Minor effect of loss to follow-up on outcome interpretation in the Swedish spine register

Background and purpose Loss to follow-up in observational studies may skew results and hamper study reliability. We evaluated the importance of loss to follow-up in the Swedish spine register. Patients Patients operated in the lumbar spine and scheduled for a postal questionnaire follow-up during part of 2016 were identified. Out of the 351 patients, 203 had responded. After multiple attempts, 115 of the 148 non-responders were reached; 68 returned the complete questionnaire; and 47 answered a brief questionnaire by phone. Analyses were made with the Chi-square test, analysis of covariance or logistic regression. Some analyses were adjusted. Results At baseline, the non-responders were younger than the responders (55 vs 61 years, p