Some of the most convenient and obvious sources of references to potentially relevant studies are existing reviews. Copies of previously published reviews on, or relevant to, the topic of interest should be obtained and checked for references to the included (and excluded) studies. As well as the Cochrane Database of Systematic Reviews (CDSR), The Cochrane Library includes The Database of Abstracts of Reviews of Effects (DARE) and the Health Technology Assessment Database (HTA Database), both produced by the Centre for Reviews and Dissemination (CRD) at the University of York in the UK. Both databases provide information on published reviews of the effects of health care. As well as being published and updated quarterly in The Cochrane Library, more up-to-date versions of these databases are available free of charge on the CRD web site, where they are updated more frequently. For example, for the issue of The Cochrane Library published in January 2007, the DARE and HTA records were supplied by CRD staff in November 2006. The January 2007 publication of The Cochrane Library was the current issue until April 2007, so the DARE and HTA records in The Cochrane Library range between being two months to five months out of date.
CRD used to produce the CRD Ongoing Reviews Database which was searchable through the UK National Research Register (NRR) but since that was archived in September 2007, records of ongoing reviews have been transferred to the HTA Database.
Reviews and guidelines may also provide useful information about the search strategies used in their development: see Box 6.2.g. Specific evidence-based search services such as Turning Research into Practice (TRIP) can be used to identify reviews and guidelines. For the range of systematic review sources searched by TRIP see:
A key source for identifying guidelines is the National Guideline Clearinghouse:
MEDLINE, EMBASE and other bibliographic databases can also be used to identify review articles and guidelines. In MEDLINE, the most appropriate review articles should be indexed under the Publication Type term ‘Meta-analysis’, which was introduced in 1993, or ‘Review’, which was introduced in 1966. Guidelines should be indexed under the Publication Type term ‘Practice Guideline’, which was introduced in 1991. EMBASE also has a thesaurus term ‘Systematic Review’, which was introduced in 2003, and ‘Practice Guideline’, which was introduced in 1994.
There is a so-called ‘Systematic Review’ search strategy or filter on PubMed under the Clinical Queries link:
It is very broad in its scope and retrieves many references that are not systematic reviews. The strategy is described as follows: “This strategy is intended to retrieve citations identified as systematic reviews, meta-analyses, reviews of clinical trials, evidence-based medicine, consensus development conferences, guidelines, and citations to articles from journals specializing in review studies of value to clinicians.”
Search strategies or filters have been developed to identify systematic reviews in MEDLINE (White 2001, Montori 2005) and EMBASE (Wilczynski 2007). Search strategies for identifying systematic reviews in other databases and for identifying guidelines are listed on the InterTASC Information Specialists' Subgroup Search Filter Resource web site.
As well as searching the references cited in existing systematic reviews and meta-analyses, reference lists of identified studies may also be searched for additional studies (Greenhalgh 2005). Since investigators may selectively cite studies with positive results, reference lists should be used with caution as an adjunct to other search methods (see Chapter 10, Section 10.2.2.3).