Loss to Follow-up Update
Heads up about an important systematic review of the effects of attrition on outcomes of randomized controlled trials (RCTs) that was recently published in the BMJ.
- Key Question: Would the outcomes of the trial change significantly if all persons had completed the study, and we had complete information on them?
- Loss to follow-up in RCTs is important because it can bias study results if the balance between study groups that was established through randomization is disrupted in key prognostic variables that would otherwise result in different outcomes. If there is no imbalance between and within various study subgroups (i.e., as randomized groups compared to completers), then loss to follow-up may not present a threat to validity, except in instances in which statistical significance is not reached because of decreased power.
The aim of this review was to assess the reporting, extent and handling of loss to follow-up and its potential impact on the estimates of the effect of treatment in RCTs. The investigators evaluated 235 RCTs published between 2005 through 2007 in the five general medical journals with the highest impact factors: Annals of Internal Medicine, BMJ, JAMA, Lancet, and New England Journal of Medicine. All eligible studies reported a significant (P<0.05) primary patient-important outcome.
The investigators did several sensitivity analyses to evaluate the effect varying assumptions about the outcomes of participants lost to follow-up on the estimate of effect for the primary outcome. Their analyses strategies were—
- None of the participants lost to follow-up had the event
- All the participants lost to follow-up had the event
- None of those lost to follow-up in the treatment group had the event and all those lost to follow-up in the control group did (best case scenario)
- All participants lost to follow-up in the treatment group had the event and none of those in the control group did (worst case scenario)
- More plausible assumptions using various event rates which the authors call the “the event incidence:” The investigators performed sensitivity analyses using what they considered to be plausible ratios of event rates in the dropouts compared to the completers using ratios of 1, 1.5, 2, 3.5 in the intervention group compared to the control group (see Appendix 2 at the link at the end of this post below the reference). They chose an upper limit of 5 times as many dropouts for the intervention group as it represents the highest ratio reported in the literature.
- Of the 235 eligible studies, 31 (13%) did not report whether or not loss to follow-up occurred.
- In studies reporting the relevant information, the median percentage of participants lost to follow-up was 6% (interquartile range 2-14%).
- The method by which loss to follow-up was handled was unclear in 37 studies (19%); the most commonly used method was survival analysis (66, 35%).
- When the investigators varied assumptions about loss to follow-up, results of 19% of trials were no longer significant if they assumed no participants lost to follow-up had the event of interest, 17% if they assumed that all participants lost to follow-up had the event, and 58% if they assumed a worst case scenario (all participants lost to follow-up in the treatment group and none of those in the control group had the event).
- Under more plausible assumptions, in which the incidence of events in those lost to follow-up relative to those followed-up was higher in the intervention than control group, 0% to 33% of trials—depending upon which plausible assumptions were used (see Appendix 2 at the link at the end of this post below the reference)— lost statistically significant differences in important endpoints.
When plausible assumptions are made about the outcomes of participants lost to follow-up in RCTs, this study reports that up to a third of positive findings in RCTs lose statistical significance. The authors recommend that authors of individual RCTs and of systematic reviews test their results against various reasonable assumptions (sensitivity analyses). Only when the results are robust with all reasonable assumptions should inferences from those study results be used by readers.
For more information see the Delfini white paper on “missingness” at http://www.delfini.org/Delfini_WhitePaper_MissingData.pdf
1. Akl EA, Briel M, You JJ et al. Potential impact on estimated treatment effects of information lost to follow-up in randomised controlled trials (LOST-IT): systematic review BMJ 2012;344:e2809 doi: 10.1136/bmj.e2809 (Published 18 May 2012). PMID: 19519891
Article is freely available at—
Supplementary information is available at—
For sensitivity analysis results tables, see Appendix 2 at—