Intended and Actual Participation in the Colorectal Cancer Screening Program

A Prospective Cohort Study With AOK Insurees

Background: The public generally has a positive view of colorectal cancer screening, but there is still room for improvement in participation rates. The aim of this study was to identify factors that are associated with intended and actual participation.

Methods: We conducted a prospective cohort study of a random sample of insurees of the AOK (a statutory health insurance carrier) in the German federal state of Lower Saxony. 50-year-old men and 55-year-old women who were eligible for their first screening colonoscopy received a written questionnaire in June 2020, three weeks after being invited to undergo colorectal cancer screening. For those who intended to do so, we used multivariable logistic regression analysis to determine any statistical associations between sociodemographic and medical characteristics and participation rates within 30 months.

Results: 82.7% of the respondents (239/298) intended to participate, and 43.3% (129/298) actually did so within 30 months. The participation rates among persons who had already decided to have a stool test or a colonoscopy were 50.7% (36/71) and 55.2% (58/105), respectively; the participation rate among undecided persons was 33.3% (19/57). The strongest association in the regression model was with an already made appointment (OR = 11.1, 95% confidence interval: [3.9; 31.8]). After exclusion of the existing-appointment variable from the regression model, living in a smaller town (OR = 2.41 [1.08; 5.35]) and a stated preference for colonoscopy (OR = 2.52; [1.20; 5.27]) were positively associated with participation. Insurees with a parent affected by colorectal cancer participated less frequently, even after adjustment for prior colonoscopies (OR = 0.31 [0.12; 0.80]).

Conclusion: The wide gap between intended and actual participation implies that there is potential for improvement in the prevention of colorectal cancer, and that certain groups of people could benefit from targeted support in making their intention to undergo screening a reality. Because of the methodological limitations of this initial investigation, its findings need to be confirmed by further studies.

Great progress has been made in colorectal cancer screening in Germany over the past 20 years. At the end of 2002, screening with stool tests was expanded to include colonoscopy, which enables the removal of precursors of colorectal cancer (1). This can prevent not only mortality but also the development of colorectal cancer (2, 3, 4). Indeed, the incidence of colorectal cancer has since fallen by 22–25% and mortality by 35–40% (5, 6). This is partly also thanks to diagnostic colonoscopies, which are performed far more frequently than screening colonoscopies (7). In 2017, the stool guaiac test, which had been in use since 1971, was replaced by the immunological stool test, which has better diagnostic accuracy and is also easier to use. Finally, the transition in 2020 to an organized program in which insurees are invited in writing by their health insurance carrier to participate in colorectal cancer screening and receive comprehensive evidence-based information to help their decision-making represented an important milestone (8, 9).

There is less success to report in terms of participation in colorectal cancer screening procedures: Over a 10-year period, 26% of eligible men and 27% of eligible women underwent colonoscopy, while 15% took at least three stool tests (10). Not least in view of the growing absolute number of colorectal cancer cases due to the demographic aging of the population—a rise from the current 62,000 to 70 000 cases is estimated by 2040 (11)—significantly higher screening participation is desirable, if only to keep the disease burden of colorectal cancer stable or reduce it further.

In Germany, approval of colorectal cancer screening is high among the population: In an online survey to test the information materials used in the colorectal cancer screening program, 55% declared their willingness to undergo colonoscopy and 75% the stool test (12, 13). At the same time, the significantly lower actual participation (uptake) of colorectal cancer screening indicates what is referred to as an intention–behavior gap, which is already well-known from other areas of prevention such as exercise and diet (14, 15). The aim of this study is to investigate intended and actual participation in colorectal cancer screening in more detail and to identify characteristics in which participants and non-participants differ in the case of intended participation.

Methods

This study uses data from the SIGMO study (funding: Innovation Fund of the German Federal Joint Committee (Gemeinsamer Bundesausschuss [G-BA], 01VSF18007); the research question was formulated a posteriori. The SIGMO study investigated the effect that additionally offering sigmoidoscopy would have on participation in colorectal cancer screening (16). Approval for the SIGMO study was granted by the Ethics Committee of the Hannover Medical School (reference number 8671_BO_K_2019) and the study is registered with the German Clinical Trials Registry (DRKS00019010). Participants gave their informed consent for the use of their data.

The SIGMO study population was made up of insurees of the AOK (a German statutory health insurance carrier) Lower Saxony who were 50 (or 60) years old (men) or 55 (or 60) years old (women) in the first quarter of 2020 and who received a letter, including gender-specific information, inviting them for colorectal cancer screening for the first time in June 2020, followed by a written study questionnaire 3 weeks later (Figure 1). For the purposes of this study, a subpopulation of insurees who were eligible for their first screening colonoscopy was included: 50-year-old men and 55-year-old women. The study design is that of a prospective cohort study. After excluding individuals who changed health insurance carrier (n = 38) or died (n = 2) over the course of the study (1/2020–6/2022) (loss to follow-up of 11.8% [40/338]), a total of 298 insurees were included (eFigure, eBox, eTable 1). A non-responder analysis was performed to compare participants (n = 298) with the baseline sample (n = 1637) (eSupplement).

Figure 1

Participation in colorectal cancer screening within 30 months

eBox

Additional information on the study population

eFigure

Flow diagram of the study population

eTable 1

Loss-to-follow-up analysis

Participation intention was recorded with the following questions: “Do you intend to participate in colorectal cancer screening?” (yes, no), “Which colorectal cancer screening procedure do you intend to participate in?” (stool test, colonoscopy, undecided) and “Have you already made an appointment for colorectal cancer screening?” (colonoscopy appointment, stool test appointment, no). Participation was determined based on billing of the specific billing codes of the German Uniform Value Scale (Einheitlicher Bewertungsmaßstab) for colorectal cancer screening in the period from January 2020 to June 2022. January 2020 was chosen as the start of follow-up, since eligibility for a screening colonoscopy began in the first quarter of 2020 on the birthday of included insurees. In the case of insurees who had already participated in colorectal cancer screening in 2020 prior to the survey and who answered “no” to the question on intention to participate, the variable was recoded as a positive intention to participate (n = 6, all women); the stool test was coded as the intended procedure for five individuals and colonoscopy for one. An already-made appointment was coded for all these insurees.

Differences between participants and non-participants were calculated for the total study population (n = 298) as well as for those with the intention to participate (n = 239) over a follow-up of 30 months (1/2020–6/2022) for the following variables: intention to participate (yes, no; stool test, colonoscopy, undecided), appointment (yes, no), sex (male, female), education based on general education level based on CASMIN (Comparative Analysis of Social Mobility in Industrial Nations) (high, medium, low/none) (17), migrant background (yes, no), size of place of residence (from 50 000, 10 000–49 999, < 10 000), employment status (full-time, part-time, not employed), self-reported experience of colonoscopy (yes, no), cancer (yes, no), colorectal cancer in a parent (yes, no), and body mass index (BMI) (< 25, 25–30, > 30). Individuals with a parent affected by colorectal cancer were stratified based on colonoscopy experience.

Multivariable logistic regression analyses were used in the subpopulation of insurees intending to participate (n = 239) in order to investigate which factors are associated with participation within 30 months. The relatively small sample size limited the number of variables in the model. Forward selection or backward elimination modeling methods resulted in an over 20% rate of missing values. Therefore, the following approach was chosen for modeling: Variables with a correlation to participation with a p-value < 0.2 were selected. Of these, the variables on education and BMI were excluded, since the model with both variables had a more than 10% rate of missing values, both variables had an OR with a p-value > 0.05, and the model without these two variables showed no relevant changes (eTable 2). Since a previous colonoscopy can affect the uptake of screening colonoscopy (following a colonoscopy with normal findings, no screening colonoscopy is required for the next 10 years), adjustments were made in the model for the variable regarding self-reported experience of colonoscopy. Statistical analyses were performed using IBM SPSS Statistics Version 29.0.1.0.

eTable 2

Association between characteristics of insurees and participation in colorectal cancer screening within 30 months among insurees intending to participate

Results

A total of 298 AOK insurees were included (eFigure). In the 30 months following the start of the first quarter of 2020, a total of 43.3% (129/298) participated in colorectal cancer screening: 22.1% in the stool test, 15.8% in colonoscopy, and 5.4% in both procedures (Figure 1, 2 and Table 1). While participation in the stool test was continuous throughout the first year, declining slightly thereafter, it was not until after invitation letters had been sent out that a significant increase was seen in colonoscopy participation from mid-2020, which then also declined from 2021 onwards. A comparison of the study population with the baseline sample shows that the latter participated less frequently in the same period with 29.3% (stool test 17.9%, colonoscopy 8.2%, both procedures 3.2%) (eSupplement).

In all, 82.7% (239/289) of respondents intended to participate in colorectal cancer screening (Figure 2). Thus, the absolute and relative difference between intended and actual participation (intention–behavior gap) is 39.4% and 47.5%, respectively. The frequency of participation varied depending on whether a decision regarding the procedure had already been taken (p < 0,001): Individuals who intended to take a stool test or undergo colonoscopy had a 50.7% and 55.2% participation rate, respectively, whereas undecided individuals had a 33.3% participation rate (Table 1). See eTable 3 for procedure-specific intended and actual participation. Individuals who reported having already made an appointment for colorectal cancer screening were the most likely to participate at 85.1% (p < 0.001).

Figure 2

Intended and actual participation in colorectal cancer screening within 30 months

Table 1

Characteristics and participation of the total study population (n = 298) as well as all individuals intending to participate

eTable 3

Procedure-specific intended and actual participation

Insurees who participated in colorectal cancer screening within 30 months more frequently expressed an intention to participate, had decided on one of the procedures, had already made an appointment, had a higher level of education, worked part-time, and more often lived in towns with a population of less than 10 000 (Table 1). Individuals with a parent affected by colorectal cancer participated less frequently (p < 0.05). This is not explained by a previous colonoscopy, since those without a previous colonoscopy were less likely to participate. When considering only insurees who intended to participate with regard to their actual participation, comparable differences were found between participants and non-participants.

The multivariable logistic regression analysis included only individuals that intended to participate. In model 1, there is a strong association between an already-made appointment and participation (OR = 11.1 [3.9; 31.8]) (Table 2). Model 1 as a whole explains a variance of 31%. In model 2, preference for colonoscopy versus no/uncertain preference (OR = 2.52, [1.20; 5.27]; p = 0.014) and size of place of residence below 10 000 versus above 50 000 inhabitants are associated with participation (OR = 2.41, [1.08; 5.35]; p = 0.032), while insurees with a parent affected by colorectal cancer are less likely to participate (OR = 0.31, [0.12; 0.80]; p = 0.016). These results should be interpreted with caution, given the sometimes very small case numbers. Model 2 has a significantly lower explained variance of 17%; an already made appointment explains the difference to Model 1 of 14%.

Table 2

Association between characteristics of insurees and participation in colorectal cancer screening within 30 months among insurees intending to participate

Discussion

For the first time, results from a prospective cohort study on intended and actual participation in a colorectal cancer screening program in Germany are available. The vast majority (82.7%) of surveyed 50-year-old men and 55-year-old women intended to participate in colorectal cancer screening; however, less than half successfully carried out their plan. Across all insurees who intended to participate, an already-made appointment, the size of their place of residence, preferred procedure, and a positive family history of colorectal cancer were associated with uptake within 30 months.

The high level of intention to participate in the study population reflects its strong approval of colorectal cancer screening. For the 82.7% of surveyed insurees who intended to participate after receiving an invitation along with the evidence-based decision aid, the decision-making process appears to have already been completed. However, to what extent insurees made an informed decision remains unclear (18). Nevertheless, only just under half of those motivated to participate in colorectal cancer screening actually did so within 2.5 years. An intention–behavior gap in France was also investigated in 2018: Here, 71% of those who intended to participate did not actually take a stool test within 5 months following a letter inviting them to do so (19). The corresponding analysis of our data revealed that 81.6% (195/239) of those who intended to participate did not successfully do so within 5 months. However, comparability is still limited: The larger gap in the AOK insurees analyzed in this study may be due in part to the COVID-19 pandemic.

A number of theoretical psychosocial and cognitive models aimed at explaining health behavior are available (20, 21). Recent studies show that in the case of colorectal cancer screening, up to half of the variance in intention to participate or participation can be explained by the Health Action Process Approach (HAPA) model (22, 23, 24). According to the HAPA model, by forming the intention to participate, the insurees in this study left the motivational phase and entered the volitional phase of actual action (25). For those who intended to participate that were in the volitional phase and were referred to as intenders, our study found that an already made appointment was the largest influencing factor. The high level of intention to participate suggests that strategies on the motivational level may not be what is most needed. The insurees already appear to be appropriately and adequately supported by the invitations and evidence-based information provided as part of the colorectal cancer screening program. Therefore, interventions should also be designed specifically for the volitional phase.

More specifically, in the volitional phase, strategies such as reminders and reducing practical barriers (for example, making appointments easier to arrange, sending out test kits) can be used to increase participation (26, 27, 28, 29). Making an appointment, in particular, appears to be a key step towards participation, meaning that interventions to ease the appointment-making process may offer the greatest potential in terms of increasing participation among insurees who essentially want to participate in colorectal cancer screening. This approach is used, for example, in organized mammography screening, whereby an appointment for the examination is sent together with the invitation. People who had already decided to undergo colonoscopy were also more likely to participate compared to people with no preference for a procedure. This suggests that a decision in favor of the more complex colonoscopy is more resolute. Conversely, a prompt decision for the stool test or colonoscopy appears to be important, which could be relevant for the further development of decision aids.

These observations show that psychological models such as the HAPA model are suitable for shedding light on previously unknown patterns of discrepancy between intention and participation and for developing interventions based on these patterns. Following on from this first analysis, further studies are needed to examine, based in particular on psychological models, who does and who does not manage to successfully implement their intention to participate; what role other factors such as place of residence play; and to what extent social inequalities in the successful implementation of an intention to participate could exacerbate health inequalities (30, 31). In this first analysis, education was not associated with participation in the multivariable model.

Individuals with a parent affected by colorectal cancer, and who thus have an above-average risk of colorectal cancer (32), were less likely to participate in colorectal cancer screening. This could be explained by the fact that individuals with first-degree relatives affected by colorectal cancer had a risk-adapted colonoscopy at the age of 40–45 years according to the guidelines (33, 34, 35). This was also the case in our study population: Individuals with an affected parent were more likely to report a previous colonoscopy than the overall study population (52% versus 38%). However, only 19% (3/16) of those with no experience of colonoscopy participated in screening. Likewise in the multivariable model, insurees with a positive family history, adjusted for previous colonoscopies, were less likely to participate. Having said that, these observations have only limited validity due to the low case number. If this finding is confirmed in further studies, individuals with an affected parent would be an important target group for interventions aimed at increasing participation (36).

The design of a prospective cohort study that records outcomes based on billing data instead of self-reported participation and a follow-up of 88% implies that the comparative analyses have high internal validity within the study population. The validity of self-reported previous colonoscopy can also be regarded as very high (37). Due to the case numbers, only participation without taking the procedure into account could be considered in a meaningful manner (eTable 3). The validity is overall limited by a selection bias and low external validity. The higher absolute frequency of participation in colorectal cancer screening by the study population reflects a response bias or healthy volunteer bias (eSupplement). Accordingly, the frequency of the intention to participate may also represent an overestimation. There is evidence that a response bias in association analyses tends to play a minor role if the relations and not the absolute frequencies are considered (38). The intention–behavior gap and the determinants of successful participation when participation is intended are unlikely to be affected by this response bias, since the gap tends to be underestimated due to the overestimation of participation. With regard to external validity, AOK insurees account for approximately a third of all people with statutory health insurance in Germany. However, according to an analysis of AOK insurees in Lower Saxony, this group differs in terms of average sociodemographic characteristics from all statutory health insurees, meaning that the results can be extrapolated to all insurees in Germany to only a limited extent (39). It remains unclear to what extent the COVID-19 pandemic affected the intention–behavior gap; however, in contrast to other screening tests, healthcare data show no decline in screening colonoscopy (40). Furthermore, we do not expect to see any significant reduction in participation as a result of the pandemic in our follow-up until mid-2022.

Outlook

The reasons for the marked difference observed between intended and actual participation need to be found urgently. Psychological models can provide valuable help in the determination of causes. The considerable intention–behavior gap offers enormous potential to increase the individual and population-related benefits of colorectal cancer screening if we manage to provide motivated individuals with targeted support to carry out their intended participation.

Acknowledgments

We would like to thank the participating AOK insurees who answered the questionnaire and gave their consent to linkage with their utilization data.

Conflict of interest statement
The authors declare that no conflict of interests exists.

Manuscript received on 11 August 2023, revised version accepted on 18 April 2024.

Translated from the original German by Christine Rye.

Corresponding author
PD Dr. med. Maren Dreier

Medizinische Hochschule Hannover, Institut für Epidemiologie, Sozialmedizin und Gesundheitssystemforschung

Carl-Neuberg-Str. 1, 30625 Hannover, Germany

Cite this as:
Dreier M, Brinkmann M, Stahmeyer JT, Hemmerling M, Krauth C, Walter U: Intended and actual participation in the colorectal cancer screening program —a prospective cohort study with AOK insurees. Dtsch Arztebl Int 2024; 121: 497–504. DOI: 10.3238/arztebl.m2024.0087

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