A systematic review of the effects of screening for colorectal cancer using the faecal occult blood test, Hemoccult.

Introduction

Colorectal cancer is a leading cause of illness and death in the Western world. In Australia, the United Kingdom, and the United States it is the commonest cancer in women after breast cancer (age standardised incidence 22-33 per 100000) and in men after prostate and lung cancer (age standardised incidence 31-47 per 100 000).[1 2] Just under half of all people affected will die from their disease.[1 2] The human and financial costs of this disease have prompted considerable research efforts to evaluate the ability of screening tests to detect the cancer at an early, curable stage. Tests that have been considered for screening include faecal occult blood tests, sigmoidoscopy, and colonoscopy.

We have reviewed the evidence about the ability of screening with the faecal occult blood test Hemoccult to reduce mortality from colorectal cancer. Since follow up in Hemoccult screening trials has not been sufficiently long to clarify the effect of detecting and removing adenomas on mortality, our results reflect the effect of detecting early colorectal cancer on mortality. As well as evaluating the effectiveness of screening, we also considered its benefits and harms. If screening is effective, for which populations would screening be likely to be of net benefit, given the potential harms? Weighing the benefits and harms of screening helps to define health policy and future research needs. We reviewed the information from the screening trials about physical harm associated with follow up colonoscopy or sigmoidoscopy. Other harmful screening effects include disruption to lifestyle, the stress and discomfort of testing and further investigations, and the anxiety caused by false positive tests.[3] This review will also be published and maintained on The Cochrane Library, an electronic publication of the Cochrane Collaboration (Oxford: Update Software).

Methods

We conducted a comprehensive search of the health literature for all controlled trials of screening for colorectal cancer by means of faecal occult blood tests. The search included correspondence with trialists for unpublished data and clarification of published results. Details of this search are available on request.[1] The trials were independently assessed for their quality by BT and PG using criteria recommended by the Cochrane Collaboration.[4] Disagreements about quality were resolved by discussion.

Data from the trials were independently extracted by BT and LI and analysed using Meta-analyst version 0.991.[5] We performed the data analysis on an "intention to screen" basis--using the groups that subjects were randomised to whether or not they were ever screened. To determine the size of the effect of screening on mortality from colorectal cancer, we estimated relative risks and risk differences, firstly for each trial and then overall, using fixed and random effects models and then used the [chi square] test for heterogeneity of effects.[5 6]

Analysis by intention to screen underestimates the effect that would be seen in those who actually attended screening. Hence, as a secondary analysis, we adjusted for attendance for screening in individual trials using a previously published method.[7] Essentially, this involves dividing the intention to screen effect (relative risk reduction) by the proportion attending.

Results

Trial characteristics

We identified four randomised controlled trials[8-11] and two non-randomised controlled trials[12 23] that evaluated the effectiveness of screening with the faecal occult blood test Hemoccult. The randomised trials involved about 330 000 people in Denmark, England, Sweden, and the United States, and the nonrandomised trials involved about 113 000 people in France and the United States. Table 1 shows the characteristics of the trials. Most trials commenced in the mid-1970s or early 1980s and involve annual or biennial Hemoccult screening.[8-11] The nonrandomised New York trial evaluated Hemoccult in addition to sigmoidoscopy, which was offered to all trial participants.[12]

[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]

The Funen, Nottingham, and Gothenburg trials randomly allocated individuals or households identified from general practitioner records or population registers to invitation to screening with Hemoccult or to control groups (table 2). The Minnesota and New York trials allocated people who had agreed to participate in the trials ("volunteers") to screening or control groups, while the Burgundy trial, which started in 1988, non-randomly allocated groups from …