Objectives

The key secondary objectives are to develop risk prediction models to quantify the risk of acquiring (i) S. aureus or (ii) P. aeruginosa ICU pneumonia during ICU stay, by using a composite score of independent risk factors identified through primary objective 1 and 2. Other secondary and exploratory objectives can be found on ClinicalTrials.gov, under identifier NCT02413242, or in the online (Additional file 1: S.01).

Study design

ASPIRE-ICU (Advanced understanding of S taphylococcus aureus and Pseudomonas aeruginosa Infections in EuRopE - Intensive Care Units) is a multi-center, prospective, observational cohort study nested within ongoing routine surveillance among ICU patients in Europe. The study is composed of two study populations, the surveillance population and study cohort population. The study cohort is nested within the larger surveillance population; this means that all data and specimens collected specifically for study cohort participants is in addition to data already captured by ways of surveillance. An overview of the schedule of procedures, including all sample collection types and time points can be found in the online (Additional file 1: Table S.02).

Study populations and recruitment

Study outcome definitions

Considering that the definition of our primary outcome, ICU pneumonia caused by S. aureus or P. aeruginosa, is very extensive, we would like to refer to the (Additional file 1: S.03) containing the full definition. In summary, ICU pneumonia is defined as pneumonia occurring ≥48 h after admission to the ICU and is confirmed by a new or worsening infiltrate on chest X-ray or CT-thorax. Furthermore, the patient must fulfill specific clinical criteria (for example abnormal temperature, production of sputum, auscultatory abnormalities, acute changes in the ventilatory support system), in addition to at least 1 microbiological criterion (positive respiratory specimen, blood culture, pleural fluid aspirate or lung tissue culture).

Site selection

In total, 30 sites in 11 countries were selected from sources including but not limited to the COMBACTE CLIN-Net and LAB-Net databases [7]. To ensure the pan-European continent is represented, there is at least one country included from the Northern / Southern / Eastern and Western region of Europe. The selected sites adequately balance different factors such as geography, background antibiotic resistance prevalence, etc.. To ensure enrollment is met within the expected timelines, back-up sites were selected for various reasons (i.e. low enrollment numbers, decline further participation) that can supplement or replace primary sites selected. For each site also a local laboratory was selected to participate in the study.

A Site Selection Committee selected sites and laboratories based on pre-defined criteria in the Site Selection Plan. Assessment of these criteria was aided by site feasibility questionnaires. Participating ICUs must have a routine S. aureus screening protocol in order to be selected for participation.

Screening should consist of a minimum of one nasal swab and one lower respiratory tract (LRT) sample analyzed locally on the day of ICU admission. The LRT sample is defined as the collection of an endotracheal aspirate (ETA) sample, or, if an ETA cannot be collected, a sputum sample may be taken. As an exception, for routine S. aureus colonization screening at ICU admission only, if both the ETA and sputum cannot be collected, a throat swab may be taken.

Statistical analysis

Quality assurance

Data will be entered in a web-based electronic data capture system that was designed for ASPIRE-ICU. The study site will enter data in the electronic data capture system from the subject’s source documents (i.e. medical chart). Information linking the subject ID to the subject’s medical file (only applicable for study cohort subjects) will be kept in a secure place at the participating study site.

Monitoring will include 100% source data verification for the first three enrolled study cohort subjects at each site, and then 10% of the remaining enrolled study cohort subjects.

Definition of pathogen-specific pneumonia

The definition of S. aureus or P. aeruginosa ICU pneumonia is based on pre-defined pneumonia criteria in combination with the presence of the bacterium in an appropriate sample around the time of diagnosis, which are aligned with endpoints being used for two randomized controlled trials for prevention of S. aureus and P. aeruginosa pneumonia [8]. Thus, in case of cultures that yield multiple possible pathogens, it may be that the diagnosis of S. aureus or P. aeruginosa pneumonia is made incorrectly. The distinction between colonization and infection is sometimes difficult to assess. There is however no ‘reference standard’ to reliably assess the causative pathogen [3]. We have contemplated quantitative cultures in all pneumonia patients, but it was not feasible to implement this at each site. Quantitative measurements will however be applied on study samples received by the central laboratory, and thus will provide additional retrospective information. Furthermore, other outcomes, such as mortality will provide objective outcome information in addition to a diagnosis of pneumonia.

Enrichment strategy

In this study, 50% of the study cohort population is S. aureus colonized at ICU admission to ‘enrich’ the study population with S. aureus carriers (in nose or lower respiratory tract), while this naturally occurs in approximately 20–25% of the ICU population [10]. This was chosen as S. aureus carriage is being studied as one of the main known risk factors for subsequent S. aureus disease, thus without enrichment, the population needed for equal precision would be much larger [9, 11, 12]. However, the limitation of this choice is that one can argue that a population as such is not representative of the general ICU population, thus incidence estimation as well as assessing risk factors for P. aeruginosa ICU pneumonia may be suboptimal. We acknowledge this, and for this reason the surveillance population was included. Their data will allow distribution of baseline factors with the study cohort in an effort to assess the ubiquity of results across both groups. Furthermore, it can serve to identify potential bias between participants and non-participants. Considering that P. aeruginosa colonization at ICU admission is relatively rare and that colonization often occurs after ICU admission, no recruitment selection for P. aeruginosa colonized subjects will take place [13].

Routine surveillance

This study utilizes routine S. aureus screening conducted at each participating site as the basis for eligibility assessment. For consistency of screening results across sites, all swabs will be plated under protocol on the same chromogenic agar plates (Colorex agar, bioTRADING Benelux) provided by the ASPIRE-ICU study team. In the original protocol, a nose swab and ETA sample (sputum sample if non-intubated) are collected at screening. However, soon after study start, the protocol was amended to include a throat sample as a LRT sample, in case ETA and sputum were both not feasible.

Future perspectives

In this era of increasing antimicrobial resistance, the research field is steadily exploring other therapies, for example prophylactic therapies such as antibody-based preventive measures. A potential advantage of monoclonal antibodies (mAbs) is that they will not encourage bacterial resistance to the same extent as antibiotics, and may even augment antibiotic effectiveness [14]. This study was designed in part to provide crucial information on the incidence, patient-related and contextual factors of ICU pneumonia caused by S. aureus and P. aeruginosa, but also inform the design of future phase III trials, that will investigate mAbs effectiveness against S. aureus and P. aeruginosa. Two large COMBACTE phase II trials, SAATELLITE (A Human Monoclonal Antibody Against Staphylococcus aureus Alpha Toxin in Mechanically Ventilated Adult Subjects) and EVADE (Effort to Prevent Nosocomial Pneumonia Caused by Pseudomonas aeruginosa in Mechanically Ventilated Subjects) have already started [8]. SAATELLITE investigates the effect of MEDI4893, a mAb targeting S. aureus alpha toxin and EVADE studies MEDI3902, which is another mAb that simultaneously targets PcrV and Psl on the surface of the P. aeruginosa bacterium, in subjects at risk for ICU pneumonia [5, 6, 8]. These targeted therapies, if proven effective, may be used in the future for patients at highest risk. This study will help to identify the subset of patients that will likely benefit most.