Two tests may be better than one in bowel cancer screening, finds study

BMJ 2012; 344 doi: (Published 31 January 2012) Cite this as: BMJ 2012;344:e783
  1. Nigel Hawkes
  1. 1London

Two sigmoidoscopy tests separated by three to five years detect roughly a third more bowel cancers than does a single test, a new study shows. But whether the benefit of detecting the extra cancers justifies the additional costs and the risks of complications, or whether it reduces mortality by a similar degree, remains unknown.

The NHS in England plans to offer flexible sigmoidoscopy screening for bowel cancer to all people from the age of 55, after a trial showed that a single such test conferred a substantial and lasting benefit. The new results, from a US trial, show that a second test increases detection of cancers by 26% in women and 34% in men.

The study draws on results from a major US trial that recruited participants between 1993 and 2001. The trial protocol offered flexible sigmoidoscopy on entry and a repeat three years later (subsequently modified to five years). Of the 77 447 people enrolled, 39 442 had two sigmoidoscopies, and the additional cancers detected by this second procedure were analysed by a team led by Joel Weissfeld of the University of Pittsburgh (Journal of the National Cancer Institute 2012;104:1-10).

The results show that a single screening identified colorectal cancer or advanced adenoma in 37.8 per 1000 people screened and that the proportion rose to 49.8 per 1000 in those who had a second test. Three quarters of those screened twice had a negative result on the first screen, and among these 18.8% tested positive the second time round. The proportion of participants undergoing further diagnostic interventions, such as colonoscopy, rose from 15.3% of those who had one sigmoidoscopy screen to 22.9% of those who had two.

What matters ultimately is whether sigmoidoscopy screening reduces cancer incidence and mortality, for which data are not yet available. Assuming that there is such a benefit, it may be impossible to distinguish between the effects of the first and subsequent screenings, the authors say. They add, “There are also reasons why repeating a flexible sigmoidoscopy screening may not be beneficial. Although repeated screening increases yields, it also increases the costs of screening, the need for diagnostic intervention, and the risks of complication.”

In an associated editorial Noel Weiss and Polly Newcomb of the University of Washington in Seattle say that the size of the study and the large numbers of cancers and adenomas identified provide statistically solid estimates to be incorporated into cost-benefit evaluations of different rescreening strategies.

The NHS screening programme currently offers faecal occult blood tests to all people aged 60 or over. The plan is to offer a single sigmoidoscopy screening to those reaching the age of 55, the offer extending up to their 60th birthday. The blood tests will continue to be offered at 60 whether or not people have chosen to be screened by sigmoidoscopy.


Cite this as: BMJ 2012;344:e783