Background: Although the concept of an "ongoing study" seems self-explanatory, it is difficult to determine whether a trial is underway.
Objective: To analyze the definitions of "ongoing clinical trial" across different clinical trial registries, methodological guidelines, and other sources.
Design and setting: This meta-research study was conducted at the Universidade Federal de São Paulo (UNIFESP), Brazil.
Methods: We performed a cross-sectional analysis of relevant clinical trial registry databases, methodological guidelines for conducting systematic reviews, and other sources that would define or regulate clinical trials.
Results: We identified various heterogeneous definitions used by eligible sources at both the start and end of a clinical trial. The starting criteria used were as follows: when the team is planning the protocol, when permission is given to conduct the study, or when the first participant is enrolled. Some sources used the time at which the last outcome data was collected as a criterion to determine the end of the trial. The International Committee of Medical Journal Editors stated that a study is still "ongoing" during the analysis process. Several sources use a vague definition or present no clear criteria for defining the start or end of a study.
Conclusion: The concept of "ongoing clinical trials" lacks a transparent and homogeneous definition across relevant sources. A consensus on this concept is important to facilitate the evaluation of available evidence and conduct research synthesis. Further efforts are necessary to determine the best definition for the start and end of a clinical trial.