Healthcare decisions for patients and public health policies should be supported by the best available research evidence. To ensure the hierarchy of the evidence, an evidence pyramid was conceptualized, with in-vitro studies and animal studies forming the bottom of the pyramid, expert opinions, case series, case reports, and observational studies forming the middle layer, and RCTs occupying the top. Systematic literature reviews (SLRs), which are a synthesis of multiple studies, including observational studies and RCTs, are conceptualized to occupy a place even above the RCTs in the evidence pyramid.[1] Thus, the evidence from SLRs is considered to be the most trustworthy form of scientific evidence as they are associated with high statistical power and precise results.[2,3]

However, there are concerns over the overproduction and poor quality of SLRs which has led to several international and multidisciplinary groups collaborating to develop guidelines.[2] Various solutions have been developed to ensure transparency and consistency in the conduct and reporting of SLRs:

  • The Cochrane Collaboration introduced a handbook to guide the conduct and reporting of SLRs.[2]
  • The Quality of Reporting of Meta-analyses (QUOROM) guideline was published in 1999[4]
  • Various portals were launched for prospective registration of SLR protocols prior to data collection, such as the PROSPERO registry in 2011[2]
  • The publication of the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) guidelines in 2009[4]

The PRISMA guidelines were developed to help reviewers report their findings completely and transparently. The PRISMA 2009 statement comprised a checklist of 27 items for reporting in SLRs including the title, abstract, methods, results, discussion, and funding. It also had a section that provided an explanation and elaboration for each item of the guideline. To understand the flow of information through the different phases, a flow chart was supplemented to ensure that aspects such as identification, screening, eligibility, and inclusion are not missed.[4]

The PRISMA guidelines were updated in 2020, and the PRISMA 2020 statement has a 27-item checklist, which also includes an abstract checklist and a revised flow diagram format. In the methods section, the protocol and registration items have moved to a new section with an additional subsection. Authors are required to present the complete search strategies for all the databases. For the ‘Selection process’ item, it is now required to report how many reviewers have screened each record and whether they have worked independently and details of any automation tools used. There are new sub-items in the results section as well.[4] The flow diagram template has been updated with optional boxes that are applicable for review updates. To reflect this, there are four variations of the flow diagram template to choose from.

In addition, several ‘extensions’ of the PRISMA Statement have been developed to facilitate reporting of different types of SLRs.[4] For example, the PRISMA-ScR Statement is developed with the aim of facilitating better reporting of scoping reviews. This extension adds a few points relevant to scoping reviews, and removes some aspects that are not relevant.[5] Next, there is the PRISMA-NMA statement which is a 32-item checklist, and has the flow diagram in a different format that suits comparison of multiple treatments using direct and indirect evidence in network meta-analyses. Likewise, there are separate PRISMA extensions for SLR abstracts, SLRs of acupuncture therapies, SLRs of diagnostic studies, SLRs for IPD (individual patient data) meta-analysis, SLR protocols, SLRs for assessing treatment harms, and so on.[6] Several new PRISMA statements are in development, including SLRs of pediatric population, and SLRs of outcome measurement instruments.[7]

The PRISMA guidelines and its extensions help ensure that SLRs are reported transparently and accurately. Today, most journals and regulatory agencies including HTA bodies require that the SLRs submitted to them strictly adhere to the relevant PRISMA guidelines, regardless of whether the SLR is conducted for academic or regulatory purposes. It is highly essential that all researchers are aware of these guidelines, and more importantly, follow the appropriate extension of the guidelines according to their research objectives, to ensure the highest quality of evidence.

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  1. Pandis N. The evidence pyramid and introduction to randomized controlled trials. Am J Orthod Dentofacial Orthop. 2011 Sep;140(3):446-7 
  2. Gopalakrishnan S, Ganeshkumar P. Systematic Reviews and Meta-analysis: Understanding the Best Evidence in Primary Healthcare. J Family Med Prim Care. 2013 Jan;2(1):9–14. 
  3. IOANNIDIS JPA. The Mass Production of Redundant, Misleading, and Conflicted Systematic Reviews and Meta-analyses. Milbank Q. 2016 Sep;94(3):485–514. 
  4. 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. Syst Rev. 2021 Dec 29;10(1):89. 
  5. Tricco AC, Lillie E, Zarin W, O’Brien KK, Colquhoun H, Levac D, et al. PRISMA Extension for Scoping Reviews (PRISMA-ScR): Checklist and Explanation. Ann Intern Med. 2018 Oct 2;169(7):467–73.

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