The "parallel-cohort RCT": Novel design aspects and application in the Kids-DOTT trial of pediatric venous thromboembolism.

In traditional randomized controlled trial (RCT) designs, patients who are ineligible for randomization are excluded from study participation. However, these patients often constitute an important subgroup of the disease population. By extending existing RCT infrastructure, efforts to evaluate such patients in parallel cohort arms would provide an efficient means of generating multi-center prospective data on natural history, which would facilitate the development of future RCTs involving these subgroups of interest. This brief report discusses principles of the parallel-cohort RCT design, describes the few published trials in which it has been employed, and highlights novel aspects of its use in the Kids-DOTT trial-an ongoing multi-center randomized trial of the duration of anticoagulant therapy for venous thrombosis in children.