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PRISMA flow diagram, checklist and guidelines explained

PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) is a reporting guideline: it tells authors what to report about a systematic review, so that readers can judge how it was done. The current version, PRISMA 2020 (Page et al., 2021), consists of a 27-item checklist and a flow diagram, and replaces the 2009 statement.

The official checklists, flow diagram templates, and supporting documents are maintained at prisma-statement.org.

Is PRISMA a methodology?

No. PRISMA is not a method for conducting a review and not a tool for judging its quality. The authors state that PRISMA 2020 is not intended to guide how a review is conducted, and that it should not be used to assess the conduct or methodological quality of reviews (other tools exist for that, such as ROBIS and AMSTAR 2). It is most useful when read before starting, so that you collect every piece of information you will later need to report.

Is PRISMA only for health research?

It was designed primarily for systematic reviews of the effects of health interventions, but the authors note that the items are applicable to reviews of other interventions, such as social or educational ones, and that many items fit reviews with other objectives. For psychology and the social sciences, the APA's MARS (Meta-Analysis Reporting Standards) are the other standard you may be asked to follow — see Publishing a meta-analysis.

The PRISMA 2020 flow diagram

The flow diagram documents, stage by stage, how the initial pool of records became the final set of included studies — the same searching and selecting process described in the meta-analysis steps:

PRISMA flow diagram: identification of records from databases and registers with removals before screening, screening with records excluded, retrieval of reports with reports not retrieved, eligibility assessment with reports excluded by reason, and studies included in the review with their reports

The diagram above is redrawn, in this site's own style, 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. Licensed under CC BY 4.0.

  • Identification: how many records were found (databases and registers), and how many were removed before screening — split into duplicates, records excluded by automation tools, and records removed for other reasons.
  • Screening: how many records were screened (title/abstract) and how many were excluded at this stage.
  • Retrieval: how many reports were sought for retrieval, and how many could not be retrieved.
  • Eligibility: how many reports were assessed for eligibility, and how many were excluded — with each exclusion reason reported and counted separately.
  • Included: how many studies were ultimately included, and how many reports those studies correspond to.

The 2020 design has up to three identification tracks that converge on the same final box: new studies identified via databases and registers, new studies identified via other methods (websites, organisations, citation searching, and similar), and, for updated reviews, previous studies carried over from the earlier version. Boxes that do not apply are removed.

Records, reports, and studies

The diagram counts three different things, and mixing them up is the most common source of confusion:

  • A record is the title or abstract (or both) of a report as indexed in a database or website.
  • A report is a document providing information about a study: a journal article, preprint, conference abstract, register entry, clinical study report, dissertation, and so on.
  • A study is an investigation that may have several reports (for instance a trial published across multiple papers). This is why "studies included" and "reports of included studies" can differ.

Which template should I use?

Use the version that matches your review. The official site offers four editable Word templates, from two independent choices:

  • new systematic review, or updated review (the updated version adds a track for the studies included in the previous version, carried straight through to the total of included studies);
  • searches of databases and registers only, or of databases, registers and other sources (websites, organisations, citation searching, and similar).

The first choice depends on whether you are reporting a new review or an update of a published one; the second on where you actually searched (Page et al., 2021).

If a journal asks for the older PRISMA 2009 diagram, check its instructions first, but note that the authors say the 2009 statement should no longer be used.

What the guidance asks you to report

The explanation and elaboration paper for PRISMA 2020 (Page et al., 2021) lists the essential elements for the flow diagram (item 16a):

  • the number of records identified, and those removed before screening (for example duplicates, or records deemed ineligible by machine classifiers);
  • the number of records screened, and excluded after screening titles or abstracts;
  • the number of reports retrieved for detailed evaluation, and the potentially eligible reports that could not be retrieved;
  • the number of retrieved reports that did not meet the inclusion criteria, with the primary reasons for exclusion (such as ineligible study design or population);
  • the number of studies and reports included in the review, plus any ongoing studies identified.

It also recommends, where applicable, saying how many records were excluded by a person and how many by automation tools, and, for an updated review, the number of studies included in the previous version. Item 16b adds that studies which might appear to meet the inclusion criteria but were excluded should be cited with the reason for exclusion. Reporting exact counts at every stage is what lets readers judge how selective — and how reproducible — the search and selection process was.

PRISMA 2020 vs PRISMA 2009: what changed?

PRISMA 2020 keeps the spirit of the 2009 checklist but updates it for new methods. Among the noteworthy changes listed by the authors:

  • A separate abstract checklist is included.
  • Protocol and registration moved to an "Other information" section, with a new sub-item on amendments.
  • Full search strategies are recommended for all databases, registers, and websites searched, not just one database.
  • The study selection item stresses how many reviewers screened each record and report, whether independently, and any automation tools used.
  • The synthesis items are split into several sub-items (methods and results), including how heterogeneity was explored and sensitivity analyses.
  • New items on certainty of the evidence, competing interests, and the availability of data, code, and other materials.

The PRISMA 2020 checklist

The checklist has 27 items (some with sub-items) in seven sections: title, abstract, introduction, methods, results, discussion, and other information. A dedicated checklist for abstracts accompanies it. The official site provides fillable versions; it is good practice to consult the items early in the writing process rather than at submission.

Extensions

The base checklist is complemented by extensions for specific designs and parts of a review. The ones most relevant to a meta-analysis are:

  • PRISMA-P (Moher et al., 2015): reporting of systematic review protocols.
  • PRISMA-S (Rethlefsen et al., 2021): reporting of literature searches.
  • PRISMA-ScR (Tricco et al., 2018): scoping reviews.
  • Extensions for network meta-analyses, individual participant data, harms, and diagnostic test accuracy reviews, as listed by the PRISMA 2020 authors.

Check the official site for the current status of each extension, since some were developed against PRISMA 2009.

Where PRISMA fits

PRISMA is one of several reporting guidelines a meta-analysis can follow, alongside APA's MARS (Meta-Analysis Reporting Standards) and MOOSE (for meta-analyses of observational studies). Which one to follow depends on the study design and, above all, on the target journal's instructions.

Learn this in depth

Applying PRISMA in practice — keeping the counts consistent from the first search to the final flow diagram, documenting exclusions as you go, and writing each checklist item — is covered step by step in the ActiLab courses and manual, currently in preparation. Join the newsletter to be notified.

References

Article Moher, D., Shamseer, L., Clarke, M., Ghersi, D., Liberati, A., Petticrew, M., Shekelle, P., Stewart, L. A., & PRISMA-P Group. (2015). Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement. Systematic Reviews, 4(1), 1. https://doi.org/10.1186/2046-4053-4-1

Article Page, M. J., McKenzie, J. E., Bossuyt, P. M., Boutron, I., Hoffmann, T. C., Mulrow, C. D., Shamseer, L., Tetzlaff, J. M., Akl, E. A., Brennan, S. E., Chou, R., Glanville, J., Grimshaw, J. M., Hróbjartsson, A., Lalu, M. M., Li, T., Loder, E. W., Mayo-Wilson, E., McDonald, S., . . . Moher, D. (2021). The PRISMA 2020 statement: An updated guideline for reporting systematic reviews. BMJ, 372, n71. https://doi.org/10.1136/bmj.n71

Article Page, M. J., Moher, D., Bossuyt, P. M., Boutron, I., Hoffmann, T. C., Mulrow, C. D., Shamseer, L., Tetzlaff, J. M., Akl, E. A., Brennan, S. E., Chou, R., Glanville, J., Grimshaw, J. M., Hróbjartsson, A., Lalu, M. M., Li, T., Loder, E. W., Mayo-Wilson, E., McDonald, S., . . . McKenzie, J. E. (2021). PRISMA 2020 explanation and elaboration: Updated guidance and exemplars for reporting systematic reviews. BMJ, 372, n160. https://doi.org/10.1136/bmj.n160

Article Rethlefsen, M. L., Kirtley, S., Waffenschmidt, S., Ayala, A. P., Moher, D., Page, M. J., Koffel, J. B., Prisma-S Group, Blunt, H., Brigham, T., Chang, S., Clark, J., Conway, A., Couban, R., De Kock, S., Farrah, K., Fehrmann, P., Foster, M., Fowler, S. A., . . . Young, S. (2021). PRISMA-S: An extension to the PRISMA statement for reporting literature searches in systematic reviews. Systematic Reviews, 10(1), 39. https://doi.org/10.1186/s13643-020-01542-z

Article Tricco, A. C., Lillie, E., Zarin, W., O'Brien, K. K., Colquhoun, H., Levac, D., Moher, D., Peters, M. D. J., Horsley, T., Weeks, L., Hempel, S., Akl, E. A., Chang, C., McGowan, J., Stewart, L., Hartling, L., Aldcroft, A., Wilson, M. G., Garritty, C., . . . Straus, S. E. (2018). PRISMA extension for scoping reviews (PRISMA-ScR): Checklist and explanation. Annals of Internal Medicine, 169(7), 467–473. https://doi.org/10.7326/M18-0850