In 2012, The Cochrane Collaboration published their Methodological Expectations of Cochrane Intervention Reviews, a set of guidelines showing the mandatory and highly desirable elements of a Cochrane systematic review. One of the MECIR statements was that all new reviews should have a PRISMA flow diagram. A PRISMA flow diagram is essentially a flow chart, showing the number of studies screened, included and excluded at each stage of the review. However, to date there has been no guidance on how to create or expand a PRISMA diagram for a Cochrane review update. A working group of Cochrane Information Specialists have recently published a paper in the Systematic Reviews journal, giving recommendations on PRISMA diagrams and review updates.
One major issue in creating a PRISMA diagram for a review update is the number of different scenarios that the diagram has to illustrate. The original review may not have any included studies or it may have many, the update may have identified new studies for inclusion or it may not. Another issue is that the search history of the original review or subsequent updates may be incomplete. A dilemma considered by the working group is whether to have a new diagram for every update of the review or whether to try to incorporate all the information into a single diagram. The working group decided that a single diagram for the current review update, with the number of included studies from the original review or previous update included in the total, was preferable.
The working group recommends that:
- Where multiple searches have been conducted for a review or review update, the results of all searches should be added together.
- For a review update, two extra boxes should be added to the PRISMA diagram. One should show the number of studies included in the original review or previous update. The other should show the new studies retrieved for the current update. If multiple searches have been conducted for the review update, then the results of all the searches should be added together.
Although PRISMA diagrams are only mandatory for new Cochrane reviews, it is best practice to include them in review updates as well. There are still outstanding issues, for example, the adapted diagram does not show the total number of references that have been screened over the lifetime of the review. This solution however, is a simple, clear way to document the flow of studies when the previous search history may not be available.
The full paper (including Figure 1, above) can be accessed at:
Stovold, E, et al., 2014: Study flow diagrams in Cochrane systematic review updates: an adapted PRISMA flow diagram. Systematic Reviews, 3:54.
Figure One is copied from the above article, published by BioMed Central and reproduced according to their guidance: http://www.systematicreviewsjournal.com/about/faq/reproduce