"No language restrictions" in database searches: what does this really mean?

The aim of this study was to investigate the coverage of non-English journals by MEDLINE((R)) and EMBASE, the two major biomedical databases used for identifying studies for possible inclusion in systematic reviews and meta-analyses. A series of searches were conducted to compare the coverage of journals in languages other than English. The results were compared against listings in Ulrich's Periodicals Directory, an authoritative source of information on periodicals published in more than 200 countries. This study has highlighted the existence of a database coverage bias, in terms of the systematic exclusion of journals from certain countries and/or in certain languages. Searching that relies only on English language databases may result in failure to find many relevant studies published in languages other than English, irrespective of the research question and the avoidance of any language restrictions.