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Prevalence of bias attributable to composite outcome in clinical trials published in 2019-2020: a systematic review

José Mário Nunes da Silva1, Juliana Ferreira Souza Conceição2, Paula Camila Ramírez3

  • 1Universidade de São Paulo, School of Public Health, Graduate Program in Epidemiology - São Paulo (SP), Brazil.

Revista Brasileira De Epidemiologia = Brazilian Journal of Epidemiology
|July 3, 2025
PubMed

Insights

Bias attributable to composite outcome (BACO) affects over 25% of clinical trials. Most trials underestimate treatment effects when using composite outcomes that include mortality, highlighting the need for objective interpretation guidelines.

Area of Science:

  • Clinical Trials
  • Biostatistics
  • Medical Research Methodology

Background:

  • Composite outcomes in clinical trials can introduce bias.
  • Bias attributable to composite outcome (BACO) is a potential issue when mortality is a component.

Purpose of the Study:

  • To investigate the prevalence of bias attributable to composite outcome (BACO) in clinical trials.
  • To assess the impact of BACO on the interpretation of trial results.

Main Methods:

  • Systematic review of randomized clinical trials published between January 2019 and December 2020.
  • Included trials with a binary composite primary outcome containing all-cause mortality.
  • Calculated the BACO index to assess the association between composite outcome effects and mortality effects.

Main Results:

  • 91 studies were included, with 25.2% exhibiting significant or suggestive BACO.
  • In 73.9% of affected studies, BACO indicated an underestimation of the treatment effect.
  • No studies showed a significant overestimation of the association due to the composite outcome.

Conclusions:

  • A significant proportion of clinical trials using composite outcomes with mortality are affected by BACO.
  • The BACO index provides an objective measure to assess bias in composite endpoints.
  • Predefined guidelines are necessary for interpreting effects on composite endpoints using criteria like the BACO index.
Abstract

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