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Alzheimer Dis Assoc Disord. 2009 Oct-Dec;23(4):333-6. doi: 10.1097/WAD.0b013e3181aba588.

MRI substudy participation in Alzheimer disease (AD) clinical trials: baseline comparability of a substudy sample to entire study population.

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Department of Neurosciences, University of California San Diego, La Jolla, 92093-0717, USA.



To determine if a self-selected group of participants who enroll in an imaging substudy of Alzheimer disease (AD) clinical trials is representative of the overall study sample.


Baseline data from 2 ongoing AD clinical trials with 402 and 313 randomized participants were analyzed. Magnetic resonance imaging substudy enrollers (166 participants in trial 1 and 161 participants in trial 2) and nonenrollers were compared on baseline demographic, medical and clinical characteristics separately for the 2 trials.


In both trials, enrollers were statistically similar to nonenrollers on most demographic and clinical measures. One study sample showed that enrollers had lower cognitive scores at baseline when compared with nonenrollers: lower Mini Mental State Examination scores (20.15+/-3.6 vs. 21.04+/-3.6, P=0.02), and higher Alzheimer's Disease Assessment Scale-cognitive domain scores (24.99+/-8.5 vs. 23.03+/-9.3, P=0.03); however, the difference was not considered clinically important and was not observed in the second trial.


The groups of individuals who agreed to participate in the imaging substudies of AD trials were remarkably comparable to the comparison groups at baseline on a wide range of demographic and clinical measures; there seems to be a minimal effect of self-selection bias. The results indicate that it may be reasonable to generalize findings in an imaging substudy to the complete the study population.

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