Introduction: The optimal approach to indexing the Cochrane Collaboration reviews to facilitate retrieval is unknown. Current indexing vocabularies may inadequately cover the reviews' concepts. Furthermore, lists of terms may not capture their context.

Objective: To determine whether current indexing vocabularies cover the reviews' content and whether their context can be represented by an indexing schema of semantic relationships.

Methods: All Systematic Reviews in Issue 2 of the Cochrane Database were included in the study. Each review was indexed by concepts such as diseases, therapies, tests, agents, outcomes or processes of care. A list of relationships between concepts was developed, and one or more term-relationship-term tuples were assigned. When possible, exact MeSH terms were used. If no exact MeSH term existed, exact matches from other standardized vocabularies were sought. If unavailable, a similar or less specific MeSH term was sought. If neither existed, a similar or less specific non-MeSH term was chosen.

Results: Of the 285 articles reviewed, 6 (2%) contained insufficient information for indexing. 70% of the indexed articles could be represented by one term-relationship-term tuple, while the remainder required more. Of the 608 indexing terms, 57% had exact, 6% similar, and 31% less specific MeSH matches. 4% had exact, 0% similar, and 2% less specific non-MeSH matches.

Discussion: An indexing schema of semantic relationships using standardized medical vocabularies represented nearly all of the current reviews. While only 57% of concepts had exact MeSH matches, more general terms existed for nearly all the rest. Future investigation is required to determine the significance of insufficiently detailed indexing terms and whether the semantic relationship approach permits effective searching.