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Immortal time bias in a retrospective study examining colorectal cancer mortality according to adherence to colonoscopy
  1. Conchubhair Winters1,
  2. Alexander Charles Ford1,2
  1. 1Leeds Institute of Medical Research at St. James's, University of Leeds, Leeds, UK
  2. 2Leeds Gastroenterology Institute, Leeds Teaching Hospitals NHS Trust, Leeds, UK
  1. Correspondence to Dr Conchubhair Winters, University of Leeds Leeds Institute of Medical Research at St James's, Leeds LS9 7TF, UK; Conchubhair.winters{at}

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We read with interest the study by Zorzi et al.1 The authors conducted a retrospective cohort study comparing outcomes between individuals with a positive faecal immunochemical test (FIT) as part of an Italian screening programme, dependent on whether they were adherent with a colonoscopy within the programme, reporting an adjusted HR of 2.03 (95% CI 1.68 to 2.44) for mortality from colorectal cancer (CRC) among non-adherent individuals during a median 4.9 years of follow-up.

Reasons why 20% of the study population were non-adherent to the programme are not discussed, other than speculation that that some individuals may have had a colonoscopy outside it. There are likely to be substantial differences between the 80% of individuals who were adherent with this screening programme and those who were not, in terms of demographic characteristics, lifestyle choices, comorbidities and deprivation indices. Although …

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  • Contributors CW and ACF drafted the manuscript. Both authors reviewed it and agreed to its publication.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; internally peer reviewed.