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Reply: Immortal time bias in a retrospective study examining colorectal cancer mortality according to adherence to colonoscopy
  1. Manuel Zorzi1,
  2. Jessica Battagello2,
  3. Claudio Barbiellini Amidei1
  1. 1 Veneto Tumour Registry, Azienda Zero, Padova, Italy
  2. 2 Azienda ULSS n 2 Marca Trevigiana, Treviso, Italy
  1. Correspondence to Dr Manuel Zorzi, Veneto Tumour Registry, Azienda Zero, 35131 Padova, Italy; manuel.zorzi{at}

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We appreciate Dr Winters and Dr Ford’s letter1 for the opportunity to clarify the role of immortal time bias that in the authors’ opinion may have affected the results of our previous article.2

Several sensitivity analyses were conducted to examine whether changing the time window at risk after positive faecal immunochemical test (FIT) affected estimated risks, but no significant differences were observed. There are two main reasons for this.

The first is intrinsically related to colorectal cancer (CRC) screening that in most cases detects initial lesions.3 4 Given progression to CRC is slow, dying within the first months of a FIT positive test from a most probably asymptomatic CRC is highly unlikely. In fact, out of 505 overall CRC deaths, only 3 (0.6%) occurred in the first month, 9 (1.8%) up to the …

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  • Contributors All authors equally contributed to data analysis and intepretation, and to the drafting of the manuscript. The final version was approved by all the authors.

  • 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.