BM-BALs were collected within 12 h of admission then thrice weekly. Each BM-BAL was screened by Gram staining for intracellular organisms and then quantitatively cultured. VAP was diagnosed using the Clinical Pulmonary Infection Score. The concordance for isolates from the BM-BAL was assessed against concurrently collected endotracheal aspirates (EA). Four hundred and twelve patients requiring a minimum of 48 h of mechanical ventilation were enrolled. Fifty patients developed 58 episodes of VAP. Concordant pathogens were found in 85% of BM-BAL specimens collected 2 days prior to VAP onset. Their antibiograms were stable over the preceding 4 days. The isolation of pathogens with colony counts >or=10(4) cfu/mL from BM-BAL performed 2 days prior to the clinical onset of VAP had a sensitivity of 84%, specificity of 50%, positive predictive value of 31% and a negative predictive value of 93% for predicting the development of VAP. BM-BAL WCC, quantification of bacterial growth and the percentage of intracellular organisms were not helpful in predicting VAP diagnosis. BM-BAL surveillance cultures are well tolerated and useful in predicting the pathogens and their antibiograms causing VAP. Diagnostic specimen collection at the time of VAP onset is still required as surveillance cultures may be negative even one day prior to VAP onset.

Determination of Risk Factors Associated With Isolation of Linezolid-Resistant Strains of Vancomycin-Resistant Enterococcus

To identify independent risk factors associated with isolation of linezolid-resistant, vancomycin-resistant Enterococcus (VRE). A retrospective, case-case-control study. A tertiary care, academic medical center.Methods. VRE isolates from clinical cultures were retrospectively analyzed for linezolid resistance during our 18-month study period. Clinical data were obtained from electronic patient records, and the risk factors associated with isolation of linezolid-resistant VRE were determined by comparison of 2 case groups with a control group. A total of 20% of the VRE isolates analyzed during the study period were linezolid resistant, and resistant isolates were most commonly recovered from the urine (40% of resistant isolates). Risk factors found to be associated with isolation of linezolid-resistant VRE were peripheral vascular disease and/or the receipt of a solid organ transplant, total parenteral nutrition, piperacillin-tazobactam, and/or cefepime. Only 25% of patients from whom linezolid-resistant VRE was isolated had previous linezolid exposure, and in the multivariate model this was not found to be a risk factor associated with the isolation of linezolid-resistant VRE. The results of this analysis suggest that there is horizontal transmission of linezolid-resistant VRE in our institution and highlight the need for improved infection control measures. Furthermore, the high incidence of linezolid-resistant VRE demands a reassessment of our empirical antibiotic selection for patients infected with VRE.

Surgical site and vascular infections: treatment and prophylaxis

Vascular infections typically include those of surgical sites, prosthetic grafts, and vascular ulcers, including some diabetic foot ulcers. Each of these infections represents a serious health concern, particularly among individuals with comorbid conditions who are at an increased risk of morbidity and mortality. Surgical site infections occur primarily as a result of contamination by skin organisms during surgery, whereas prosthetic graft infections result typically from a progressive wound infection. Diabetic foot ulcers and infections are especially complicated and difficult to treat. They occur in individuals with systemic illness that has compromising effects on the nervous, vascular, musculoskeletal, and immunologic systems. Vascular infections, like those elsewhere in the body, reflect an imbalance between the host and bacteria. Efforts to limit or prevent the likelihood of patients developing these infections centre on reducing the bacterial inoculum by means of asepsis and antisepsis. As well as size of the bacterial inoculum, the bacterial properties of pathogenicity and resulting virulence are also significant. The most frequent pathogenic bacteria encountered in surgical patients are Gram-positive cocci (e.g. Staphylococcus aureus and streptococci). Strains with multiple antibiotic resistance (e.g. meticillin resistant S. aureus [MRSA], S. epidermidis, and vancomycin-resistant enterococci [VRE]) can cause significant surgical site infection problems. Local resistance patterns and surveillance efforts are essential to ensure appropriate empiric antibiotic selection for prophylaxis or treatment.

Mixed community-acquired lower respiratory tract infections

Although mixed infections are known to be clinically relevant in conditions such as nosocomial pneumonia and ventilator-related pneumonia, it is increasingly recognized that a substantial number of community-acquired lower respiratory tract infections may also be attributed to more than one pathogenic organism. A better definition of the true incidence of mixed infections in community-acquired lower respiratory tract infections is partly derived from recent advances in available diagnostic methods (eg, molecular biology). Two points still must be determined: whether the presence of a mixed infection is associated with altered outcomes and whether empirical antibiotic selection should be modified to account for potential polymicrobial infections.

Consequences of Combining Cystic Fibrosis— and Non-Cystic Fibrosis-Derived Pseudomonas aeruginosa Antibiotic Susceptibility Results in Hospital Antibiograms

In preparing hospital antibiograms for individual organisms and antibiotics, laboratories often combine susceptibility data for isolates from a variety of sources and patient types. If results from patients with known resistance patterns that vary from normal are included, the overall susceptibility value for the institution could be misleadingly skewed. To assess the degree of bias introduced into a hospital antibiogram by combining cystic fibrosis (CF) and non-CF isolates of Pseudomonas aeruginosa to produce one hospital-wide percent susceptible figure for each tested antibiotic. A retrospective analysis was conducted of an academic, tertiary care medical center's microbiology database. We examined quarterly and annual susceptibility data from 2004, comparing non-CF data with combined susceptibility data for 10 antibiotics within each quarter, as well as those reported in the annual antibiogram. Differences were assessed for statistical significance using chi(2) testing with Bonferroni correction. Large differences were observed between non-CF and combined percent susceptible data in the 4 quarters (aminoglycosides 3% vs 20%, fluoroquinolones 2% vs 18%, respectively) and when comparing annual non-CF (n = 191) with annual combined (n = 266) data. With the annual figures, these differences were frequently statistically significant (70% vs 58%, 91% vs 83%, 85% vs 70%, and 72% vs 60% for gentamicin, tobramycin, amikacin, and gatifloxacin/levofloxacin, respectively; all p< or =0.01). Combining CF and non-CF P. aeruginosa susceptibility into one percent susceptibility value for all isolates may produce figures that underestimate the activity of some antibiotic classes against non-CF isolates. Clinicians may make less than optimal empiric antibiotic selection choices based on such data.