How to Create an Effective PRISMA Flow Diagram

If you’ve ever read a systematic review and/or meta-analysis, you may have noticed that the authors followed the “PRISMA guidelines”. But what exactly does that mean?

According to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) website, “PRISMA is an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses. PRISMA primarily focuses on the reporting of reviews evaluating the effects of interventions, but can also be used as a basis for reporting systematic reviews with objectives other than evaluating interventions (e.g. evaluating etiology, prevalence, diagnosis or prognosis).”

What is a PRISMA flow diagram?

A PRISMA flow diagram is an image that shows the flow of information in a systematic review or meta-analysis. PRISMA stands for Preferred Reporting Items for Systematic Reviews and Meta-Analyses, which is an evidence-based set of reporting guidelines developed to improve the transparency and quality of reporting in these types of studies.

A PRISMA flow diagram provides a visual overview of the different stages in a systematic review or meta-analysis, including the identification, screening, eligibility assessment, and inclusion of studies. It helps researchers and readers understand the study selection process in the review and any reasons for excluding certain studies.

The flow diagram typically starts with the initial number of studies identified through database searches. It then outlines the number of studies remaining after removing duplicates and the number of studies excluded at each stage based on predefined eligibility criteria. Finally, it shows the number of studies included in the final analysis.

The PRISMA flow chart is an important component of a systematic review or meta-analysis, as it provides a clear overview of the study selection process. It allows readers to assess the rigor and comprehensiveness of the review and helps researchers avoid bias or potential errors in study selection.

What are the benefits of using PRISMA?

For authors:

• Transparency: PRISMA flow charts enhance the transparency of the research process and help authors clearly communicate the steps taken to identify, screen, and include/exclude studies, promoting transparency and reproducibility.
  
• Methodological quality: A PRISMA flow diagram helps authors maintain a clear and organized record of their study selection process, which is crucial in assessing the reliability and validity of the review.
  
• Reduced bias: A PRISMA flow diagram documents the number of studies identified, screened, and included/excluded, with the reasons for exclusion. This transparency helps readers evaluate potential bias and assess the generalizability of the findings.
  
• Enhanced reporting: A PRISMA flow diagram supplements the written description of the study selection process, allowing authors to present a comprehensive and visually appealing summary of their work. This visual representation helps readers quickly understand the selection of studies and the reasons for their inclusion or exclusion.
  
• Compliance with guidelines: Many academic journals and research institutions require authors to adhere to PRISMA guidelines when conducting systematic reviews or meta-analyses. 

For Journal Peer Reviewers and Editors: 

• Evaluation of study selection process: PRISMA flow diagrams allow reviewers and editors to evaluate the study selection process followed by the authors. The flow diagram provides a clear overview of the number of records identified, screened, and included/excluded at each stage, enabling reviewers to assess the quality and comprehensiveness of the review. 
  
• Assessment of bias and generalizability: By reviewing the PRISMA flow chart, reviewers can examine the reasons for excluding studies from the review. This information allows them to evaluate potential sources of bias and assess the generalizability of the findings. 
  
• Reproducibility and transparency: PRISMA flow diagrams enhance the reproducibility and transparency of research. Reviewers and other researchers can easily follow the flow diagram to repeat the steps taken by the authors in selecting studies for inclusion.
  
• Compliance with guidelines: PRISMA flow diagrams are aligned with the PRISMA guidelines and checklist. Reviewers can use the flow diagram to ensure that the authors have complied with these guidelines and followed appropriate reporting standards.
  
• Assessment of methodological quality: Reviewers can use the PRISMA flow diagram as a tool to evaluate the methodological quality of the systematic review or meta-analysis. They can ensure that all relevant studies have been included and irrelevant ones excluded. 

What is included in a PRISMA flow diagram?

 A PRISMA flow diagram includes several elements and provides a visual representation of the study selection process in a systematic review or meta-analysis. Blow are the key components typically included in a PRISMA flow diagram:

1) Identification

The flow diagram starts with the initial number of records identified through various sources such as literature databases, manual searches, or other means.

Flow diagram from:  Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ 2021;372:n71. doi: 10.1136/bmj.n71. For more information, visit: http://www.prisma-statement.org/

2) Screening

It shows the number of records remaining after removing duplicates. This stage involves screening the titles and abstracts of the identified records to assess their relevance to the research question or objective.

3) Eligibility

The flow diagram indicates the number of studies that passed the initial screening and proceeded to the next stage. At this stage, full-text articles of potentially relevant studies are assessed against predetermined eligibility criteria.

4) Inclusion/Exclusion 

It presents the number of studies included in the systematic review or meta-analysis. These studies have met the predefined eligibility criteria and are considered suitable for further analysis. The flow diagram highlights the number of studies excluded at different stages and the reasons for their exclusion. Common reasons for exclusion include irrelevance to the research question, inadequate study design, insufficient data, or failure to meet specific inclusion criteria.

Flow diagram from:  Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ 2021;372:n71. doi: 10.1136/bmj.n71. For more information, visit: http://www.prisma-statement.org/

5) Data Extraction

Depending on the study design and objectives, the flow diagram may include a stage for data extraction, where the number of studies included for data extraction is recorded. Data extraction involves extracting relevant information from the included studies, such as study characteristics, outcomes, and effect sizes.

6) Synthesis

While not always included in the flow diagram, some flow diagrams may show the number of studies included in the synthesis or analysis phase. This stage involves synthesizing the findings of the included studies, often through statistical methods, to draw overall conclusions.

Flow diagram from:  Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ 2021;372:n71. doi: 10.1136/bmj.n71. For more information, visit: http://www.prisma-statement.org/

PRISMA flow diagram examples and templates

Examples of and templates for PRISMA flow diagrams can be found on the PRISMA webpage.

PRISMA flow chart best practices

When creating a PRISMA flow diagram, it is important to follow certain best practices to ensure accuracy, clarity, and adherence to the PRISMA guidelines. 

Before creating the flow diagram, thoroughly read and understand the PRISMA guidelines. Familiarize yourself with the recommended reporting items and the flow diagram structure specified in the guidelines.

Avoid overcrowding the flow diagram with excessive text or unnecessary details. Use concise and clear descriptions for each stage, ensuring that readers can easily follow the flow of information.

Include the necessary information in each step of the flow chart. This typically includes the number of studies or records at each stage (n= ), reasons for exclusion (if applicable), and the final number of studies included in the analysis.

Use consistent terminology throughout the flow diagram. Ensure that the terms used in the flow diagram match those used in the text. 

If possible, provide detailed information on the reasons for excluding studies at each stage. This can include specific eligibility criteria not met, study design limitations, or any other relevant details that justify exclusion. However, be mindful of space constraints and the need for readability.

Double-check the accuracy of the flow diagram by cross-referencing it with the study selection process described in the text. Ensure that the flow diagram accurately represents the steps taken and the number of studies at each stage.

 If any changes occur during the review process (e.g., updated searches, additional screening rounds), update the flow diagram accordingly to reflect the most current information accurately.

Challenges with PRISMA flow diagrams

While PRISMA flow diagrams have several benefits, there are also some challenges associated with their creation and interpretation. Here are some common challenges:

• Complexity of the study selection process: The study selection process in systematic reviews or meta-analyses can be complex, involving multiple screening rounds, eligibility criteria, and decision-making criteria. Representing this complexity in a concise and clear flow diagram can be challenging, especially when dealing with a large number of studies.
  
• Space constraints: Journals often have limitations on the space available for figures and flow diagrams. PRISMA flow diagrams need to be designed to fit within these limitations, which can make it difficult to include all relevant information and maintain readability.
  
• Limited information in the flow diagram: PRISMA flow charts provide a summary of the study selection process but may not capture all the details and nuances of the inclusion and exclusion decisions. The flow diagram alone may not fully explain the reasons behind each decision, which may require referring to the main text for more comprehensive information.
  
• Evolving review process: Systematic reviews and meta-analyses can be iterative processes, involving ongoing updates, additional searches, or changes in inclusion/exclusion criteria. Keeping the flow diagram up to date with these changes can be challenging, especially if multiple versions of the flow diagram exist.
  
• Interpretation challenges: Readers and reviewers may interpret the flow diagram differently, leading to potential misunderstandings or misinterpretations. The flow diagram should be designed and described in a way that minimizes ambiguity and maximizes clarity.

Final thoughts

PRISMA flow diagrams benefit both authors and readers by promoting transparency, methodological quality, reduced bias, enhanced reporting, and compliance with guidelines. They contribute to the overall quality and credibility of the systematic review or meta-analysis, enabling readers to assess the reliability and validity of the study selection process. 

PRISMA flow diagrams remain valuable tools for summarizing and visualizing the study selection process in both systematic reviews and meta-analyses.

For more in-depth information about PRISMA flow charts, check out the article titled PRISMA 2020 explanation and elaboration: updated guidance and exemplars for reporting systematic reviews in the BMJ.