The Basque Country’s colorectal cancer screening programme was associated with a 50 per cent reduction in mortality from this cause among adults aged 50 to 69

The Basque Country’s screening programme is one of the programmes with the highest take-up rates in Europe, which makes it particularly suitable for assessing the benefits of these prevention strategies. Although this study is retrospective and therefore not suitable for establishing a causal relationship, it is highly relevant, with a rigorous design and a very large sample and follow-up period, which allows for robust conclusions to be drawn.

The results show that, within a few years of the screening programme being introduced, a reduction in mortality related to colorectal cancer began to be observed. This reduction in mortality may be due to a decrease in the incidence of this cancer as a result of the identification and subsequent removal of precursor lesions (advanced adenomas), as well as the early detection of cancer, as demonstrated by the high percentage of cases detected at early stages of development. In short, this study confirms the benefits of colorectal cancer screening programmes and therefore provides a further incentive to increase participation in them.

The study is of a very high standard, but we must not lose sight of the fact that it is an observational study and the evidence it generates must be put into context. In this case, the context is that the effectiveness of colorectal cancer screening has been demonstrated through randomised clinical trials (one of the most reliable sources of evidence for establishing causality). Therefore, although – as the authors note in the discussion – there are factors that may have changed during the study period and which may have contributed to the decline in mortality from colorectal cancer (for example, health behaviours or access to the healthcare system), it is reasonable to assume that screening has made a significant contribution to the decline in colourectal cancer-specific mortality.

The main implication of this study is that it highlights the importance of a public health system with robust policies on cancer prevention and screening. In the case of the Basque Country, the colorectal cancer screening programme has been in place without interruption since 2009, which has helped to generate the benefits reported in this study.

Assessment of the press release

The press release accurately summarises the main findings of the study, but there are some substantive issues that should be corrected before it is published, as they could lead to a distorted interpretation of the study:

1. The authors themselves explicitly state that the observational and retrospective design prevents any causal inference from being drawn. Throughout the manuscript, the term ‘association’ is used consistently, never ‘direct causality’. The press release, however, states in the headline and in the body: ‘the programme… has reduced mortality… by 30 per cent’ and ‘has prevented more than 6,000 deaths’. However, its observational-retrospective design precludes causal inference. I recommend including ‘associated with a reduction’ rather than ‘has reduced’ or ‘has prevented’.
2. The reference in the first paragraph to savings on surgery, hospital admissions and cancer treatments is not supported by the data presented in this study, which is purely epidemiological (mortality) and contains no cost component. Attributing this to the JAMA Network Open study would be inaccurate and could prompt an awkward question from a journalist asking for the specific figure; therefore, I would avoid this comment in the press release.
3. The figure of “more than 6,000 deaths prevented” does not appear anywhere in the manuscript or in the tables/figures, and should therefore not be presented as a result of the study.

Assessment of the study

This is a large-scale population-based study with a very long follow-up period: it includes virtually the entire population of the Basque Country over 21 consecutive years, allowing the phenomenon to be observed at different stages of the screening programme and even during the disruption to healthcare caused by COVID-19. The data are drawn from high-quality population registers (the Basque Institute of Statistics and the screening programme’s own register), which reduces the risk of selection bias or loss to follow-up.

From a methodological perspective, the study employs a robust analytical design, using age-adjusted rates, segmented regression to objectively identify the point at which the trend changed, and counterfactual projections that estimate what would have happened had the screening programme not existed. Furthermore, it incorporates an unscreened reference group (those under 50 years of age) as an internal control and confirms its results through sensitivity analyses (excluding deaths from COVID-19). It is this combination of a large sample size, long-term follow-up and the triangulation of several independent analytical approaches that lends greater weight to the observed association, although, as in any observational study, it cannot establish direct causality with the same certainty as a randomised trial.

As for the most relevant findings, the study shows that the reduction in colorectal cancer mortality was particularly marked in the target population (aged 50–69), with a 50 per cent decrease compared with pre-screening rates, well above that observed in other population-based European programmes, which typically report reductions of 9–10 per cent in the first decade of follow-up. This highly favourable result is largely explained by the high and sustained participation in the Basque programme, which remained at around 70 per cent throughout the period, together with an adherence rate to diagnostic colonoscopy following a positive test of over 90 per cent.