Abstract
PurposeThe relatively large number of participants lost to follow-up (attrition) in spinal registers calls for studies that investigate the features of these individuals and their possible outcome. The aim was to explore the effect of attrition on patient-reported outcome in patients undergoing degenerative lumbar spine surgery. Three groups were studied: spinal stenosis (LSS), disc herniation (LDH) and degenerative disc disorder (DDD).MethodsPatients who underwent surgery for degenerative lumbar spine conditions during 2008–2012 according to registration in the Swespine national register were eligible for the study. Non-respondents were registered in Swespine prior to surgery, but not at follow-up. Swespine data were merged with hospital data from seven Swedish regions (65% of the population), Statistics Sweden, the National Patient Register and the Social Insurance Agency.Baseline characteristics of non-respondents were described and compared to those of the respondents. Coefficients from regression analyses on PROM values for respondents were used to estimate the levels of PROM values for non-respondents, assuming the same effects of baseline characteristics for the two subgroups. Regression analyses were then conducted to identify variables associated with non-response. The results from the regression analyses were used to predict outcomes for patients with the characteristics of a non-respondent. Primary outcome variable in LSS and LDH was Global Assessment for leg pain, and in DDD, Global Assessment for back pain.ResultsAge, sex, educational level, smoking, living alone, being born outside the EU, previous spine surgery and unexpected events before follow-up were factors that were significantly associated with non-response. Being born inside, the EU was important in all of the studied groups (LSS: OR 0.61 p = < 0.000; LDH: OR 0.68 p = 0.001; DDD: OR 0.58 p = 0.04). For spinal stenosis patients, an unexpected event appeared particularly important (OR 3.40, p = 0.000). The predicted outcome of non-respondents was significantly worse than for respondents (LSS: 75.4% successful outcome vs. 78.7%; LDH: 53.9% vs. 58.2%; DDD: 62.7% vs. 67.5%. P-value in all groups = < 0.000).ConclusionAttrition in Swespine cannot be ignored, as non-respondents were predicted to have worse outcome. The effect of attrition bias should always be considered when contemplating outcome recorded in a quality register with patients lost to follow-up.
Highlights
In all types of clinical studies, the ultimate goal is to have complete follow-up data of all included patients
In contrast to the previous studies, the current paper examines a large nation wide sample and a larger set of factors, through the linkage of Swespine data to several other registers, and outcome estimation of non-respondents is performed in an algorithmic multiple regression model
To avoid too large a number of tables, only the Lumbar Spinal Stenosis (LSS) cohort is presented in the main document and the results for the remaining groups can be found as an online resource (ESM_1)
Summary
In all types of clinical studies, the ultimate goal is to have complete follow-up data of all included patients. Attrition by non-response at follow-up may threaten the validity of Karolinska Institute, Stockholm, Sweden 4 Department of Orthopaedics, Sahlgrenska University. Registers tend to demonstrate rather large proportions of non-respondents [1]. The Swespine national register systematically collects information on patients undergoing spine surgery. It has a good coverage and completeness, and approximately 25% non-respondents at follow-up at one year [2]. Do these non-respondents jeopardize the validity of Swespine data?
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