One in 20 Australians will develop colorectal cancer (CRC),1 and almost half of these people will die from the disease.2 Survival from CRC is highly stage-dependent. Localised disease has a 94% 5-year survival rate;3 however, using current approaches, only 35% of CRC is diagnosed at this stage.4 Screening to detect presymptomatic individuals in the at-risk age group reduces CRC mortality.5 In addition, nearly all CRCs arise from adenomatous polyps; these typically grow slowly, with a median time to development of cancer of 10 years. As a result, removal of polyps through endoscopic polypectomy may reduce CRC risk by up to 90% in those screened.6

Although CRC screening is desirable and advocated by expert groups,5 it is not widespread. The most appropriate initial screening test — faecal occult blood testing (FOBT), flexible sigmoidoscopy, or colonoscopy — remains a subject of debate.5

Because the success of screening programs hinges on participation rates and compliance, methods for recruitment of participants need to be established. Participation in CRC screening using FOBT has been reported to improve when subjects are invited by their own general practitioners.7,8 However, less is known about participation in colonoscopic screening programs. Therefore, we explored participation in colonoscopic screening by comparing two recruitment strategies: selection of subjects drawn from a sample of general practice databases, followed by invitation by the subject’s own GP, versus invitations by the investigators to subjects drawn at random from the electoral roll.

In the electoral roll arm, 566 people were selected. Forty-six were excluded for being on the ACT Cancer Registry, so 520 invitations were sent, resulting in 120 colonoscopies being performed (Box 2). Thus, for every four subjects invited, one screening colonoscopy was performed. In the general practice arm, 913 names were selected from the GPs’ databases, leading to 361 invitations after review of patients’ details, and resulting in 111 colonoscopies (Box 2). Thus, for every three invitations, one screening colonoscopy was performed. This difference in proportions was significant (P = 0.025).

After name selection in the general practice sample, 60% (552/913) were not invited; 309 (56%) of those not invited were classified as “archived”, as their files had been archived after 3 years of inactivity. These selections could not be electronically filtered at the time of name selection, but were not retrievable; consequently, it was not known whether these patients had changed GPs. GPs had various archiving practices, with two practices contributing 271 (88%) of these 309 files. Three of the six general practices required extensive manual reviewing of records to select people for invitation.

Expert groups uniformly recommend screening for colorectal cancer in people with average risk. Randomised clinical trials have demonstrated a reduction in CRC mortality of 15%–33% after screening by FOBT followed by colonoscopy in subjects testing positive.12-14 The evidence that screening by colonoscopy alone reduces CRC incidence is weaker, coming from cohort studies6,15 and a small randomised trial,16 although the level of prevention is 80%–90%. Colonoscopic screening is the preferred strategy of the American Cancer Society and the American College of Gastroenterology.5 The National Health and Medical Research Council recommends either biennial FOBT or 5-yearly flexible sigmoidoscopy for people at average risk.11

The efficacy of screening by either FOBT or colonoscopy is influenced by participation rates. To our knowledge, there are no results available from general population sample screening trials using colonoscopy; although large-scale studies have been published, subjects were referred or recruited from specialised populations.17-19 One study reported 45% participation among 7005 veterans, with 54% of those invited declining involvement.17 Initial participation in FOBT (performance of the first round of tests) ranges from 50% to 67%;14,20 participation rates decline thereafter, with 20%–50% of initial participants completing further examinations.21,22 Ongoing participation in FOBT screening outside clinical trials is typically less than 30%.23

In this study, we compared two population samples — from the ACT electoral roll and from GPs’ patient databases. The two recruitment approaches yielded similar participation rates, with 35% of the electoral roll sample participating and 40% participating from the general practice sample (note: these estimates include non-responders, who could not be assessed for eligibility, in the denominator). These rates are lower than initial participation in FOBT, but compare favourably with the 10-year compliance.20

We caution that the ACT has a relatively well-educated, high-income population that may respond more favourably to screening requests than the rest of Australia.24 However, evidence from the Australian population suggests that socioeconomic status does not affect participation in cancer screening.25,26

We encountered several limitations to GP-based recruitment. General practice databases were diverse, with half of the practices requiring manual review of records to select people for invitation. In addition, GPs had various archiving practices. Many potential subjects may have been excluded from the invitation round because their files had been archived. We could not determine if these patients had left the practice or were merely “inactive”. The labour of manual record sorting could be traded off against better eligibility. However, about 15% of records of general practice invitees did not contain histories of recent colorectal screening tests — a major source of ineligibility. Furthermore, recruitment through general practice databases does not ensure that the population will be proportionally represented, as our comparison to the 2001 matched ACT Census population estimates showed. Equity is a key principle of population screening,27 and these findings suggest that GP-based recruitment could reduce equity compared with a broader population approach.

The GPs we recruited to the study were enthusiastic. Others have reported that engaging GP participation can be an obstacle to screening studies.28,29

A weakness of the electoral roll approach was that more than a third of those invited were ineligible for colonoscopy screening (Box 2). On the other hand, a centralised invitation process allows control of the flow of people attending screening centres. During the study, this control was used to ensure resources were well managed.

The electoral roll sample showed that 19% of the population aged 55–74 years had a colonoscopy in the 10 years before recruitment. This provides a practical estimate to be used with modelled estimates in determining the costs of population CRC screening.

Our study was designed to test the observation from FOBT studies7,8 that an invitation for screening from a subject’s usual GP rather than a central organisation is more likely to lead to screening participation. Supporting this hypothesis, an Australian study also reported superiority of GP recruitment to an aspirin prevention study, with participation of 1 in 6 invitees (general practice) versus 1 in 17 (electoral roll).30 In contrast, we found that subjects invited by GPs were only slightly more likely to reply or attend the first visit for colonoscopic screening. Not surprisingly, those attending from GP invitations were more likely to meet eligibility criteria, but eligible invitees were no more likely to actually attend for colonoscopy. Therefore, the source of invitation played little role in the likelihood of a subject responding or attending. For consideration of design of large-scale population screening, the benefit of recruitment from general practices (superior eligibility) must be offset against the increase in recruitment work required, and in the likely uneven recruitment by region, and consequent effect on equity.

Colon cancer screening is highly cost-effective compared with other accepted healthcare interventions.31 The overall cost and personnel requirements of colonoscopic screening are sensitive to compliance rates and the prevalence of adenomatous polyps in the screened population.32 We have not presented an assessment of costs or personnel requirements, but our data may be useful for baseline assumptions.

This study provides useful data in relation to recruitment for large-scale CRC screening, particularly in the Australian setting. Electoral roll recruitment is simple, effective and yields better population coverage than general practice recruitment alone. General practice recruitment by itself has several limitations, and would need to be supported by other programs. Nevertheless, regardless of recruitment process, we found that it is possible to formally engage the community, GPs, gastroenterologists and healthcare organisations in population-based colorectal screening.

2 Recruitment and participation of subjects in colonscopy screening