© 2014 The Cochrane Collaboration. This is the protocol for a review and there is no abstract. The objectives are as follows: To determine the diagnostic accuracy of the plasma and CSF Abeta42 index tests for distinguishing Alzheimer's disease dementia from each of the other forms of dementia in people who meet the general criteria for a dementia syndrome. To investigate the heterogeneity of test accuracy in the included studies. We expect that heterogeneity will be likely and that it will be an important component of the review. The potential sources of heterogeneity, which will be used as a framework for the investigation of heterogeneity, include target population, index test, target disorder and study quality and are detailed in the analysis section.