Impact of severity of illness bias and control group misclassification bias in case-control studies of antimicrobial-resistant organisms.

BACKGROUND: Case-control studies often analyze risk factors for antibiotic resistance. Recently published articles have illustrated that randomly selected control-patients may be preferable to those with the susceptible phenotype of the organism. A possible methodologic problem with randomly selected control-patients is potential bias due to control group misclassification. This occurs if some control-patients did not have clinical cultures performed and thus might have been unidentified case-patients. If this bias exists, these studies might be expected to report lower odds ratios (ORs) because control-patients would be more like case-patients. OBJECTIVE: To analyze potential biases that might arise due to control group misclassification and potentially larger selection biases that may be introduced if control-patients are required to have at least one clinical culture. PATIENTS: One hundred twenty case-patients, 770 control-patients in group 1, and 510 control-patients in group 2. METHODS: Two case-control studies. Case-patients had clinical cultures positive for imipenem-resistant Pseudomonas aeruginosa. The first group of control-patients were random. The second group of control-patients were identical to those in group 1 except being required to have at least one clinical culture. RESULTS: Univariate analyses showed higher ORs for case-patients versus control-patients in group 1 (imipenem [OR, 12.5], piperacillin-tazobactam [OR, 3.7], and vancomycin [OR, 4.7]) as compared with case-patients versus control-patients in group 2 (imipenem [OR, 8.0], piperacillin-tazobactam [OR, 2.5], and vancomycin [OR, 3.0]). CONCLUSION: Requiring control-patients to have at least one clinical culture introduces a selection bias likely because it eliminates patients with less severe illness.     

Determination of risk factors associated with isolation of linezolid-resistant strains of vancomycin-resistant Enterococcus.

OBJECTIVE: To identify independent risk factors associated with isolation of linezolid-resistant, vancomycin-resistant Enterococcus (VRE). DESIGN: A retrospective, case-case-control study. SETTING: 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. RESULTS: 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. CONCLUSIONS: 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.     

The wound care team: a new source of group a streptococcal nosocomial transmission.

BACKGROUND: In August 2001, the Centers for Disease Control and Prevention (CDC) notified the Texas Department of Health (TDH) of an unusually high number of wounds infected with group A streptococci (GAS) in an acute care facility. The TDH initiated an investigation, ultimately identifying 28 cases of non-pharyngeal, non-community-acquired GAS that had occurred between December 2000 and August 2001 and resulted in 3 deaths and 4 nonfatal cases of invasive disease. Ten specimens were sent to the CDC for emm typing; all isolates were emm type 114. However, the source of the outbreak could not be confirmed through laboratory testing at that time. METHODS: A case-control study was conducted comparing the 10 case-patients with 52 control-patients with wounds that were not infected with GAS. Age, gender, type of wound, underlying medical conditions, and treatment by the wound care team were examined for association with GAS infection. RESULTS: The odds of having wound care team treatment versus not having it were 424.2 (95% confidence interval, 19.0 to 9,495.2) among case-patients when compared with control-patients. No other risk factor showed this magnitude of association. CONCLUSIONS: This study provided overwhelming epidemiologic evidence that the wound care team was the means of transmission. One year later, when two patients receiving wound care were concurrently diagnosed as having GAS, a member of the wound care team was found to be GAS positive for the matching emm type. This is the first report of a GAS hospital outbreak linked to a wound care team.     

Clustering of Serratia marcescens infections in a neonatal intensive care unit.

OBJECTIVES: To study clusters of infections caused by Serratia marcescens in a neonatal intensive care unit (NICU) and to determine risk factors for S. marcescens infection or colonization. DESIGN: Genotyping of S. marcescens isolates was performed by pulsed-field gel electrophoresis (PFGE). A retrospective case-control study was conducted. SETTING: A tertiary-care pediatric hospital with a 16-bed NICU. PATIENTS: All neonates with at least one culture positive for S. marcescens in the NICU during December 1999 to July 2002. Case-patients (n = 11) treated in the NICU during December 1999 to February 2000 were included in the case-control study. Neonates treated in the NICU for at least 72 hours during the same period with cultures negative for S. marcescens were used as control-patients (n = 27). RESULTS: S. marcescens was cultured from 19 neonates; 9 were infected and 10 were colonized. PFGE analysis identified three epidemic strains; each cluster consisted of identical isolates, except one isolate in the first cluster that was different. The risk factors identified were low birth weight, prematurity, prolonged respiratory therapy, prolonged use of antibiotics, and maternal infection prior to delivery. Overcrowding and understaffing were recorded simultaneously with the clusters. CONCLUSIONS: PFGE analysis showed three independent clusters. Several factors contributed to spread of the epidemic strains: (1) there were many severely premature and susceptible neonates, (2) the NICU was overcrowded during the clusters, and (3) transmission was likely to occur via the hands of staff. Cohorting and improvement of routine infection control measures led to the cessation of each cluster.     

Importance of control group selection for evaluating antimicrobial use as a risk factor for methicillin-resistant Staphylococcus aureus bacteremia.

OBJECTIVE: We investigated the importance of control group selection during an evaluation of antimicrobial use as a risk factor for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia at our institution. METHODS: We performed a case-control study. A case was defined as any patient admitted between January 1997 and May 2001 who developed nosocomial MRSA bacteremia. We used two control groups; control group I consisted of patients with nosocomial methicillin-susceptible S. aureus (MSSA) bacteremia and control group II included only patients without bacteremia. We matched control-patients to case-patients using age, gender, time at risk, and hospital ward. Data collected on all patients included demographics, comorbidities, antibiotic use, time at risk, length of stay, severity of illness, and outcome. RESULTS: We evaluated 63 patients (21 in each group). The three groups were well matched regarding age, gender, underlying diseases, and severity of illness. Patients in the MRSA group were more likely to have received a fluoroquinolone and had a higher mean number of days of fluoroquinolone use than did patients in the MSSA group (P = .027 and P = .015, respectively). However, all measures of fluoroquinolone use were similar for case-patients and for control-patients who did not have nosocomial bloodstream infection. CONCLUSIONS: Control group selection is important in evaluating antimicrobial use as a risk factor for MRSA bacteremia. Using control-patients infected with MSSA, rather than uninfected control-patients, may overestimate the association between antimicrobial use and MRSA infection.     

Chloramphenicol resistance in vancomycin-resistant enterococcal bacteremia: impact of prior fluoroquinolone use?

OBJECTIVE: The prevalence of vancomycin-resistant enterococci (VRE) has increased markedly during the past decade. Few data exist regarding the epidemiology of resistance of VRE to chloramphenicol, one of the few therapeutic options. DESIGN: Survey and case-control study. SETTING: A 725-bed, tertiary-care academic medical center and a 344-bed urban community hospital. PATIENTS: Hospitalized patients with blood cultures demonstrating VRE. METHODS: We examined the trends in the prevalence of chloramphenicol resistance in VRE blood isolates at our institution from 1991 through 2002 and conducted a case-control study to identify risk factors for chloramphenicol resistance among these isolates. RESULTS: From 1991 through 2002, the annual prevalence of chloramphenicol-resistant VRE increased from 0% to 12% (P < .001, chi-square test for trend). Twenty-two case-patients with chloramphenicol-resistant VRE bloodstream isolates were compared with 79 randomly selected control-patients with chloramphenicol-susceptible VRE. Independent risk factors for chloramphenicol-resistant VRE were prior chloramphenicol use (odds ratio [OR], 10.9; 95% confidence interval [CI95], 1.72-68.91; P = .01) and prior fluoroquinolone use (OR, 4.74; CI95, 1.15-19.42; P = .03). Chloramphenicol-resistant VRE isolates were more likely to be susceptible to beta-lactams and resistant to tetracycline than were chloramphenicol-susceptible VRE isolates. CONCLUSIONS: Significant increases in the prevalence of chloramphenicol-resistant VRE may limit the future utility of chloramphenicol in the treatment of VRE infections, and close monitoring of susceptibility trends should continue. The association between fluoroquinolone use and chloramphenicol-resistant VRE, reflecting possible co-selection of resistance, suggests that recent dramatic increases in fluoroquinolone use may have broader implications than previously recognized.     

Prior antimicrobial therapy and resistance of Enterobacter Citrobacter and Serratia to third generation cephalosporins

During a 6-month period all inpatients from whom Enterobacter, Citrobacter or Serratia. had been isolated were reviewed and information on selected variables recorded. Two groups, one including 19 patients with organisms resistant to third generation cephalosporins and the other 111 patients with susceptible organisms were compared. In the initial analysis, the mean number of antimicrobials received in the prior 2 months was the variable most strongly associated with isolation of resistant organisms (2.6 +/- 1.5 vs 1.5 +/- 1.6; P = 0.002). When patients who had received no antimicrobials were omitted from the analysis, the mean number of antimicrobials was similar (2.6 +/- 1.5 vs 2.3 +/- 1.5; P = 0.19). Comparisons of antimicrobials received in the prior 2 months showed only cefoxitin (9/70 vs 7/19; P = 0.016) and cefotaxime (4/70 vs 5/19; P = 0.008) to be associated with isolation of resistant organisms. These data suggest that, at our institution, antimicrobial therapy with an extended spectrum cephalosporin is an important risk factor for subsequent acquisition of an organism resistant to third generation cephalosporins.     

Further development of the case-only design for assessing gene-environment interaction: evaluation of and adjustment for bias

BACKGROUND: The case-only study for investigating gene-environment interactions provides increased statistical efficiency over case-control analyses. This design has been criticized for being susceptible to bias arising from non-independence between the genetic and environmental factors in the population. Given that independence is critical to the validity of case-only estimates of interaction, researchers frequently use controls to evaluate whether the independence assumption is tenable, as advised in the literature. Our work investigates to what extent this approach is appropriate and how non-independence can be accounted for in case-only analyses. METHODS: We provide a formula in epidemiological terms that illustrates the relationship between the gene-environment association measured among controls and the gene-environment association in the source population. Using this formula, we conducted sensitivity analyses to describe the circumstances in which controls can be used as proxy for the source population when evaluating gene-environment independence. Lastly, we generated hypothetical cohort data to examine whether multivariable modelling approaches can be used to control for non-independence. RESULTS: Our sensitivity analyses show that controls should not be used to evaluate gene-environment independence in the population, even when the baseline risk of disease is low (i.e. 1%), and the interaction and independent effects are moderate (i.e. risk ratio = 2). When the factors are associated, it is possible to remove bias arising from non-independence using standard statistical multivariable techniques in case-only analyses. CONCLUSIONS: Even when the disease risk is low, evaluation of gene-environment independence in controls does not provide a consistent test for bias in the case-only study. Given that control for non-independence is possible when the source of the non-independence can be conceptualized, the case-only design may still be a useful epidemiological tool for examining gene-environment interactions.     

广州市儿童诺如病毒胃肠炎临床流行病学分析

目的 研究广州市5岁以下儿童秋冬季诺如病毒(NVs)急性胃肠炎临床特征及危险因素.方法 NVs急性胃肠炎临床特征的探讨采用软状病毒(RV)急性胃肠炎进行频数匹配的两样本研究,危险因素研究采用1:1配对病例对照研究.结果 NVs胃肠炎最早出现的症状是呕吐47.0%(48.102),主要临床症状有呕吐80.3%(82.102)、发热52.9%(54.102)、脱水23.5%(24.102)和腹痛7.8%(8.102)等,与RV感染胃肠炎相似.NVs感染胃肠炎发病的危险因素是吃生冷食品,可疑危险因素是同胃肠炎患者接触及家庭内有腹泻患者.保护因素是勤剪指甲.结论 NVs感染胃肠炎的临床特征及轻重程度与RV相似.其疾病负担仅次于轮状病毒;防止食物或水受到NVs污染以及避免同胃肠炎患者接触,保持良好的个人卫生习惯是预防NVs感染胃肠炎的有效措施.     

An outbreak of Pseudomonas aeruginosa pneumonia and bloodstream infection associated with intermittent otitis externa in a healthcare worker.

OBJECTIVES: To investigate an outbreak of Pseudomonas aeruginosa pneumonia and bloodstream infection among four neonates, determine risk factors for infection, and implement preventive strategies. DESIGN: Retrospective case finding; prospective surveillance cultures of patients, personnel, and environmental sites; molecular typing by pulsed-field gel electrophoresis; and a matched case-control study. PATIENTS AND SETTING: Neonates in the level-III neonatal intensive care unit of a tertiary-care pediatric institution. INTERVENTIONS: Cohorting of patients with positive results for P. aeruginosa, work restrictions for staff with positive results, implementation of an alcohol-based hand product, review of infection control policies and procedures, and closure of the unit until completion of the investigation. RESULTS: Seven (4%) of 190 environmental cultures and 5 (3%) of 178 cultures of individual healthcare workers' hands grew P. aeruginosa. All four outbreak isolates and one previous bloodstream isolate were genotypically identical, as were the P. aeruginosa isolates from the hands and external auditory canal of a healthcare worker with intermittent otitis externa. Four of 5 case-patients versus 5 of 15 matched control-patients had been cared for by this healthcare worker (P = .05). The healthcare worker was treated and no further cases occurred. CONCLUSIONS: These findings suggest that a healthcare worker with intermittent otitis externa may have caused this cluster of fatal P. aeruginosa infections, adding the external ear to the list of colonized body sites that may serve as a source of potentially pathogenic organisms.     

Multidrug-resistant Pseudomonas aeruginosa bloodstream infections: risk factors and mortality

We retrospectively studied patients diagnosed with P. aeruginosa bloodstream infections (BSIs) in two Italian university hospitals. Risk factors for the isolation of multidrug-resistant (MDR) or non-MDR P. aeruginosa in blood cultures were identified by a case-case-control study, and a cohort study evaluated the clinical outcomes of such infections. We identified 106 patients with P. aeruginosa BSI over the 2-year study period; 40 cases with MDR P. aeruginosa and 66 cases with non-MDR P. aeruginosa were compared to 212 controls. Independent risk factors for the isolation of MDR P. aeruginosa were: presence of central venous catheter (CVC), previous antibiotic therapy, and corticosteroid therapy. Independent risk factors for non-MDR P. aeruginosa were: previous BSI, neutrophil count <500/mm3, urinary catheterization, and presence of CVC. The 21-day mortality rate of all patients was 33·9%. The variables independently associated with 21-day mortality were presentation with septic shock, infection due to MDR P. aeruginosa, and inadequate initial antimicrobial therapy.     

An outbreak of methicillin-resistant Staphylococcus aureus infections related to central venous catheters for hemodialysis.

OBJECTIVES: To determine risk factors for hemodialysis catheter-related bloodstream infections (HCRBSIs) and investigate whether use of maximal sterile barrier precautions would prevent HCRBSIs. SETTING: Tertiary-care medical center hemodialysis unit. DESIGN: Open trial with historical comparison and case-control study of risk factors for HCRBSIs. METHODS: Prospective surveillance was used to compare HCRBSI rates for 1 year before and after implementation of maximal sterile barrier precautions. A case-control study compared 50 case-patients with HCRBSI with 51 randomly selected control-patients. RESULTS: The HCRBSI rate was 1.6% per 100 dialysis runs (CI95, 1.1%-2.3%) in the first year and 0.77% (CI95, 0.5%-1.1%) in the second year (P = .0106). The most frequent cause of HCRBSI was MRSA in the first year (15 of 32) and MSSA in the second year (13 of 18). Ten MRSA blood isolates in the first year were identical by PFGE. Diabetes mellitus was a risk factor for HCRBSI. Age, gender, site of hemodialysis central venous catheter (CVC), other underlying diseases, coma score, APACHE II score, serum albumin level, and cholesterol level were not associated with HCRBSI and did not change between the 2 years. Hospital stay was prolonged for case-patients (32.78 +/- 20.96 days) versus control-patients (22.75 +/- 17.33 days), but mortality did not differ. CONCLUSIONS: Use of maximal sterile barrier precautions during the insertion of CVCs reduced HCRBSIs in dialysis patients and seemed cost-effective. Diabetes mellitus was associated with HCRBSI. An outbreak of MRSA in the first year was likely caused by cross-infection via medical personnel.     

A matched case-control study of convenience store robbery risk factors

Convenience store clerks have been shown to be at high risk for assault and homicide, mostly owing to robbery or robbery attempts. Although the literature consistently indicates that at least some environmental designs are effective deterrents of robbery, the significance of individual interventions and policies has differed across past studies. To address these issues, a matched case-control study of 400 convenience store robberies in three metropolitan areas of Virginia was conducted. Conditional logistic regression was implemented to evaluate the significance of various environmental designs and other factors possibly related to convenience store robbery. Findings indicate that numerous characteristics of the surrounding environment and population were significantly associated with convenience store robbery. Results also showed that, on a univariate level, most crime prevention factors were significantly associated with a lower risk for robbery. Using a forward selection process, a multivariate model, which included cash handling policy, bullet-resistant shielding, and numerous characteristics of the surrounding area and population, was identified. This study addressed numerous limitations of the previous literature by prospectively collecting extensive data on a large sample of diverse convenience stores and directly addressing the current theory on the robbers' selection of a target store through a matched case-control design.     

Risk factors for and clinical outcomes of bloodstream infections caused by extended-spectrum beta-lactamase-producing Klebsiella pneumoniae.

OBJECTIVE: To evaluate risk factors and treatment outcomes of bloodstream infections caused by extended-spectrum beta-lactamase-producing Klebsiella pneumoniae (ESBL-KP). DESIGN: Retrospective case-control study. Stored blood isolates of K. pneumoniae were tested for ESBL production by NCCLS guidelines, double-disk synergy test, or both. SETTING: A 1,500-bed, tertiary-care university hospital and referral center. PATIENTS: Sixty case-patients with bacteremia due to ESB-KP were compared with 60 matched control-patients with non-ESBL-KP. RESULTS: There were no significant differences in age, gender, APACHE II score, or underlying diseases between the groups. Independent risk factors for infections caused by ESBL-KP were urinary catheterization, invasive procedure within the previous 72 hours, and an increasing number of antibiotics administered within the previous 30 days. Complete response rate, evaluated 72 hours after initial antimicrobial therapy, was higher among control-patients (13.3% vs 36.7%; P = .003). Treatment failure rate was higher among case-patients (35.0% vs 15%; P = .011). Overall 30-day mortality rate was 30% for case-patients and 28.3% for control-patients (P = .841). Case-patients who received imipenem or ciprofloxacin as a definitive antibiotic had 10.5% mortality. The mortality rate for initially ineffective therapy was no higher than that for initially effective therapy (9.1% vs 11.1%; P = 1.000), but statistical power was low for evaluating mortality in the absence of septic shock. CONCLUSION: For K. pneumoniae bacteremia, patients with ESBL-KP had a higher initial treatment failure rate but did not have higher mortality if antimicrobial therapy was appropriately adjusted in this study with limited statistical power.     

Risk factors for and impact of infection or colonization with aztreonam-resistant Pseudomonas aeruginosa.

OBJECTIVE: To identify risk factors for infection or colonization with aztreonam-resistant Pseudomonas aeruginosa and examine the impact of this organism on mortality. DESIGN: A case-control study was performed to identify risk factors for infection or colonization with aztreonam-resistant P. aeruginosa. A cohort study was subsequently performed to examine the impact of aztreonam resistance on outcomes. SETTING: A tertiary referral center in southeastern Pennsylvania.Participants. Inpatients with a clinical culture positive for P. aeruginosa between January 1, 1999, and December 31, 2000. RESULTS: Of 720 P. aeruginosa. isolates, 183 (25.4%) were aztreonam-resistant and 537 (74.6%) were aztreonam susceptible. In a multivariable model, prior fluoroquinolone use (adjusted odds ratio [aOR], 1.81 [95% confidence interval {CI}, 1.17-2.80]), prior use of an antianaerobic agent (aOR, 1.56 [95% CI, 1.06-2.29]), and renal insufficiency (aOR, 1.59 [95% CI, 1.10-2.29]) were associated with infection or colonization with aztreonam-resistant P. aeruginosa, while older age (aOR, 0.98 [95% CI, 0.97-0.99] per year of age) was negatively associated with infection or colonization with this organism. In-hospital mortality was higher among subjects infected or colonized with aztreonam-resistant P. aeruginosa, compared with those who were infected or colonized with aztreonam-susceptible P. aeruginosa (25.7% vs 16.8%; P=.009), but in multivariable analysis, no significant association was found between infection or colonization with aztreonam-resistant P. aeruginosa and mortality. CONCLUSIONS: Curbing the use of fluoroquinolones and antimicrobials with antianaerobic activity may be an effective strategy to limit the emergence of aztreonam-resistant P. aeruginosa.     

An outbreak of Pichia anomala fungemia in a Brazilian pediatric intensive care unit.

OBJECTIVE: To report an outbreak of Pichia anomala fungemia that occurred in a Brazilian pediatric intensive care unit (ICU) from October 2002 to January 2004. DESIGN: Unmatched case-control study. METHODS: We randomly selected four control-patients for each case-patient from a list of all patients admitted to the ICU for at least 48 hours during the outbreak. A second control group was composed of all consecutive patients with nosocomial candidemia in the ICU during the outbreak. An environmental study was performed, and genetic relatedness among the clinical isolates was characterized by randomly amplified polymorphic DNA assay. RESULTS: During the study period, 1,046 children were admitted to the pediatric ICU, 17 of whom developed P. anomala fungemia (attack rate, 1.6%). The median age was 1.1 years, and the main underlying conditions were congenital malformations (35.3%) and neoplastic diseases (11.8%). The overall mortality rate was 41.2%. Two patients received no antifungal treatment; all of the others were treated with amphotericin B. On multivariate analysis, only the presence of a central venous catheter was significantly associated with P. anomala fungemia. The yeast was not found on healthcare workers' hands or in the environment. Molecular studies showed that the outbreak was caused by a single strain. The distribution of risk factors was similar between patients with P. anomala fungemia and control-patients with candidemia. CONCLUSIONS: This study highlights the importance of P. anomala as an emerging nosocomial fungal pathogen. Patients with P. anomala fungemia seem to have risk factors in common with those who have candidemia.     

洛阳市儿童医院多药耐药菌感染的分布及耐药性分析

目的 分析多药耐药菌分布、耐药性及易感因素,指导临床用药.方法 对2010年度监测到的全院多药耐药菌及药敏结果进行分析,同时追踪其易感因素.结果 共分离出病原菌1033株,其中多药耐药菌461株,检出率为44.6％;分离到的多药耐药菌主要为产ESBLs大肠埃希菌及耐甲氧西林凝固酶阴性葡萄球菌(MRCNS),分别占42.5％、26.2％;多药耐药菌对大多数常用抗菌药物耐药.结论 结合医院药物敏感试验,婴幼儿革兰阴性多药耐药菌感染宜选用头孢哌酮/舒巴坦、亚胺培南;革兰阳性多药耐药菌感染宜选用夫西地酸、替考拉宁、万古霉素.     

基于昆虫共生菌沃尔巴克氏体的蚊媒和蚊媒病控制研究进展

沃尔巴克氏体（Wolb。chia）是一种在自然界节肢动物体内广泛存在、能经卵传递的革兰阴性胞内共生菌。估计约65％的昆虫种类和28％的蚊虫种类天然携带沃尔巴克氏体。当携带沃尔巴克氏体的雄蚊与不携带或者携带不同沃尔巴克氏体型的雌蚊交配,雌蚊产的卵将不会孵化,该性状称为胞质不相容性（cytoplasmicincompatibility,CI）。胞质不相容性赋予携带沃尔巴克氏体的雌蚊生殖优势,从而使其可以扩散到蚊群中去;同时沃尔巴克氏体在蚊媒体内能对多种人类病原体（如登革热病毒、黄病毒和疟原虫等）产生抗性。基于以上特性可建立2种虫媒病控制的新策略：①将野生的传病蚊媒改变成对人类病原体具有抗性的蚊虫,从而阻断蚊媒病的传播（种群替换）;②持续诱导引起胞质不相容性的交配,从而压制或区域性根除传病蚊媒（种群压制）。     

Validity of the cross-sectional study for the ascertainment of nosocomial infection risk factors

This study was carried out in order to assess the validity of the pure cross-sectional study in the ascertainment of nosocomial infection risk-factors. The results yielded by two designs (cross-sectional and case-control) are compared. A cross-sectional design was performed in a tertiary hospital. 592 patients were studied, 38 of whom were nosocomially infected. The clinical information on all the patients included in this design was reviewed after hospital discharge. A matched case-control study was nested in the population cross-sectionally surveyed. 66 cases (28 additional patients developed a hospital infection) and 132 controls were selected. Odds ratios (ORs) for the risk factors analyzed by both designs were compared. There were no significant differences between the estimates yielded by both designs; however, a trend of lower OR estimates for the cross-sectional study was seen, which may be important for risk factors not strongly related to (low relative risk) nosocomial infection. Several factors which might account for the results observed (random error, bias introduced by matching) are discussed. It is suggested that pure cross-sectional designs for the study of risk factors of nosocomial infection may introduce a negative (toward-the-null) bias.