title: A 9-year follow-up study of participants and nonparticipants in sigmoidoscopy screening: importance of self-selection.
abstract: BACKGROUND
  Self-selection may compromise cost-effectiveness of screening programs. We hypothesized that nonparticipants have generally higher morbidity and mortality than participants.
  
  
  METHODS
  A Swedish population-based random sample of 1,986 subjects ages 59 to 61 years was invited to sigmoidoscopy screening and followed up for 9 years by means of multiple record linkages to health and population registers. Gender-adjusted cancer incidence rate ratio (IRR) and overall and disease group-specific and mortality rate ratio (MRR) with 95% confidence intervals (95% CI) were estimated for nonparticipants relative to participants. Cancer and mortality rates were also estimated relative to the age-matched, gender-matched, and calendar period-matched Swedish population using standardized incidence ratios and standardized mortality ratios.
  
  
  RESULTS
  Thirty-nine percent participated. The incidence of colorectal cancer (IRR, 2.2; 95% CI, 0.8-5.9), other gastrointestinal cancer (IRR, 2.7; 95% CI, 0.6-12.8), lung cancer (IRR, 2.2; 95% CI, 0.8-5.9), and smoking-related cancer overall (IRR, 1.4; 95% CI, 0.7-2.5) tended to be increased among nonparticipants relative to participants. Standardized incidence ratios for most of the studied cancers tended to be >1.0 among nonparticipants and <1.0 among participants. Mortality from all causes (MRR, 2.4; 95% CI, 1.7-3.4), neoplastic diseases (MRR, 1.9; 95% CI, 1.1-3.5), gastrointestinal cancer (MRR, 4.7; 95% CI, 1.1-20.7), and circulatory diseases (MRR, 2.3; 95% CI, 1.2-4.2) was significantly higher among nonparticipants than among participants. Standardized mortality ratio for the studied outcomes tended to be increased among nonparticipants and was generally decreased among participants.
  
  
  CONCLUSIONS
  Individuals who might benefit most from screening are overrepresented among nonparticipants. This self-selection may attenuate the cost-effectiveness of screening programs on a population level.
authors.affiliation: Division of Surgery, Department for Clinical Science, Intervention and Technology, Karolinska Institutet at Karolinska University Hospital, K53, Huddinge, 141 86 Stockholm, Sweden. johannes.blom@ki.se
authors.forename: Johannes
authors.initials: J
authors.lastname: Blom
authors.forename: Li
authors.initials: L
authors.lastname: Yin
authors.forename: Annika
authors.initials: A
authors.lastname: Lidén
authors.forename: Anders
authors.initials: A
authors.lastname: Dolk
authors.forename: Bengt
authors.initials: B
authors.lastname: Jeppsson
authors.forename: Lars
authors.initials: L
authors.lastname: Påhlman
authors.forename: Lars
authors.initials: L
authors.lastname: Holmberg
authors.forename: Olof
authors.initials: O
authors.lastname: Nyrén
mesh_terms: D002423:Cause of Death; D015179:Colorectal Neoplasms / Q000401:mortality* / Q000517:prevention & control*; D005260:Female; D005500:Follow-Up Studies; D005770:Gastrointestinal Neoplasms / Q000401:mortality; D015438:Health Behavior; D006801:Humans; D015994:Incidence; D008175:Lung Neoplasms / Q000401:mortality; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D016012:Poisson Distribution; D012042:Registries; D012812:Sigmoidoscopy / Q000706:statistics & numerical data*; D012907:Smoking / Q000009:adverse effects; D013548:Sweden / Q000453:epidemiology