The validity of systematic reviews rests upon the efficiency and scope of the methods used for identifying RCTs. It has long been recognised that Medline identifies an insufficient number of RCTs for the purpose of developing systematic reviews, and thus lays emphasis on the importance of handsearching. This is a very slow and time-consuming process. These limitations lead to the consideration of other methods, and the use of other electronic registers such as Embase.

It is widely believed that Embase also provides an incomplete record of RCTs, and will have limitations similar to those found in Medline. However, it is possible that the two electronic databases are not co-extensive, so RCT retrieval may be enhanced by a search of both systems.

We have compared the efficiency of Embase and Medline with respect to the identification of asthma RCTs published in Thorax and the American Review of Respiratory Disease, over a span of 5 years, from 1989 to 1993 inclusive. A common search strategy was needed as Medline uses MeSH for subject retrieval, while Embase relies on keywords. Therefore, identical text word search terms were used in both registers for identifying the RCTs. These were placebo* OR trial* OR random* OR single blind OR single-blind OR double blind OR double-blind OR comparative study OR controlled study. The table below shows the numbers and percentages of asthma RCT records identified in each register in comparison to those found during handsearching.


Journals Handsearched Embase Medline Combined
Thorax 53 48 (91%) 42 (79%) 50 (94%)
ARRD 93 77 (83%) 77 (83%) 84 (90%)
Total 146 125 (86%) 119 (82%) 134 (92%)


Overall the retrieval rate was similar for the two databases. These were not co-extensive, with the result that the pick-up rate of RCTs rose by 10% compared to that achieved by Medline alone. It appears that combining the searches of different databases increases the number of RCTs identified.