Why update a review?

The main aim of a Cochrane review is to provide the 'best available' and most up-to-date evidence which can be used by consumers, clinicians and policy makers to bring about improvements in health care. Clinical practice guidelines are often based on Cochrane or other systematic reviews and increasingly funding agencies internationally are requiring systematic reviews to justify the need for new clinical trials (and other forms of research). As a result, in the last decade the number of systematic reviews published either electronically (through the Cochrane Collaboration) or in paper journals has greatly increased.

However, since evidence on a given subject is generally dynamic and continually evolving as new research becomes available, systematic reviews run the risk of becoming out of date and even misleading. To date, there is little empirical data available to allow informed decisions about what is a reasonable and efficient approach to updating evidence in Cochrane reviews, although some guidelines do exist.

We do know that incorporating additional studies as they become available may, on occasions, change the results of a review. However, there is a risk that updating evidence too frequently may result in introducing a different form of bias related to the slower publication (or even non-publication) of studies with negative and inconclusive results.

You may be able to gauge from doing the review whether the research relevant to your review is being published frequently, suggesting your review should be updated sooner, or if you should wait a while.

In addition, other developments in your research field may result in you needing to update your review. Some examples may be better tools or markers for characterising sub-groups, new treatment regimens and/or comparisons, or new outcome measures (or refined measurement methods of existing outcomes).

In contrast, there may be areas where new evidence or data are not emerging and a review prepared many years earlier is still current and valuable. In these cases updating your review may be unnecessary and wasteful.

Cochrane reviews are unique in that reviewers are committed to not only preparing systematic reviews of evidence but also to maintaining (and updating) these reviews on a regular basis. The current recommendation is that your review should be updated at least every two years, or else a commentary added to it to explain why the update has not been done.