Background: Searching clinical trial registries is recommended as a standard component of information retrieval for systematic reviews. In this context, as a meta-registry containing data from 17 (national) registries, the International Clinical Trials Registry Platform (ICTRP) Search Portal of the World Health Organization is a potentially valuable information source.

Discussion: The use of the ICTRP Search Portal is affected by various problems, which can be classified as follows: 1) limited search functions; 2) inconsistent implementation of search functions in the standard and advanced search interfaces; 3) problems caused by linking related records; 4) partly incomplete data pool; 5) error messages during complex search queries, as well as other inconsistencies; and, 6) provision of insufficient information to users on changes to search interfaces and functions. The presentation will describe the various problems and give advice on how to deal with them in practice.

Conclusions: The ICTRP Search Portal is faced with structural problems that make it difficult to conduct a comprehensive search. We recommend dispensing with the advanced search function at the moment as the sensitivity achieved is insufficient. As long as the problems described persist, we also recommend additional searches in other registries such as ClinicalTrials.gov or EU-CTR.