Would loss to follow-up bias the outcome evaluation of patients operated for degenerative disorders of the lumbar spine?, Acta Orthopaedica, Informa Healthcare

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Would loss to follow-up bias the outcome evaluation of patients operated for degenerative disorders of the lumbar spine?
A study of responding and non-responding cohort participants from a clinical spine surgery registry

February 2011, Vol. 82, No. 1 , Pages 56-63 (doi:10.3109/17453674.2010.548024)

Tore K Solberg ^1,², Andreas Sørlie^1,², Kristin Sjaavik^1,², Øystein P Nygaard^2,³, and Tor Ingebrigtsen^1,⁴

¹Department of Neurosurgery, University Hospital of Northern Norway, Tromsø

²Norwegian Registry for Spine Surgery (NORspine), Northern Norway Regional Health Authority, Tromsø

³National Centre for Spinal Disorders and Department of Neurosurgery, University Hospital of St. Olav, Trondheim

⁴Institute of Clinical Medicine, University of Tromsø, Norway

Correspondence: tore.solberg@unn.no

Background and purpose 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.

Interpretation 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.