title: Would loss to follow-up bias the outcome evaluation of patients operated for degenerative disorders of the lumbar spine?
abstract: OBJECTIVE
  Loss to follow-up may bias the outcome assessments of clinical registries. In this study, we wanted to determine whether outcomes were different in responding and non-responding patients who were included in a clinical spine surgery registry, at two years of follow-up. In addition, we wanted to identify risk factors for failure to respond.
  
  
  METHODS
  633 patients who were operated for degenerative disorders of the lumbar spine were followed for 2 years using a local clinical spine registry. Those who did not attend the clinic and those who did not answer a postal questionnaire-for whom 2 years of outcome data were missing-and who would be lost to follow-up according to the standard procedures of the registry protocols, were defined as non-respondents. They were traced and interviewed by telephone. Outcome measures were: improvement in health-related quality of life (EQ-5D), leg pain, and back pain; and also general state of health, employment status, and perceived benefits of the operation.
  
  
  RESULTS
  We found no statistically significant differences in outcome between respondents (78% of the patients) and non-respondents (22%). Receipt of postal questionnaires (not being summoned for a follow-up visit) was the strongest risk factor for failure to respond. Forgetfulness appeared to be an important cause. Older patients and those who had complications were more likely to respond.
  
  
  CONCLUSIONS
  A loss to follow-up of 22% would not bias conclusions about overall treatment effects and, importantly, there were no indications of worse outcomes in non-respondents.
authors.affiliation: Department of Neurosurgery, University Hospital of Northern Norway, Tromsø, Norway. tore.solberg@unn.no
authors.forename: Tore K
authors.initials: TK
authors.lastname: Solberg
authors.forename: Andreas
authors.initials: A
authors.lastname: Sørlie
authors.forename: Kristin
authors.initials: K
authors.lastname: Sjaavik
authors.forename: Øystein P
authors.initials: ØP
authors.lastname: Nygaard
authors.forename: Tor
authors.initials: T
authors.lastname: Ingebrigtsen
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015982:Bias; D015331:Cohort Studies; D005500:Follow-Up Studies; D006801:Humans; D007405:Intervertebral Disc Displacement / Q000601:surgery*; D059012:Lost to Follow-Up; D008159:Lumbar Vertebrae / Q000601:surgery*; D008875:Middle Aged; D017063:Outcome Assessment, Health Care; D011446:Prospective Studies; D012042:Registries; D012307:Risk Factors; D012959:Socioeconomic Factors; D013130:Spinal Stenosis / Q000601:surgery*; D011795:Surveys and Questionnaires; D055815:Young Adult