1 School of Biomedical and Chemical Sciences, The University of Western Australia, Queen Elizabeth II Medical Centre, Nedlands, Perth; 2 School of Population Health, The University of Western Australia, Clifton Street, Nedlands, Perth; 3 Division of Microbiology, The West Australian Centre for Pathology and Medical Research, Nedlands, Perth, Western Australia, 6009
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Abstract |
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Keywords: Clostridium difficile, systematic review
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Introduction |
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Despite publication of numerous articles implicating almost all antibiotics with CDAD in hospitalized patients, it is still not clearly understood which antibiotics, or antibiotic classes, in particular are important and how these interact with other risk factors. Many narrative reviews of the subject have been published in recent years; however, only one systematic review that included a meta-analysis of the data has been published.11 It was noted in this review that most of the studies were small, indicating that the quality of CDAD epidemiological studies may be questionable. The pooling of data from observational studies of low internal validity can produce spurious results, particularly if bias and confounding are present.12 The aim of our study was to conduct a systematic review of epidemiological studies in order to determine the validity of reported associations of antibiotics with CDAD.
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Materials and methods |
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Computerized searches of MEDLINE (19662001) and EMBASE (19882001) were undertaken and the articles identified were downloaded to Endnote version 4.0. Search keywords used in the extraction were: Clostridium difficile; human; diarrhoea or colitis; antibiotic or antimicrobial; case control studies, cohort studies, prospective studies or retrospective studies. Literature cited in identified articles was examined for further studies. Assessment of the overall quality of the eligible studies was undertaken by critically reviewing each study in terms of study design, selection and information bias, confounding, precision and external validity.
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Results |
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Study quality
The use of inappropriate control groups, the presence of bias, confounding, misclassification and lack of precision in the effect estimates were common problems identified in the study. Several studies used controls selected from populations that did not necessarily represent the source population from which the cases had come. Most commonly, patients that had been tested for C. difficile during their hospital admission and were negative were used.1830 One study used C. difficile carriers31 and another used patients with other nosocomial infections.32 Table 1 summarizes the results from the remaining 33 studies judged to have used appropriate comparison groups.
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Studies commonly failed to report sources of exposure information, raising the possibility of observer bias.24,3437,39,40,46,5052 Of the remaining studies, all obtained exposure information from medical records. The routine recording of antibiotic and medication exposures, procedures and diagnoses in medical records allows accurate information to be obtained; however, variation in the quality of the medical record may result in missing data, as acknowledged by Katz et al.22 Non-differential misclassification of exposures could not be excluded from most studies due to the method of data collection used. For example, the use of standardized data collection instruments, awareness of the purpose of the study by the abstractor, the use of single or multiple abstractors and whether the abstractors were blind to disease status, was poorly reported.
Misclassification of disease could not be excluded in many studies owing to the sensitivity and specificity of laboratory tests used to diagnose C. difficile infection. The most common test used was the direct detection of cytotoxin from faecal samples, which may have resulted in the inclusion of false-negatives among the controls, particularly in studies that used C. difficile-negative patients from laboratory records for comparison.1823,2527,29,30 Other studies included patients that were identified through the culture of C. difficile from stool samples, enhancing sensitivity but potentially including false-positives owing to the low specificity of culture.33,36,39,41,44,46 Others only included as cases those that had both positive cultures and cytotoxin results.34,37,43,53,54 Only three studies used culture of a toxigenic isolate of C. difficile as part of their case definition.13,24,38 This definition is considered to be the most appropriate for epidemiological studies.55 Others used enzyme immunoassay (EIA) tests to detect either toxin A40,47,49,5660 or toxins A and B28,31,50,61 from faecal samples. Several studies failed to use a definition of diarrhoeal symptoms, or diarrhoea was stated with no clarification.19,20,24,25,32,37,4547,54,57,58,61,62 Of particular concern in some retrospective studies was the use of symptom definitions that required detail regarding the number of diarrhoeal episodes per day, which would be difficult to accurately obtain from notes in the medical record,21,27,29,48,49,51,52,56,59,63 although Katz et al.22 reported a 94% success rate. Because such information is not consistently recorded in medical records, some retrospective studies used the stool description, e.g. unformed, to enhance case ascertainment.30,64
The potential confounding effects of age, length of stay and severity of underlying illness, which may be associated with exposure to antibiotics and development of CDAD, were not adequately assessed in most studies. Age was most commonly considered either through restriction of the study population,18,24,28,29,37,50,58 matching27,40,43,44,47,49,56,60,65 or multivariate analysis.13,22,53,54,61,63,64 Some studies restricted the study population to those with a common underlying condition24,27,35,39,48,50,54,60 or an intensive care stay,20,31,64 or those who underwent particular procedures.32,33,41,47,51,52,61,62 This approach may have controlled for severity of underlying illness to some extent.
Many of the studies had small sample sizes that may have limited the precision of their estimation of effect (see Table 1). This applied in particular to cross-sectional studies and cohort studies, which commonly had small numbers of CDAD outcomes.
Evidence for the association of antibiotics with CDAD
Overall, 41 out of 48 studies found an increased risk of CDAD in hospitals associated with antibiotic exposure, but all had major weaknesses. For total antibiotics, exposure odds ratios (ORs) ranged from 2.8659 to 6.9236 in studies that used appropriate controls. A relative risk (RR) of 2.48 was recorded in one cohort study.33 For specific antibiotics, or antibiotic classes, ORs ranged from 2.1242 to 4254 for clindamycin, and from 3.8442 to 2656 for third-generation cephalosporins.
Only two studies were identified that had less serious threats to validity, and from which the reported results were considered the most reliable. McFarland et al.53 found an increased risk for CDAD after cephalosporin exposure for up to 1 week (RR 2.07, 95% CI 1.066.62) and penicillin exposure for between 1 and 2 weeks (RR 3.62, 95% CI 1.288.42) adjusted for age and severity of disease using Horns index. A moderately large cross-sectional study by Chang & Nelson63 provided precise estimates of effect for clindamycin (OR 4.22, 95% CI 2.118.45) and increased numbers of antibiotics (OR 1.49, 95% CI 1.231.81) adjusted for age, length of stay and proximity to patients with CDAD, but did not adjust for co-morbidities.
The ability to generalize study results to other populations was limited because of serious threats to internal validity in most of the studies. The association between CDAD and clindamycin, cephalosporins, penicillins and the number of antibiotics a patient received could be interpreted as representative of a more general association in other hospitalized patients.
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Discussion |
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Almost half of the studies were described by their authors as cohort or prospective studies, although they were regarded in this review as cross-sectional studies because they did not meet the epidemiological classification of a cohort study.66 Cross-sectional studies are generally poor for testing causal hypotheses, owing to the potential for length-biased sampling (a person with a longer duration of illness will have a greater chance of being selected) and the use of measurements of current rather than past exposures, which may prevent the separation of cause and effect. Several cross-sectional studies identified in this review identified cases prospectively (incident cases), thus minimizing length-biased sampling, and collected risk factor information from the medical record rather than relying on self-reporting. However, the main limitation found with many of these studies was the low number of CDAD outcome events, which may have resulted in an imprecise estimate of effect.
The choice of controls in casecontrol studies of CDAD has been a contentious issue recently, with arguments for and against the use of symptomatic C. difficile-negative patients as controls.67,68 In order to accurately measure the risk of acquiring a disease if exposed to a particular factor, cases should be compared with controls selected from the source population, i.e. subjects who would have been included in the case group if they had developed the disease. Therefore, restricting the selection of controls to patients investigated for C. difficile is not representative of the source population from which the cases arose. This is likely to produce a result biased to the null either because exposures are likely to be similar for CDAD or other forms of nosocomial diarrhoea, or because of the classification of false-negatives as controls, depending on the sensitivity of the laboratory test used.
The different methods for laboratory diagnosis of CDAD used throughout the studies reviewed have implications for the misclassification of disease that can, as previously noted, produce a biased effect estimate. Direct detection of C. difficile cytotoxin (toxin B) from faecal specimens using mammalian tissue culture lines is considered the standard diagnostic test, and was the one used most commonly in the studies identified. However, this method lacks sensitivity due to inactivation of the cytotoxin in 20% of samples during storage and transport.69 Recent evidence indicates that C. difficile and its spores are affected relatively little by storage conditions compared with cytotoxin.70 In addition, 60% of cytotoxin-negative faecal samples contain a cytotoxin-producing strain of C. difficile, suggesting a false-negative laboratory result.71,72 These methods are also time consuming, and EIAs that provide rapid results have been developed. Commercial EIAs for toxin A were used by several studies; however, this approach also requires culturing of C. difficile for adequate toxin detection.73 Furthermore, toxin AB+ variant C. difficile strains, found to account for 3% of isolates sent to a reference laboratory in the UK,74 would be missed using this method. Although toxin AB+ strains occur at a low prevalence, there is concern that these variant strains will spread through hospitals undetected where toxin A testing is used alone. To overcome this, EIAs that detect both toxins have been developed. These perform well against faecal cytotoxin tissue culture, but are still less sensitive than culture of toxigenic C. difficile.7577 Culture of C. difficile followed by detection of cytotoxin, either by tissue culture or immunoassay, has recently been recommended for confirmation of diagnosis in patients with cytotoxin-negative faecal samples.69,78 Therefore, in epidemiological studies of CDAD, culture of C. difficile followed by toxin demonstration is important in order to accurately identify cases.
In this study, we systematically reviewed the quality of published studies that investigated antibiotics as risk factors for hospital-acquired CDAD in order to decide whether a meta-analysis could be reliably conducted. A previous systematic review concluded that the meta-analysis approach enabled the ranking of antibiotics in relation to the risk of C. difficile infection.11 In our study, a critical review of studies that analysed antibiotics as risk factors for CDAD was undertaken in terms of study design, selection and information biases, control of confounding, precision of the effect estimate and external validity. Although our review was limited by including only published studies and excluding non-English articles, only two studies were identified that were considered to provide valid evidence for the role of antibiotics in hospital-acquired CDAD. Therefore, based on our findings we do not agree with Bignardi11 that a meta-analysis can be reliably conducted in order to assess the relationship between antibiotics and C. difficile.
Future studies designed to investigate the aetiology of hospital-acquired CDAD need to be aware of epidemiological principles. Investigators need to define a clear hypothesis when designing such studies so that appropriate controls are selected, an adequate sample size is used, bias minimized and confounding controlled. Adherence to these principles will ensure that risk factors are accurately identified and their association with CDAD validly and precisely estimated.
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Footnotes |
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References |
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