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Neurology supplements are not peer-reviewed. Information contained in Neurology supplements represent the opinions of the authors and are not endorsed by nor do they reflect the views of the American Academy of Neurology, Editor-in-Chief, or Associate Editors of Neurology.

Dropouts and refusals in observational studies

Lessons for prevention trials

From the Division of Neurology, Department of Medicine, University of British Columbia, Vancouver, BC, Canada.

Address correspondence to Dr. Howard Feldman, Division of Neurology, UBC Hospital, S 192-2211 Wesbrook Mall, Vancouver, BC, V6T 2B5, Canada; e-mail: hfeldman{at}interchange.ubc.ca

The success of prevention trials of Alzheimer disease and other dementias (AD/dementia) hinges on their ability to recruit and retain at-risk study populations. Losing subjects is a threat to the power to detect a treatment effect and, potentially, to the validity of these studies. Observational cohort studies accumulate data around participant outcomes that can help to inform the design of future prevention trials. Our objectives were to investigate the rates of refusal and dropout within observational cohort studies and to evaluate their characteristics and impact. This study examined data from the Canadian Cohort Study of Cognitive Impairment and Related Dementias (ACCORD), a 2-year observational cohort study of patients newly referred to dementia clinics. The sample included 124 Not Cognitively Impaired (NCI) and 342 Cognitively Impaired Not Demented (CIND) subjects. Subjects who refused initial neuropsychological (NP) testing were compared to those who completed NP testing and subjects who dropped out to those who attended follow-up. Refusal was common, with 40% of subjects not completing neuropsychological testing at baseline. Dropout was also substantial, with 55% lost to the 2-year follow-up. Subjects who refused NP testing were significantly older and less educated. CIND refusers had lower cognitive and functional scores at entry and a 2-year progression rate to dementia twice as high as that of non-refusers. CIND dropouts also had lower baseline cognitive and functional scores. These observations suggest that dropouts and refusals in prevention trials include those subjects who are at high risk for progression to AD/dementia. Targeted strategies to retain these individuals within prevention studies will be needed to achieve sufficient study power and validity.

This research was funded by the Medical Research Council of Canada (now CIHR) and the Pharmaceutical Manufacturers Association of Canada, MR PMAC program (Grant No. PA14197). Sponsors of the PMAC included Sandoz, Smithkline, Beecham, Pfizer Canada Inc., Hoffman LaRoche Ltd., Janssen Pharmaceutical Inc., and Astra. C. Jacova is funded by a post-doctoral CIHR/MSFHR Training Fellowship in Neurobiology and Behavior. H. Feldman receives support through CIHR operating grant IAP-73376 "Preventing AD and Related Dementias."

Publication of this supplement was supported by an educational grant from Beaufour Ipsen.

Disclosure: The sponsor has provided H.F. with an honorarium for his participation in this project.

Neurology supplements are not peer-reviewed. Information contained in Neurology supplements represents the opinions of the authors and is not endorsed by nor does it reflect the views of the American Academy of Neurology, Editor-in-Chief, or Associate Editors of Neurology.