Risk factors for invasive fungal disease in critically ill adult patients: a systematic review

Introduction Over 5,000 cases of invasive Candida species infections occur in the United Kingdom each year, and around 40% of these cases occur in critical care units. Invasive fungal disease (IFD) in critically ill patients is associated with increased morbidity and mortality at a cost to both the individual and the National Health Service. In this paper, we report the results of a systematic review performed to identify and summarise the important risk factors derived from published multivariable analyses, risk prediction models and clinical decision rules for IFD in critically ill adult patients to inform the primary data collection for the Fungal Infection Risk Evaluation Study. Methods An internet search was performed to identify articles which investigated risk factors, risk prediction models or clinical decisions rules for IFD in critically ill adult patients. Eligible articles were identified in a staged process and were assessed by two investigators independently. The methodological quality of the reporting of the eligible articles was assessed using a set of questions addressing both general and statistical methodologies. Results Thirteen articles met the inclusion criteria, of which eight articles examined risk factors, four developed a risk prediction model or clinical decision rule and one evaluated a clinical decision rule. Studies varied in terms of objectives, risk factors, definitions and outcomes. The following risk factors were found in multiple studies to be significantly associated with IFD: surgery, total parenteral nutrition, fungal colonisation, renal replacement therapy, infection and/or sepsis, mechanical ventilation, diabetes, and Acute Physiology and Chronic Health Evaluation II (APACHE II) or APACHE III score. Several other risk factors were also found to be statistically significant in single studies only. Risk factor selection process and modelling strategy also varied across studies, and sample sizes were inadequate for obtaining reliable estimates. Conclusions This review shows a number of risk factors to be significantly associated with the development of IFD in critically ill adults. Methodological limitations were identified in the design and conduct of studies in this area, and caution should be used in their interpretation.


Introduction
In the past, invasive fungal disease (IFD) was more commonly found in patients who were neutropenic, had received a solid organ transplant or had been treated with corticosteroids or cytotoxic agents. Increasingly, IFD is now more likely to occur in nonneutropenic patients in critical care units [1]. The majority of IFD in the critical care setting is due to Candida species [2,3]. In 2006, the Health Protection Agency (HPA) estimated that over 5,000 cases of invasive Candida species infections occur in the UK each year and that around 40% of these occur in critical care units [4]. An epidemiological survey in six UK sentinel hospitals reported that 45% of Candida bloodstream infections occur in the critically ill [5]. IFD in critically ill patients is associated with increased morbidity and mortality at a cost to both the individual and the National Health Service [6,7].
A number of randomised controlled trials (RCTs) have evaluated antifungal prophylaxis in nonneutropenic, critically ill patients, predominantly with either fluconazole [8][9][10][11][12] or ketoconazole [13][14][15][16]. Several systematic reviews and meta-analyses of these studies have been performed [17][18][19][20][21][22]. These reviews reveal that, across the individual studies, patient groups were heterogeneous, ranging from high-risk surgical patients [11,12,16] to those with septic shock [8] or acute respiratory distress syndrome [13,15]. All of the patient groups, however, were at high risk of IFD, with rates in the control arms typically being over 10%. Despite this heterogeneity, the RCTs demonstrated a remarkably homogeneous effect of antifungal prophylaxis on the risk of proven IFD with a suggested reduction in all-cause mortality [17]. The question, therefore, is not whether antifungal prophylaxis is effective, but rather how to select an appropriate group of high-risk patients to receive prophylaxis, as indiscriminate use of antifungal agents is likely to promote drug resistance and drive up cost.
The Fungal Infection Risk Evaluation (FIRE) Study was undertaken with the aim of developing and validating a risk model to identify critically ill nonneutropenic patients at high risk of IFD who would benefit from antifungal prophylaxis (UK Clinical Research Collaboration registered ID number 42) [https://www.icnarc.org/ CMS/ArticleDisplay.aspx?ID=8234e564-5902-de11-b27f-0015c5e673e7&root=RESEARCH&categoryID=70422f67-6983-de11-9a46-002264a1a658]). The first step in model development was to prospectively gather data on risk factors for IFD for this patient group. This paper reports the results of a systematic review performed to identify and summarise the important risk factors from published multivariable analyses, risk prediction models and clinical decision rules for IFD in critically ill adult patients to inform the primary data collection in the FIRE Study.

Materials and methods
An internet search was performed using MEDLINE (1950 to 2008, SilverPlatter WebSPIRS, http://www.ovid. com/site/products/tools/silverplatter/sp_webspirs.jsp; Ovid/Wolters Kluwer Health, New York, NY, USA), Embase (1947 to 2008, http://www.embase.com/; Silver-Platter WebSPIRS) and CINAHL (1960 to 2008, EBSCOhost, http://www.ebscohost.com/cinahl/; EBSCO Publishing, Ipswich, MA, USA) to identify published English-language articles which (1) investigated the predictive value of risk factors for IFD in critically ill adult patients, or (2) developed or evaluated a risk score or risk prediction model for IFD in critically ill adult patients or (3) developed or evaluated a clinical decision rule or patient algorithm for use of antifungal prophylaxis in critically ill adult patients. Three search phrases were combined: 'fungal disease and treatment', 'patient population' and 'risk factors/risk models/clinical rules' (see Additional file 1 for search strategy).
Articles were identified in a staged process whereby titles were initially screened for potential eligibility by a single reviewer (GE). Abstracts and full texts of those potentially eligible were then assessed by two reviewers (HM and JS) independently and were included if the following criteria were met: (1) evaluation of multiple risk factors, a scoring system or a clinical decision rule for IFD in critically ill patients; (2) inclusion of a control group consisting of patients without IFD or any other systemic infection and (3) study of adult humans (age > 18 years). Any disagreements between the reviewers were resolved by a third (DH). Following review of abstracts, we obtained full-text articles for all that were eligible for inclusion. At this point, members of the FIRE Study Steering Group (see Acknowledgements), as clinical experts in the field, were contacted to determine if any relevant articles were missed.
Data were extracted onto standardised data extraction sheets independently by two reviewers (HM and JS; data extraction sheets available on request). The following data were abstracted from each article: study design, method of data collection, clinical setting, population characteristics, method of analysis, risk factors reported, outcome (types and definitions of IFD) and strength of association demonstrated. Data were gathered on the adjusted ORs, 95% CIs, and P-values for each of the studied risk factors.
The methodological quality of the reporting of the selected articles was assessed independently by two reviewers (HM and JS) using a set of questions addressing both general and statistical methodologies. Given that no gold standard method exists for the methodological assessment of risk factor studies, the questions were drawn from research from a published quality assessment method for randomised and nonrandomised studies [23], as well as from research on reporting of prognostic models in the oncology field [24,25].
Eight questions assessed the general methodology: study objectives, outcome description, patient characteristics, number of centres recruited, existence of an a priori analysis plan, adjustment for known risk factors, rationale behind risk factor inclusion and definition of risk factors. In assessing whether study results were adjusted for known risk factors, the factors considered were severity of illness, length of stay, antibiotics use, receipt of total parenteral nutrition, immunosuppressant use, diabetes, renal dysfunction or renal replacement therapy, central venous catheter use and major surgery. These risk factors were selected based on expert clinical opinion. A study was recorded as adjusting for a majority of the known risk factors if six or more of the nine risk factors were accounted for.
Three questions assessed the statistical methodology: adequacy of sample size, risk factor selection and model strategy choice. Adequacy of sample size was established using the generally held rule of ten events per variable [25]. All risk factors included in the statistical modelling, including those excluded from multivariable modelling following univariable screening, were included in the calculation of events per variable. 'Risk factor selection' referred to how risk factors were entered into the multivariable logistic regression model. The selection process was based on univariable analysis, previous literature or investigator choice, or no selection strategy, whereby all risk factors were entered into the model. 'Model strategy' consisted of either forward selection, backward elimination or no stepwise process, whereby all risk factors were kept in the model. If details on the risk factor selection and model strategy were absent, then they were labelled as unclear.

Results
The electronic search identified a total of 1,864 citations ( Figure 1). After screening of titles, 165 articles were selected for abstract and full-text review, and 152 of these that were potentially eligible were excluded because they failed to meet the inclusion criteria. Among these, 109 did not assess multiple risk factors, a scoring system and/or a clinical decision rule; 122 articles had a control group which had a systemic infection; and 5 were not adult human studies. Some articles were excluded for multiple reasons. No additional articles were identified by the clinical experts consulted.
The 13 articles that met the inclusion criteria fell into three groups: 8 articles examined risk factors, 4 developed a risk prediction model or clinical decision rule and 1 was an evaluation of a clinical decision rule. Two of the articles utilised data from the same study: the EPCAN Study [26,27]. There were three case-control and nine cohort studies with varying inclusion criteria, including age and length of stay in the critical care unit.   Table 1.

Analysis of risk factors
The risk factors examined varied between the studies. Table 2 reports all the risk factors that were identified as statistically significantly associated with IFD in one or more of the 10 studies (11 articles) that carried out a multivariable analysis. Table 3 reports all significant risk factors that were examined, and the number of studies in which these were associated with IFD, on univariable and multivariable analyses. Candidate risk factors, in descending order of the number of studies in which the risk factor was significantly associated on multivariable analysis, are described below.

Total parenteral nutrition
Six of the twelve studies [26,27,29,30,32,34,35] assessed total parenteral nutrition as a risk factor, and all found a significant association with IFD on univariable analysis. Of the six studies, four [26,27,29,30,34] also found a significant association on multivariable analysis.

Renal replacement therapy
Seven studies [26,27,30,[32][33][34][35][36] examined renal replacement therapy as a risk factor for IFD, of which five [26,27,30,32,34,35] found a significant association on univariable analysis. Three studies [26,30,35] demonstrated a significant association on multivariable analysis. Only one of the two EPCAN articles demonstrated a significant result on multivariable analysis. The type and exposure time to dialysis varied across the studies. Some looked at preadmission dialysis, and others examined haemofiltration in the unit.

Infection and sepsis
Five studies [27,29,30,33,36] examined the relationship between infection and sepsis and IFD, three [27,30,36] of which demonstrated an association on multivariable analysis. The source and site of infection varied across the studies. One examined bacterial infection and bacteraemia without specifying type and source of infection [36]. Another examined enteric bacteraemia, which included Enterococcus, Bacteroides and other Gramnegative bacilli bloodstream infections [30]. One demonstrated an association with severe sepsis, although the infection source was not specified [27].

Mechanical ventilation
Five studies [26,27,29,32,34,36] examined the association between receipt of mechanical ventilation and IFD. Two of the five studies [32,36] reported a significant association on multivariable analysis. Both timing and duration of mechanical ventilation varied across the studies, with one study examining mechanical ventilation on day 3 of critical care unit admission [32] and the other on day 10 [36].

Diabetes
Four studies [27,28,35,36] examined whether a medical history of diabetes mellitus was a risk factor for IFD. Two of the four studies [35,36] demonstrated a significant association on both univariable and multivariable analyses.

APACHE II or APACHE III score
Eight studies [26][27][28][29]31,[33][34][35] examined whether either the Acute Physiology and Chronic Health Evaluation II (APACHE II) score or APACHE III score was a risk factor for IFD. Two [31,33] of the eight studies demonstrated a significant association on both univariable and multivariable analyses.

Other risk factors
A number of other risk factors were identified as being significantly associated with IFD on multivariable analysis in single studies. These included cardiopulmonary bypass time, acute renal failure, broad-spectrum antibiotic use, red blood cell transfusions, antifungal medication use, central venous catheter use, diarrhoea and peripheral catheter use ( Table 2). Of note, two studies [26,37] examined the association between neutropenia and IFD, neither of which demonstrated a significant association. Similarly, none of the five studies [27,28,[33][34][35] looking at immunosuppressant use demonstrated an association with IFD.

Risk prediction models and clinical decision rules
Four of the studies developed a risk prediction model or clinical decision rule (Table 4), and one evaluated a clinical decision rule for IFD, all in the critical care setting. León et al. [27] developed and validated a risk prediction model from which they derived a bedside scoring Ostrosky-Zeichner et al. [37] developed a number of clinical decision rules for IFD in the critical care setting. Their study was a retrospective chart review of 2,890 patients from 12 participating centres with 88 cases of IFD. Several clinical decision rules, with varying combinations of risk factors, were developed and tested. The best-performing rule consisted of the following risk factors: any systemic antibiotic, presence of a central venous catheter. In addition, their rule included at least two of the following risk factors: total parenteral nutrition, any dialysis, any major surgery, pancreatitis and  use of steroids or other immunosuppressants. The sensitivity and specificity of the model were 34% and 90%, respectively. Paphitou et al. [35] developed and validated a number of clinical decision rules from a single-centre, retrospective cohort study of 327 critically ill patients comprising 9 cases of proven IFD and 27 probable or possible cases. Several combinations of risk factors were evaluated, of which any combination of diabetes mellitus, new-onset haemodialysis, use of total parenteral nutrition or receipt of broad-spectrum antibiotics was considered the most useful. The model had a sensitivity of 78% to 83% and specificity of approximately 50%.
Pittet et al. [33] developed a number of clinical decision rules based on intensity of Candida colonisation from a single-centre, prospective cohort study of 29 critically ill patients with significant Candida colonisation, of whom 11 had severe Candida infection. The bestperforming rule, developed post hoc to give perfect discrimination in the small data set, was a Candida corrected colonisation index (ratio of highly positive fungal screening samples to the total number of samples) of 0.4 or more.
Piarroux et al. [38] evaluated the clinical decision rule developed by Pittet et al. [33] whereby patients admitted to a single surgical intensive care unit were screened for fungal colonisation and preemptively treated with fluconazole if the Candida corrected colonisation index was 0.4 or more. On the basis of a review of same-centre historical controls from a time period prior to prophylaxis, a reduction of unit-acquired IFD from 2.2% to 0% (P < 0.001) was reported.

Reporting of methodological assessment
The included studies varied with respect to their methodological quality (Tables 5 and 6). All 12 studies reported objectives, main outcomes and characteristics of the selected study patients ( Table 5). The majority of the studies were carried out in at least two critical care units. The analysis was defined a priori in ten (83%) of the twelve studies, and the majority of known risk factors were accounted for in ten (91%) of the eleven studies in which multivariable analyses were conducted. The study by Piarroux et al. [38] evaluated a clinical decision rule and therefore did not carry out a risk factor analysis. Risk factors were poorly defined in over  half of the studies, and a rationale for inclusion was missing in over two-thirds of the studies. Reporting of the statistical modelling was generally poor (Table 6), and it was usually impossible to determine exactly the number of variables that were considered as candidate predictors in each paper. Reported methods often stated, for example, 'Risk factors examined included...', but it was not clear whether the subsequent list was exhaustive, and risk factors could often be determined only from those reported in the results, which in some cases were only those selected by a modelling process or only those that were statistically significant. The numbers of risk factors reported in Table 6 are therefore approximate and, in many cases, a minimum. Some studies split data into development and validation samples, but did not report how many of the events were in the development sample. However, even upon assessing the models on the minimum number of variables included as indicated by the report and the number of events in the full sample (and therefore the maximum events per variable), all of the papers had a strong likelihood of presenting results that were overfitted to the data. Taking into account all variables considered in the statistical modelling (including those screened out on univariable analysis), the largest studies had around four events per variable, and a number of studies had examined at least as many risk factors as there were events in the data set, giving values of one event per variable or less. No studies reached the predefined threshold of ten events per variable. Roughly half of the studies based their decision regarding which risk factors to include in their multivariable analysis on univariable analysis, whereas the reporting in the remaining studies was insufficient to determine risk factor selection. In terms of modelling strategies, one-third of the studies used a forward selection process, one-third used a backward elimination process, and for the remaining third, it was unclear from the reporting which modelling strategy was used.

Discussion
Thirteen articles were identified which investigated risk factors, risk prediction models or clinical decisions rules for invasive fungal disease in critically ill adult patients. Of these 13, 8 articles examined risk factors specifically, 4 developed risk prediction models or clinical decision rules and 1 evaluated a clinical decision rule. In this systematic review, the following risk factors were found in multiple studies to be significantly associated with IFD: surgery, total parenteral nutrition, fungal colonisation, renal replacement therapy, infection and sepsis, mechanical ventilation, diabetes and APACHE II or APACHE III score. Cardiopulmonary bypass time, acute renal failure, broad-spectrum antibiotics, red blood cell transfusion, antifungal medication, central venous catheters, diarrhoea and peripheral catheter use were also found to be statistically significant, but each in single studies only. The risk prediction model and clinical decision rule studies employed all of the risk factors found to be significant in multiple studies reported above, apart from mechanical ventilation, and in addition included pancreatitis and immunosuppressant use. Risk factor definitions varied across studies, with many studies offering no definition at all. Risk factor selection process and modelling strategy also varied across studies, and no studies had an adequate sample size for the multivariable analyses. None of the selected studies described the degree of missing data or of how missing data would be handled in the analysis. Some reported numbers of patients included in each model, but reasons for exclusion were unclear.
The risk prediction models and clinical decision rules identified in this review have a number of factors that limit their usefulness for guiding early decision-making regarding antifungal prophylaxis. First, with regard to the patient populations studied, models and rules were developed and evaluated using data from patients whose length of stay in the critical care unit was four [35,37], five [38] or seven days [27]. This would have helped to identify high-risk populations; however, the performance of these models and rules, if applied at an earlier time point in the critical care stay, cannot be determined. Some models and rules were developed on the basis of patients with Candida colonisation only [27,33], and consequently they could be used only to guide empiric  50 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a therapy and not true prophylaxis. Second, with regard to the statistical modelling, models are likely to be overfitted because of the small number of events used for model development.
Stepwise selection of risk factors is likely to have resulted in model coefficients that were too large and measures of model performance that were optimistic [39]. Despite having been developed in higher-risk populations identified by longer ICU stays, the specificity of the rules was generally low and hence their use to guide treatment could result in overuse of antifungal drugs, with costs both financial and in terms of increased resistance. No studies have adequately addressed the costeffectiveness of using clinical decision rules to guide delivery of antifungal therapy. The only study that gave any consideration to costs was that by Paphitou et al. [35], who estimated the number needed to treat and associated cost to prevent one case of IFD assuming a relative risk of 0.5 and a cost of $100/day for antifungal prophylaxis. The most promising rule on these criteria had a number needed to treat of six to ten and an associated cost of $12,000 to $21,000 per case prevented. It is difficult to compare the performance of clinical decision rules across the different studies, owing to the variation in inclusion criteria, outcome definitions and prevalence of the outcome. However, it should be noted that the performance measures are likely to be favourably biased because of overfitting to the data and lack of external validation. Since the end date of our systematic review, three studies have been published validating risk prediction models or clinical decision rules identified in this review. León et al. validated their risk prediction model, the Candida Score [27], among a new prospective cohort of 892 admissions with Candida colonisation with a stay of at least 7 days in one of 36 multidisciplinary ICUs in Spain, Argentina and France [40]. As expected, the performance of the score was not as good in the validation sample with an area under the receiver operating characteristic curve of 0.77 compared with 0.85 in the development data. Based on a cut-off of a score of 3 or more, the sensitivity was 78% (81% in development data), the specificity was 66% (74% in development data) and positive and negative predictive values were 14% and 98%, respectively (not reported in development data). Playford et al. [41] validated four clinical decision rules, the best rule from Ostrosky-Zeichner et al. [37] and a subsequent revision of this published in abstract form, and the two best rules from Pittet et al. [33], in a prospective cohort of 615 patients admitted for at least 72 hours to 4 multidisciplinary ICUs in Australia. Performance of the clinical prediction rules was worse than in the development data sets, and the authors recommended that to identify a sufficiently high risk population to consider for antifungal treatment would require a combination of the clinical risk factors from Ostrosky-Zeichner et al. [37], together with measures of colonisation from Pittet et al. [33]. Most recently, Hermsen et al. [42] set out to validate the clinical decision rules of Paphitou et al. [35] and Ostrosky-Zeichner et al. [37] in a case-control study of 88 cases and 264 matched controls staying at least 4 days in a single multidisciplinary ICU in the United States. Rather than validate the rules as published, Hermsen et al. fitted new conditional logistic regression models using the risk factors from these rules, rendering their results incomparable with the original publications. It is worth noting, however, that a number of the risk factors included in the rules (surgery, pancreatitis, haemodialysis and diabetes) were not found to be significantly associated with risk of IFD.
This review is the first to systematically evaluate and assess the quality of the literature on risk factors for IFD. Rigorous search methods and a tailored quality assessment tool were combined to produce a high-quality systematic review. As search strategies are designed for identifying randomised controlled trials rather than risk factor studies, a comprehensive search strategy including multiple medical subject heading terms and keywords describing risk, risk prediction models and clinical decision rules were employed. Furthermore, abstracts and full-text articles were reviewed, and data extracted, by two investigators independently to ensure that all relevant articles and data were captured. There is currently no validated gold standard or single recommended instrument for methodological assessment of risk factor studies, so a combined methodological assessment was developed for this review and tailored to assess the specific areas of risk factor studies which were considered to be important.
One limitation of our review is that the heterogeneity of the selected articles precluded any meta-analysis. Study objectives differed between the studies, with some assessing a specific clinical decision rule and some examining a range of risk factors. The ways in which the risk factors and outcomes were defined also differed, and different inclusion criteria were imposed across the studies, making combination of results inappropriate. The existence of publication bias is always a possibility in systematic reviews, but many risk factors were shown to be nonsignificant on multivariable analysis, indicating that negative as well as positive results were represented in the studies. In the univariable analysis however, it was difficult to identify which risk factors were nonsignificant, as the full list of factors examined was not always made clear.

Conclusions
In conclusion, this review has shown a number of risk factors to be significantly associated with the development of IFD in critically ill adults. However, this review has highlighted numerous methodological limitations in the design and conduct of studies in this area, and as such it is suggested that caution should be used in their interpretation. These results form an important underpinning for a large, publicly funded, prospective cohort study, the FIRE Study, which aims to develop and validate a risk model to effectively target antifungal prophylaxis to critically ill nonneutropenic patients at high risk of IFD. The first step in model creation was to prospectively gather data on risk factors and outcomes for this patient group, which was guided by the results of this review.

Key messages
◆ IFD in critically ill patients is associated with increased morbidity and mortality at a cost to both the individual and the healthcare system. ◆ Thirteen articles which investigated risk factors, risk prediction models or clinical decisions rules for IFD in critically ill adult patients were identified. ◆ Multiple risk factors were found to be associated with IFD on univariable and multivariable analyses. ◆ Studies varied in terms of objectives, risk factors, definitions and outcomes.