Use of multiple databases together with additional search strategies is often suggested to search relevant references for systematic reviews. (1,2) For instance, the Cochrane Handbook recommends using at least MEDLINE and Cochrane Central as well as EMBASE, when available, to search randomized controlled trials (RCTs). (3) However, using multiple databases can be strenuous and time consuming owing to the database-specific syntax of search strategies and differences of field codes and proximity operators between interfaces. Another difficulty is the different thesaurus terms between databases that may hamper translation. In addition, it is inconvenient for reviewers to screen more and possibly irrelevant titles and abstracts. Last but not least, limited access and subscriptions make the process all the more tedious and challenging. (4)

In contrast, some studies exist in the literature that investigate the value of using multiple databases for different topics. Some of these studies report no effect on the outcome by searching more than one databases, thus proving just one database to be sufficient. (5,6) While others have reported a single database to be insufficient to retrieve all references for systematic reviews. (7) Majority of articles on this topic base their conclusions on the coverage of databases, (8) while many have failed to identify an acceptable number of databases to be searched. (9) Having said that, the presence of an article in a database does not guarantee it will be found in a search in that database. Therefore, the ideal database or a certain number of databases to be searched for relevant references for a systematic review remains unclear.(4)

A recent prospective study has done some research in this area with an aim to determine the combination of databases to be searched for systematic reviews to obtain efficient results by means of minimizing the burden for the investigators and not the validity of the research by missing relevant references.(4)  This study recommended the biomedical searches to be performed using a combination of the following four databases, viz. EMBASE, MEDLINE (plus Epub ahead of print), Web of Science Core Collection, and Google Scholar. Use of this combination showed 93% of the systematic reviews to obtain levels of recall to be considered acceptable (> 95%). Unique results from specialized databases that closely match systematic review topics indicated their use whenever applicable, for e.g. PsycINFO for reviews in the fields of behavioural sciences and mental health or CINAHL for reviews on the topics of nursing and/or allied health.(4)

Similarly, researchers at Erasmus University Medical Center (MC) have developed a methodology for generating comprehensive search strategies. (10) This methodology encompasses all steps of the search process, starting with a question and resulting in thorough search strategies in multiple databases. Researchers believe that this can prove to be a robust method to create high-quality, vigorous searches in multiple databases in a relatively short time frame. (10)

Another systematic review has also showed that searching Medline alone for systematic reviews of exercise or other unconventional therapies is likely to be inadequate, while additional specialised databases along with checking reference lists and contacting experts can prove to be most effective for including all relevant papers in the review. (11)

However, skills and experience of the searcher is an important aspect that can play a role in the efficacy of the search strategies being used. (12) Non-structured searches and searches with lower recall may even miss out relevant references. This can be solved with additional efforts like hand/cursory searching, looking up references, and contacting key players, which might add the extra references in the search.(4)

To sum it all up, there need to be ways to optimize multiple databases and/or combinations in order to include all the relevant references in systematic reviews. Some researchers might suggest a combination of a few databases, depending on the topic/area of research in a particular systematic review. Majority of evidence available in the literature states that searching only one database may prove to be insufficient, thus leading to missing references. In addition, checking reference lists as well as contacting key experts to include all the necessary information and insights is recommended.

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  1. Levay P, Raynor M, Tuvey D. The contributions of MEDLINE, other bibliographic databases and various search techniques to NICE public health guidance. Evid Based Libr Inf Pract 2015; 10:50–68.
  2. Beyer FR, Wright K. Can we prioritise which databases to search? A case study using a systematic review of frozen shoulder management. Health Inf Libr J 2013; 30:49–58.
  3. Higgins JPT, Green S. Cochrane handbook for systematic reviews of interventions: The Cochrane Collaboration, London, United Kingdom. 2011.
  4. Bramer WM, Rethlefsen ML, Kleijnen J, et al. Optimal database combinations for literature searches in systematic reviews: a prospective exploratory study. Syst Rev 2017; 6(1):245.
  5. Aagaard T, Lund H, Juhl C. Optimizing literature search in systematic reviews—are MEDLINE, EMBASE and CENTRAL enough for identifying effect studies within the area of musculoskeletal disorders? BMC Med Res Methodol 2016; 16:161.
  6. Rice DB, Kloda LA, Levis B, et al. Are MEDLINE searches sufficient for systematic reviews and meta-analyses of the diagnostic accuracy of depression screening tools? A review of meta-analyses. J Psychosom Res 2016; 87:7–13.
  7. Bramer WM, Giustini D, Kramer BMR. Comparing the coverage, recall, and precision of searches for 120 systematic reviews in Embase, MEDLINE, and Google Scholar: a prospective study. Syst Rev 2016; 5:39.
  8. Hartling L, Featherstone R, Nuspl M, Shave K, Dryden DM, Vandermeer B. The contribution of databases to the results of systematic reviews: a cross-sectional study. BMC Med Res Methodol 2016; 16:1–13.
  9. Ross-White A, Godfrey C. Is there an optimum number needed to retrieve to justify inclusion of a database in a systematic review search? Health Inf Libr J 2017; 33:217–24.
  10. Bramer WM, de Jonge GB, Rethlefsen ML, et al. A systematic approach to searching: an efficient and complete method to develop literature searches. J Med Libr Assoc 2018; 106(4):531–541.
  11. Stevinson C, Lawlor DA. Searching multiple databases for systematic reviews: added value or diminishing returns? Complement Ther Med 2004; 12(4):228-32.
  12. Rethlefsen ML, Farrell AM, Osterhaus Trzasko LC, et al. Librarian co-authors correlated with higher quality reported search strategies in general internal medicine systematic reviews. J Clin Epidemiol 2015; 68:617–626.

Written by: Ms. Tanvi Laghate

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