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Open Access

Study Protocol

Coenrollment of critically ill patients in PROSPECT: A protocol and statistical analysis plan

  • Alex Thabane ,

    Roles Conceptualization, Methodology, Writing – original draft, Writing – review & editing

    thabanla@mcmaster.ca

    Affiliation Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada

    ⨯
  • Diane Heels-Ansdell,

    Roles Methodology, Writing – original draft, Writing – review & editing

    Affiliation Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada

    ⨯
  • Nicole Zytaruk,

    Roles Methodology, Writing – original draft, Writing – review & editing

    Affiliation Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada

    ⨯
  • Deborah Cook,

    Roles Conceptualization, Supervision, Writing – original draft, Writing – review & editing

    Affiliations Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada, Department of Medicine, McMaster University Medical Center, Hamilton, Ontario, Canada

    ⨯
  • for the PROSPECT Investigators and the Canadian Critical Care Trials Group

    Membership of the PROSPECT Investigators and the Canadian Critical Care Trials Group is provided in the S2 File.

    ⨯

Abstract

Introduction

The enrollment of a patient into more than one study (i.e., coenrollment) has risks which warrant exploration, particularly with respect to possible effects on trial outcomes. This pre-planned secondary analysis will examine the sensitivity of treatment effects to coenrollment in an international critical care trial (Probiotics: Prevention of Severe Pneumonia and Endotracheal Colonization Trial (PROSPECT).

Objective(s)

The primary objective is to evaluate the sensitivity of the effect of probiotics on the primary endpoint of VAP to patient coenrollment in at least one other study. The secondary objectives are to describe the characteristics of coenrolled patients and the studies they were coenrolled into; examine any differences in baseline traits; understand differences in center-level characteristics between coenrolling and non-coenrolling centers; identify factors associated with coenrollment; and explore the relationship between coenrollment status and the incidence of adverse events.

Methods

We developed a protocol and statistical analysis plan (SAP) for this secondary analysis involving the conduct of a Cox regression model, including treatment allocation, coenrollment status, and the interaction between the two as independent variables. We also describe our planned statistical analyses for the secondary objectives, involving descriptive statistics, univariable analyses, and multivariable analyses.

Ethics and dissemination

The results of this study will be published in a peer-reviewed journal focused on critical care research or trial methodology, and presented at local, national, and international conferences. As a secondary analysis, this study will not undergo secondary research ethics board approval. Data will be presented in aggregate and without patient identifiers.

Citation: Thabane A, Heels-Ansdell D, Zytaruk N, Cook D, for the PROSPECT Investigators and the Canadian Critical Care Trials Group (2026) Coenrollment of critically ill patients in PROSPECT: A protocol and statistical analysis plan. PLoS One 21(3): e0345656. https://doi.org/10.1371/journal.pone.0345656

Editor: António Machado, Universidade dos Açores Departamento de Biologia: Universidade dos Acores Departamento de Biologia, PORTUGAL

Received: April 11, 2025; Accepted: March 9, 2026; Published: March 23, 2026

Copyright: © 2026 Thabane et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: No datasets were generated or analysed during the current study. All relevant data from this study will be made available upon study completion.

Funding: This work was funded by the Canadian Institutes of Health Research, Canadian Frailty Network, Physician Services Incorporated, Hamilton Academic Health Sciences Organization and Academic Medical Organization of Southwestern Ontario, as well as St. Joseph’s Healthcare Hamilton and McMaster University. iHealth provided the blinded study product. Dr. Deborah Cook holds a Canada Research Chair in Knowledge Translation in Critical Care. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Probiotics have been suggested as a cost-effective strategy for preventing ventilator-associated pneumonia (VAP) in critically ill patients [1], with several systematic reviews of randomized controlled trials reporting evidence of efficacy [2–4]. However the Probiotics: Prevention of Severe Pneumonia and Endotracheal Colonization Trial (PROSPECT) [5], which included 2,650 critically ill patients randomized to receive either the probiotic Lactobacillus rhamnosus GG or a placebo, found no effect of probiotics on the development of ventilator-associated pneumonia (VAP). This finding has called into question the utility of probiotics for the prevention of VAP.

Given the incongruence between the PROSPECT findings and prior randomized controlled trials, it is important to assess the robustness of the primary analyses in PROSPECT through sensitivity analyses [6]. One variable which could have modified the results is coenrollment, defined as the simultaneous or sequential enrollment of one patient into more than one study. While coenrollment confers benefits in terms of research efficiency, several concerns have been raised regarding the coenrollment of patients into multiple studies, including the interaction of study drugs and the subsequent impact of those interactions on treatment effect estimates [7–9]. Along with other possible consequences for patients, families, clinicians, and research teams, coenrollment may attenuate, neutralize, or inflate treatment effects observed in a trial. Exploring the sensitivity of the treatment effect of probiotics to coenrollment will help strengthen the robustness of the conclusion that probiotics have no clinical value in the prevention of VAP.

We present a protocol and statistical analysis plan for secondary analysis of the PROSPECT dataset with the objective of examining the sensitivity of the effect of probiotics on the primary outcome of ventilator-associated pneumonia (VAP) to coenrollment.

Methods

Study design

This study is a pre-planned secondary analysis of PROSPECT, a multi-center, international, randomized placebo-controlled trial [5].

Objectives

Primary objective.

The primary objective is to evaluate the sensitivity of the effect of probiotics on the primary endpoint of VAP to patient coenrollment.

Secondary objectives.

The secondary objectives are to describe the characteristics of coenrolled patients and the studies they were coenrolled into; explore differences in baseline patient characteristics between coenrolled and non-coenrolled patients; explore differences in center-level characteristics between coenrolling and non-coenrolling centers; identify patient- and center-level factors associated with patient coenrollment; and explore the relationship between coenrollment and adverse events.

Summary of PROSPECT Trial

Eligibility criteria.

Patients included in PROSPECT met the following inclusion criteria, as outlined in the trial protocol and statistical analysis plan (SAP) [10]:

  1. Adults ≥ 18 years of age admitted to a medical, surgical, or trauma intensive care unit (ICU).
  2. Receiving invasive mechanical ventilation, estimated to be required for ≥72 hours.

Patients were excluded from participation for meeting any one of the following criteria:

  1. Invasive mechanically ventilated >72 hours at the time of screening.
  2. Potential increased risk of iatrogenic probiotic infection including specific immunocompromised groups: HIV < 200CD4 cells/µL, chronic immunosuppressive medications, previous transplantation at any time, chemotherapy in the last 3 months, absolute neutrophil count < 500 cells/µL.
  3. Risk for endovascular infection: rheumatic heart disease, congenital valve disease, surgically repaired congenital heart disease, unrepaired cyanotic congenital heart disease, valvular replacement (mechanical or bio-prosthetic), previous or current endocarditis, permanent endovascular devices (e.g., endovascular grafts, inferior vena cava filters, dialysis vascular grafts), tunnelled hemodialysis catheters, pacemakers, or defibrillators. The following were not exclusions: coronary artery stents or bypass grafts, mitral valve prolapse, bicuspid aortic valve, temporary catheters (central venous, peripherally inserted, extra-corporeal life support-related) or neurovascular coils.
  4. Primary diagnosis of severe acute pancreatitis.
  5. Percutaneously inserted feeding tubes in situ.
  6. Strict contraindication or inability to receive enteral medications.
  7. Intent to withdraw advanced life support.
  8. Previous enrolment in this trial or current enrollment in a potentially confounding trial.

Recruitment & randomization.

Patients were recruited from 44 intensive care units ICUs in Canada, the United States, and Saudi Arabia, beginning in October 2013 and ending in October 2020. A priori written informed consent or deferred consent was obtained from each patient or their substitute decision-maker, per our consent guidelines and the local ethics approval. Patients were randomly allocated in a 1:1 ratio via a computer-based random number generator to receive either the study intervention (i.e., the probiotic Lactobacillus rhamnosus GG) or an enteral placebo [10]. Randomization was stratified by center and by admission category (i.e., medical, surgical, trauma) [10].

Interventions.

Patients in the probiotics group received 1 × 1010 colony forming units of Lactobacillus rhamnosus GG (i-Health, Inc.) in one capsule suspended in tap or sterile water, administered through a nasogastric or orogastric feeding tube [10]. Patients in the placebo group were randomized to receive an identical capsule containing microcrystalline cellulose, delivered the same way [10]. Patients received their allocated treatment for 60 days in the ICU, or until ICU discharge, death, or isolation of Lactobacillus spp. in a culture from a sterile site or if the sole or predominant organism in a culture from a non-sterile site [10].

Primary Endpoint.

The primary endpoint of PROSPECT was VAP, defined as the presence of new, progressive, or persistent radiographic infiltrate on chest radiographs after at least 2 days of mechanical ventilation, plus at least 2 of the following criteria: a fever above 38˚C; white blood cell count less than 3 × 106/L or exceeding 10 × 1010/L; or purulent sputum [5].

Blinding

Patients, bedside clinicians, investigators, and research coordinators were blinded to the treatment allocation during this trial. While the statistician involved in the primary analysis of the trial was blinded to the treatment allocation until the main analysis was complete, for this pre-planned secondary analysis study, the statistical team will be unblinded to the treatment allocation.

Statistical analysis

To meet our primary objective, we plan to conduct a statistical analysis similar to that performed in the primary trial – a Cox regression model adjusted for center and admission diagnosis, with an independent variable of treatment and dependent variable of VAP [5] – but including as additional independent variables patient coenrollment status and the interaction between treatment allocation and coenrollment status. This analysis will allow us to statistically test the sensitivity of the effect of probiotics to coenrollment. We define coenrollment as the simultaneous or sequential enrollment of a patient into at least one other study. We will report the hazard ratios (HRs) with corresponding 95% confidence intervals (CIs) between coenrolled and non-coenrolled patients in both treatment groups and report the interaction p-value between treatment allocation and coenrollment status. We hypothesize that coenrollment will not influence the treatment effect of probiotics on the primary endpoint of VAP, which is in line with previous critical care studies on coenrollment [8,11,12].

As a secondary objective, we plan to describe the characteristics of coenrolled patients, including the number of coenrolled patients and number of studies in which patients were coenrolled, by treatment allocation and overall. We also plan to describe the characteristics of coenrolled studies (i.e., studies into which PROSPECT patients were coenrolled), including the informed consent model of these studies, study design, study affiliation, and funding source. Descriptive statistics will be reported as counts (percentages) and means (standard deviations [SDs]) for categorical and continuous variables, respectively.

We plan descriptive statistics to describe the differences in baseline patient characteristics between coenrolled and non-coenrolled patients. Patient-level variables of interest include: age; APACHE II score [13]; Clinical Frailty Score [14]; inotrope or vasopressor infusion; dialysis use; individual granting informed consent. We similarly have planned descriptive statistics to explore differences in center-level characteristics between coenrolling and non-coenrolling centers. Center-level variables of interest include: center size (i.e., number of ICU beds); years of PROSPECT participation; year of PROSPECT initiation; site investigator trial experience; lead research coordinator trial experience; hospital type; and location.

To identify factors associated with patient coenrollment, we plan to conduct a multi-level logistic regression analysis, with a random intercept for center to account for clustering at the center-level. The dependent variable will be coenrollment status. We will include the following center-level variables as independent variables: center size, years of PROSPECT participation, site investigator trial experience, lead research coordinator trial experience, and hospital type (academic vs university). We will also include the following patient-level variables: age, sex, APACHE II score, and individual granting informed consent (substitute decision-maker vs. patient). We will report odds ratios (ORs) with corresponding 95% CIs and p-values. Given familiarity and prior analysis of this trial [5], we are confident about very little missing data. The exception is the variable of Clinical Frailty Score [14] which we will exclude from the regression as it was not collected during the pilot and vanguard phase of the trial [15,16]. Based on existing coenrollment literature in the field of critical care [8,12], we hypothesize that coenrollment will be positively associated with less seriously ill patients (as measured by the APACHE II score), larger center size, informed consent granted via substitute decision-maker, academic centers, and centers with high site investigator and lead research coordinator experience. We hypothesize that no association will be found between years in which the center participated in PROSPECT, patient age, or sex.

To explore the association between coenrollment status and adverse events, we will report the rate of adverse by coenrollment status (counts with percentages) and conduct a Fisher’s exact test comparing the rate of adverse events between coenrolled and non-coenrolled patients, reporting p-values. We hypothesize that coenrollment will have no association with adverse events.

The analysis will be performed by the lead author (AT), to be replicated independently by the trial analyst (DH) who will be blinded to the results. We will interpret results cautiously and in light of confidence intervals, considering analyses to be exploratory in nature. A summary of the statistical analysis plan is presented in S1 File.

Sample size calculation

The sample size is conditional on the number of patients enrolled in PROSPECT. The analysis for our primary objective involves a Cox regression model to test the interaction between treatment status and coenrollment. With respect to independent variables, the typical rule of thumb for number of events per variable is a minimum of 10 (although some research indicates between 5–9 events per variable provides comparable outcomes) to avoid an overfitted model [17]. With a total of 121 VAP events and a sample size of 2,650 patients, we believe we have enough observations and events to conduct this analysis.

Timeline

This study is a secondary analyses of data from a large, international randomized trial [5]. While participant recruitment, data collection, and results for the primary trial have been reported, the conduct of this secondary data analyses has yet to begin. We anticipate that this study, requiring data analyses and manuscript development, will be completed and submitted for publication in mid-2025.

Ethics and safety considerations

As a secondary analysis of an RCT database requiring no additional information and analyzing only anonymized data, this study will not undergo secondary research ethics board (REB) approval. The initial trial was approved by Health Canada, the REBs of all participating hospitals, and Public Health Ontario [10]. Data will be presented in aggregate and without any patient identifiers to ensure the anonymity of patients.

Dissemination

We plan to publish the results of this study in a peer-reviewed journal focused on critical care or trial methodology. The first author (AT) and co-primary investigator for the trial (DC) will lead knowledge translation efforts, mainly through the presentation of the results at local and national conferences. Efforts will be made to discuss the findings and their implications with relevant stakeholders (e.g., critical care trialists, REB members, patients, substitute decision-makers).

Discussion

While the issue of coenrollment is relatively unexplored in the field of critical care, we have previously examined patterns, predictors, and consequences of coenrollment in two international critical trials: the OSCILLation for ARDS Treated Early Trial (OSCILLATE) and the PROphylaxis for ThromboEmbolism in Critical Care Trial (PROTECT) [8,12]. In neither trial did coenrollment influence any of the safety or efficacy outcomes [8,12]. However, multivariable analyses of factors associated with coenrollment yielded some differences in results between the two studies. In the ARDS trial, age, site investigator and research coordinator trial experience, center size, and country were associated with coenrollment in multivariable analyses [8]; in the thromboprophylaxis trial, coenrollment was not associated with age, but was associated with disease severity, center size, individual granting informed consent, center affiliation with a research consortium, and research coordinator experience [12]. While differences in model design (i.e., the selection of independent variables included in the model) may explain some of these discordant findings, this can be taken as evidence of the complexity of coenrollment decision-making and conditional context. Exploration of factors associated with coenrollment in each trial is therefore warranted.

Coenrollment is handled variably by ICUs and institutions. Policies against the coenrollment of patients have been previously reported [18], suggesting hesitation due to possible negative effects. Beyond treatment interactions, increased patient burden continues to be a key apprehension when considering coenrollment [18]. Understanding the effects of coenrollment in trials such as PROSPECT, and the factors associated with coenrollment, will help critical care trialists better understand this practice, including the types of patients unlikely to be coenrolled, and may help investigators provide patients and research sites with accurate information on the possible benefits and harms of coenrollment.

This study has several strengths. PROSPECT was unique in capturing detailed coenrollment data, including the number of coenrollment events and characteristics of coenrolled studies, which is uncommon among critical care trials. PROSPECT also collected factors associated with coenrollment in other studies, such as investigator and trial coordinator expertise [12], which permits exploring whether these factors are associated with coenrollment in PROSPECT. Further, our proposed multi-level logistic regression analysis will allow evaluation of the independent association of several patient- and center-level factors with coenrollment whilst controlling for correlation in observations within study sites. Controlling for within-site correlation has the potential to improve the validity of the model, subsequently leading to more accurate estimations of association. This study could offer a prototype analysis to explore coenrollment in future randomized trials.

This study will be limited by an inability to explore the timing of coenrollment (i.e., simultaneous enrollment vs. sequential enrollment in another study), as this information was not available. We will also be unable to explore reasons for declining coenrollment as this information was not collected, limiting our ability to comment on consent decision-making patterns relative to coenrollment. Our findings will need to be interpreted in light of the field of critical care, and may not apply to all types of trials in other populations or settings.

Conclusion

This study will generate evidence about the consequences, patterns, and predictors of coenrollment in a large critical care trial, contributing to the wider discussion on the patterns, harms, and benefits of coenrollment.

Supporting information

S1 File. Statistical Analysis Table.

https://doi.org/10.1371/journal.pone.0345656.s001

(DOCX)

S2 File. PROSPECT and Canadian Critical Care Trials Group Members.

https://doi.org/10.1371/journal.pone.0345656.s002

(PDF)

Acknowledgments

We are grateful to the patients and families participating in this trial, as well as the collaborating Research Coordinators and Investigators, and bedside clinicians who supported this work. The trial was designed by the PROSPECT Steering Committee, the PROSPECT Investigators and Research Coordinators and the Canadian Critical Care Trials Group. We would like to thank the Methods Center staff for their expertise with PROSPECT data management, including Nicole Zytaruk, Lois Saunders, Shelley Anderson-White, Alyson Takaoka, Mary Copland, France Clarke, Lori Hand, Megan Davis, Neala Hoad, Melissa Shears and Kristine Wachmann.

References

  1. 1. Branch-Elliman W, Wright SB, Howell MD. Determining the ideal strategy for ventilator-associated pneumonia prevention. Cost-benefit analysis. Am J Respir Crit Care Med. 2015;192(1):57–63. pmid:25871807
  2. 2. Siempos II, Ntaidou TK, Falagas ME. Impact of the administration of probiotics on the incidence of ventilator-associated pneumonia: a meta-analysis of randomized controlled trials. Crit Care Med. 2010;38(3):954–62. pmid:20016381
  3. 3. Cheema HA, Shahid A, Ayyan M, Mustafa B, Zahid A, Fatima M, et al. Probiotics for the prevention of ventilator-associated pneumonia: an updated systematic review and meta-analysis of randomised controlled trials. Nutrients. 2022;14(8):1600. pmid:35458162
  4. 4. Sun Y-C, Wang C-Y, Wang H-L, Yuan Y, Lu J-H, Zhong L. Probiotic in the prevention of ventilator-associated pneumonia in critically ill patients: evidence from meta-analysis and trial sequential analysis of randomized clinical trials. BMC Pulm Med. 2022;22(1):168. pmid:35484547
  5. 5. Johnstone J, Meade M, Lauzier F, Marshall J, Duan E, Dionne J, et al. Effect of probiotics on incident ventilator-associated pneumonia in critically ill patients: a randomized clinical trial. JAMA. 2021;326(11):1024–33. pmid:34546300
  6. 6. Thabane L, Mbuagbaw L, Zhang S, Samaan Z, Marcucci M, Ye C, et al. A tutorial on sensitivity analyses in clinical trials: the what, why, when and how. BMC Med Res Methodol. 2013;13:92. pmid:23855337
  7. 7. Cook DJ, Blythe D, Rischbieth A, Hebert PC, Zytaruk N, Menon K, et al. Enrollment of intensive care unit patients into clinical studies: a trinational survey of researchers’ experiences, beliefs, and practices. Crit Care Med. 2008;36(7):2100–5. pmid:18552686
  8. 8. Cook DJ, Ferguson ND, Hand L, Austin P, Zhou Q, Adhikari NKJ, et al. Coenrollment in a randomized trial of high-frequency oscillation: prevalence, patterns, predictors, and outcomes*. Crit Care Med. 2015;43(2):328–38. pmid:25393702
  9. 9. Myles PS, Williamson E, Oakley J, Forbes A. Ethical and scientific considerations for patient enrollment into concurrent clinical trials. Trials. 2014;15(1):470.
  10. 10. Johnstone J, Heels-Ansdell D, Thabane L, Meade M, Marshall J, Lauzier F, et al. Evaluating probiotics for the prevention of ventilator-associated pneumonia: a randomised placebo-controlled multicentre trial protocol and statistical analysis plan for PROSPECT. BMJ Open. 2019;9(6):e025228. pmid:31227528
  11. 11. Clarke F, Hand L, Deane A, Zytaruk N, Hardie M, Arabi Y, et al. Coenrollment in a critical care trial: Characteristics and consequences. Contemp Clin Trials. 2025;154:107938. pmid:40379131
  12. 12. Cook D, McDonald E, Smith O, Zytaruk N, Heels-Ansdell D, Watpool I, et al. Co-enrollment of critically ill patients into multiple studies: patterns, predictors and consequences. Crit Care. 2013;17(1):R1. pmid:23298553
  13. 13. Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med. 1985;13(10):818–29. pmid:3928249
  14. 14. Rockwood K, Song X, MacKnight C, Bergman H, Hogan DB, McDowell I, et al. A global clinical measure of fitness and frailty in elderly people. CMAJ. 2005;173(5):489–95. pmid:16129869
  15. 15. Cook DJ, Johnstone J, Marshall JC, Lauzier F, Thabane L, Mehta S, et al. Probiotics: Prevention of Severe Pneumonia and Endotracheal Colonization Trial-PROSPECT: a pilot trial. Trials. 2016;17:377. pmid:27480757
  16. 16. Johnstone J, Meade M, Marshall J, Heyland DK, Surette MG, Bowdish DM, et al. Probiotics: Prevention of Severe Pneumonia and Endotracheal Colonization Trial—PROSPECT: protocol for a feasibility randomized pilot trial. Pilot Feasibility Stud. 2015;1(1).
  17. 17. Vittinghoff E, McCulloch CE. Relaxing the rule of ten events per variable in logistic and Cox regression. Am J Epidemiol. 2007;165(6):710–8. pmid:17182981
  18. 18. Harron K, Lee T, Ball T, Mok Q, Gamble C, Macrae D, et al. Making co-enrolment feasible for randomised controlled trials in paediatric intensive care. PLoS One. 2012;7(8):e41791. pmid:22870249