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How to Write and Publish a Systematic Review 

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Welcome to our blog, where we explore the robust structure of systematic reviews – the gold standard in evidence synthesis. Together, we’ll dissect their components, unveil their significance, and learn how to write a systematic review, especially for beginners.

In this blog, we will explore the robust structure of systematic reviews – the gold standard in evidence synthesis.

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What is the difference between a systematic review and a narrative review?

A review article or literature review is a type of research that summarizes and aggregates available evidence in a particular topic. There are various kinds of reviews, however, the 2 main types are Narrative Reviews and Systematic Reviews. The techniques and methods used to decide the process of article selection for summarization will dictate the kind of review article produced.

  • Narrative Reviews: These do not necessarily have a methods section or a strictly defined search strategy and instead rely (usually) on a degree of expertise on the topic to perform study selection. Experts in the field are generally invited by journal editors to write a review about the topic they are experts in.

Here is an example of a review article:

GERD and Barrett’s esophagus as indications for revisional surgery after sleeve gastrectomy: experience of a bariatric center of excellence IFSO-EC and narrative review.

  • Systematic Reviews: As the name implies, are systematic, and follow a very strict methodology, governed by the Preferred Reporting Inventory for Systematic Reviews and Meta-Analysis (PRISMA). This ensures that the search, as well as selection process, is systematic, transparent, and reproducible (any researcher can get the same results if they redid the study).

Here is an example of a systematic review:

Warfarin versus direct oral anticoagulants for treating left ventricular thrombus: a systematic review and meta-analysis.

This example on DOAC vs. Warfarin for ventricular thrombus not only performs a PRISMA-compliant review that’s reproducible, and thus systematic, but also performs an aggregate analysis, resulting in a combined effect size from a meta-analysis.

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What are the types of Systematic Reviews?

In general, there are 2 ways to summarize evidence; qualitative and quantitative.

  • Qualitative summarization means that the results of identified studies are graded based on factors such as quality or level of evidence and briefly described, but there is no “re-analyzing” or “pooled analysis” of data. In this example, authors mention the findings of each included study in each sub-heading, but perform no quantitative analysis and thus offer no aggregate effect size.

Evictions and Infant and Child Health Outcomes: A Systematic Review

  • Quantitative summarization refers to the combination of the results of various studies to generate one combined result known as the effect size. This is called a meta-analysis. As mentioned above, the study on DOAC vs Warfarin for ventricular thrombus not only performs a PRISMA-compliant review that’s reproducible, and thus systematic, but also performs an aggregate analysis, resulting in a combined effect size from a meta-analysis.

Warfarin versus direct oral anticoagulants for treating left ventricular thrombus: a systematic review and meta-analysis.

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What are the Steps in Writing a Systematic Review?

A Systematic Review can be approached by adhering to the following steps:

  1. Finding a research idea
  2. Performing the feasibility test
  3. Identifying search strategy
  4. Identifying inclusion/exclusion criteria
  5. Identifying variables
  6. Performing the search
  7. Screening studies
  8. Collecting data
  9. Bias assessment
  10. Meta-analysis
  11. Writing the paper

How do I come up with ideas or topics for a Systematic Review?

If you have access to a mentor who’s knowledgeable in a field you’re interested in, you can approach them to see if they can shed any insight on any current interesting or relevant questions. Otherwise, you can look at recent publications to see new, emerging topics as well as look for gaps in knowledge that could point to further research. Another way is to look for older reviews that are ~5 years old and look for new findings in these areas.

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How do I turn an idea into a research question?

Typically, research questions follow the PICO format. This stands for Patients, Interventions, Controls and Outcomes, which can help you organize your topic into an actual question.

Ideally, reviews cover patients with the same disease and stage and compare an intervention group with a control group and have a standardized outcome. For example, this paper on warfarin vs DOAC for left ventricular thrombus:

Warfarin versus direct oral anticoagulants for treating left ventricular thrombus: a systematic review and meta-analysis.

has the following PICO:

P: Patients with left ventricular thrombus
I: DOAC
C: Warfarin
O: Thrombus Resolution

It’s important to ensure that studies included have a standardized measurement of outcomes. For example, thrombus resolution could be defined as no persistent thrombus after X amount of time as visualized on an ultrasound. When different measurements of outcomes are used, this creates heterogeneity which may negatively impact results by introducing bias.

What is a feasibility test and how do I perform it?

A good feasibility test will tell you 2 key pieces of information necessary for writing a successful Systematic Review.

  1. Is there enough literature, and
  2. Is there space in the literature?

Key aspect 1 refers to there being enough actual studies to perform a review. If your topic is a super rare disease with only 3 published cases, then there’s probably not enough literature available.

Key aspect 2 refers to any similar recently published papers. For example, if you spot a review published last year that included 15 studies, and only 1 new study has been published, then there isn’t much space for a new review as it’s unlikely to contribute any new findings.

To perform this, perform scoping searches in databases such as PubMed related to the topic you want to develop and make note of the volume and recent reviews.

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How do I identify a search strategy?

The search strategy is one of the most important components, as it will be the key factor that determines the overall pool of studies that you will screen for inclusion. A good search strategy can be refined in sequential steps by adding (or removing) keywords. Search strategies typically search for terms in the title and/or abstract and bridge terms using Boolean operators such as AND or OR as well as NOT.

Example: The search strategy from this article Warfarin versus direct oral anticoagulants for treating left ventricular thrombus: a systematic review and meta-analysis, is the following:

“(left ventricular thrombus) AND (treatment); (left ventricular thrombus) AND (warfarin); (LV thrombus) AND ((anticoagulation) OR (vitamin K antagonist)); and (left ventricular thrombus) AND (direct oral anticoagulant).” 

This was taken from the ‘search strategy and study selection’ from the Methods section.

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How do I Identify inclusion/exclusion criteria?

Selection criteria are essential as they are the safeguard that guarantees only studies of enough quality and content make it into your review. As the term implies, these criteria are a set of characteristics, typically related to the study design and population included that will determine whether each individual study will be included or not in the review.

Some reviews will opt only to analyze randomized trials, while others are laxer on the quality of studies included.

How do I Identify variables for analysis?

This is one of the million-dollar questions! Fortunately, there are various ways to determine which variables are the most appropriate. If you have a mentor available, they might be the best source of relevance for what may or not be interesting to analyze.

If you don’t, don’t fret, you can look at prior meta-analyses as well as pay attention to the literature for repetition of outcomes to determine which ones are the most important based on the frequency of appearance.

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What kinds or types of variables can I analyze in a meta-analysis?

In theory, meta-analysis can aggregate the results of pretty much anything, ranging from incidence to diagnostic accuracy and beyond, however, the most established methods and the most frequently analyzed variables are continuous data (operative time, blood pressure, length of stay) and dichotomous data (number of events such as death, transfusions, etc.).

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How do I screen studies for a Systematic Review?

Screening studies refers to the process of appraising a study title and abstract to determine whether your pre-established criteria for inclusion are met or not before taking a full dive into the actual study. You can use Excel to generate a list of included and excluded studies or employ external applications such as Zotero, Covidence, Rayyan, and so on.

How do I extract data?

Extracting data is relatively simple, you look through your included studies, attempting to identify your variables of interest and save them, usually by copying them into a large Excel file that contains the aggregate summary of findings of all the studies you’ve analyzed.

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How do I rank and grade evidence in a Systematic Review?

As you’ve probably already encountered, not every study is equal, not even RCTS! Studies of all kinds are prone to mistakes that lead to errors called biases, which are design or methodological errors that can lead to altered results. In the case of RCTs, there is a very helpful tool by Cochrane called “Risk of Bias” or RoB for short. For non-RCTs, there are more scales and tools available. However, the most frequently used are the Newcastle Ottawa score, GRADE and the Oxford score.

What software or programs can I use to do a Meta-Analysis?

The quantitative summary part of a systematic review can be done in a plethora of software, which ranges from free to expensive, from easy-to-use to complex. The most used programs are, however, Cochrane’s Review Manager, which is free, relatively easy to use, and somewhat limited; and R, which is also free, but very hard to use.

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What are the components of a Systematic Review?

Like most scientific articles, Systematic Reviews follow a common structure composed of an Introduction, Methods, Results, Discussion and Conclusion.

Introduction: As the name implies, this is where you set the foundation of the overlying topic that’s going to be addressed as well mentioning the gaps in the literature and how your study is going to address them. Typically, introductions should be a 3-4 paragraph section with a reverse pyramid structure, starting broad about the topic in general and narrowing down the focus into your research topic in the second/third paragraphs of the introduction.

Methods: This section is like a recipe, making sure that anyone reading your paper can clearly follow the process you employed in designing your study as well as gathering data, deciding which data was included and why, as well as any statistical methods employed. Typically, in Systematic Reviews, the methods section follows the PRISMA guidelines which include the following:

  • Systematic Search, Date and Databases
  • Inclusion and Exclusion Criteria
  • Outcomes of Interest
  • Screening and Data abstraction
  • Bias Grading and Analysis

Results: Here, the results will be presented objectively, without any speculation as to why these occurred (this goes in the discussion). The actual presentation will vary on whether the review includes a meta-analysis, which will include reporting of the forest plots and effect sizes calculated.

Discussion: Unlike the results section, which must be an objective narration of findings, the discussion section is a place where you can explore and theorize why your findings occurred, contrast them to prior literature and expand on the identified limitations of your study. Authors also usually choose to include a further directive like “ further studies should explore X or Y finding”.

Conclusion: In brief, you should mention the overall most significant findings related to the primary objective of your study.

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How do I write the introduction for a Systematic Review?

Introductions typically have 3-4 paragraphs, which introduce the overall overlying topic, introduce the problem and briefly explain the gap in the literature as well as how your study is addressing this gap.

How do I write the methods section for a Systematic Review?

In general, the methods section is one of the most important sections of a systematic review but it’s also one of the most straightforward to write. You can use the PRISMA criteria as an outline for the points that must be covered.

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How do I present the findings of a meta-analysis?

Meta-analysis is presented with a graph called a forest plot, which shows the individual effect sizes of each study as well as an aggregate effect size. When describing this in a results section, it’s important to note the effect size, the p values as well as the confidence intervals. 

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How do I write the discussion section for a Systematic Review?

The most basic structure for a good discussion includes stating the most important findings and trying to relate or explain these findings as well as comparing them against underlying literature from the same or similar topics.

Do I need to be an expert to write a Systematic Review?

No! But a degree of expertise is needed, whether yours or a mentor, to best define the research question and the selection criteria.

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How many people do I need to perform a Systematic Review?

Strictly speaking, at least 2 people are needed to perform a Systematic Review because it requires at least 2 independent reviewers to screen and extract data to identify possible mistakes.

What databases should I include in a Systematic Review?

There’s a multitude of databases indexing studies from thousands of journals, however, without a doubt, the most important one is PubMed.

You should consider additional databases in your search such as Scopus, Web of Science and Google Scholar.

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In brief, the literature search will allow you to get an overall idea of the quantity as well as quality of available literature on a topic. An easy way to start is to go to PubMed, Google Scholar or a similar database and search for broad terms of what you’re interested in.

For example, “heart failure” “aortic stenosis”, “SLE”. Then, skim through the articles. Afterward, add key terms based on your interest, for example, “management”, “treatment”, and “diagnosis”. A detailed guide on how to perform a literature search can be found in our research course and our systematic review course.

Are Randomized Clinical Trials the only includible studies in a Systematic Review?

Not necessarily! If there are no, or few trials on your topic you can include retrospective studies, or whichever kind of study you determine appropriate for your review. While RCTs are usually the most desirable, rare topics frequently don’t have RCTs and retrospective studies become acceptable.

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Where can I learn more about how to perform a Systematic Review?

You can check out our detailed course on how to perform a systematic review here. The course will cover every question we answered here with detailed live lessons in addition to multiple exercises and assignments to help you learn better.

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I hope this blog has offered a solid foundation for crafting your first systematic review and meta-analysis. Best wishes on your research endeavor, and please feel free to contact us with any inquiries. We’re here to help!

By David Hinojosa, MD

Dr. Hinojosa completed a Research Fellowship at Harvard Medical School/Massachusetts General Hospital and is currently a Urology resident at Baylor College of Medicine. He has published over 40 articles, of which 20 are Systematic Reviews and Meta-Analysis in top journals across various specialties such as general surgery, vascular surgery, neurosurgery, urology among others. He is one of the instructors of the live Systematic Review Course that will cover everything you need to know about systematic reviews and meta-analyses!

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