Understanding and Preventing Loss to Follow-up: Experiences from the Spinal Cord Injury Model Systems
1Kim H, 2Cutter G, 3George B, 2Chen Y
1Duke Clinical Research Institute, Durham, NC, USA; 2University of Alabama at Birmingham, Birmingham, AL, United states; 3Thomas Jefferson University, Philadelphia, PA, United states
One of the most critical threats to the validity of longitudinal research is the bias caused by study attrition. Loss to follow-up (FU) seems to be inevitable and usually depends on the length of FU and the complexity of the study protocol. To improve the generalizability of study findings and better inform the public, it is crucial to know what potential sources of bias and imprecision might be present in a dataset with incomplete FU. If the loss to FU is predictable, prevention efforts can be focused on those individuals at high risk of non-participation. The goal of this study was to improve our ability to draw valid conclusions from longitudinal research, particularly regarding long-term outcomes and survival, by identifying mechanisms for reducing study attrition.
We identify demographic and clinical factors associated with loss to FU at post-injury years 1-35 among 25,871 people with spinal cord injury (SCI) enrolled in the National Spinal Cord Injury Database (NSCID). Loss to FU was defined as no research information obtained from participants who were eligible for the planned data collection. Generalized linear mixed models were used for analysis of factors at each post-injury year.
The loss to FU rates were 23.1% and 32.9% for post-injury years 1 and 5, respectively, and remained >40% between post-injury years 20 and 35. The FU rate varied by study sites and was improved in recent injury cohorts. People who were more seriously injured and those who attained higher levels of education were more likely to return for FU than their counterparts. People who were at risk of being marginalized in society (non-whites, those with less education, the unemployed, victims of violence, and those with no health insurance) had the highest odds of being lost to FU across all post-injury years.
The present study provides evidence that highlights the importance of incorporating strategies to maximize FU participation from initial enrollment in the NSCID. Important demographic and injury characteristics associated with loss to FU can be obtained at study entry. These individuals can then be targeted for strategies to maintain their participation at FU. These approaches should be targeted based upon the specific risk factors exhibited by participants. Furthermore, this study provides evidence that centers differ in retention, but that overall retention has increased over time, suggesting that improvements can be made. Because it is easier to retain participants than to relocate them once they have been lost to FU and because reducing attrition strengthens the value of longitudinal data, developing and validating effective interventions to ensure participant retention is critical.
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