"The largest analysis to date of amyloid plaques in peoples brains confirms that the presence of the substance can help predict who will develop Alzheimers and determine who has the disease. Two linked studies published Tuesday in JAMA also support the central early role in Alzheimers of beta amyloid the protein that creates plaques. Data from nearly  people on five continents shows that amyloid can appear  to  years before symptoms of dementia httphealth.nytimes.comhealthguidesdiseasedementiaoverview.htmlinlinenytclassifier that the vast majority of Alzheimers patients have amyloid and that the ApoE gene known to increase Alzheimers risk greatly accelerates amyloid accumulation. The findings also confirm that amyloid screening by PET scan or cerebral spinal fluid test can help identify people for clinical trials of drugs to prevent Alzheimers. Such screening is increasingly used in research. Experts say previous trials of antiamyloid drugs on people with dementia failed because their brains were already too damaged or because some patients not screened for amyloid may not have had Alzheimers. The papers indicate that amyloid imaging is important to be sure that the drugs are being tested on people who have amyloid said Dr. Roger Rosenberg the director of the Alzheimers Disease Center at the University of Texas Southwestern Medical Center at Dallas who wrote an editorial httpjama.jamanetwork.comarticle.aspxarticleid about the studies. Advertisement Dr. Samuel Gandy an Alzheimers researcher at Mount Sinai Hospital who was not involved in the research said doctors can feel fairly confident that amyloid is due to Alzheimers. But he and others cautioned against screening most people without dementia because there is not yet a drug that prevents or treats Alzheimers and amyloid scans are expensive and typically not covered by insurance. Dr. Gandy also said that findings that amyloid could appear as early as age  suggested that any medications to prevent the disease must be especially safe because well be exposing people to a drug for decades when they are healthy. The researchers scrutinized data from numerous studies. One analysis httpjama.jamanetwork.comarticle.aspxarticleid involved people without dementia who had normal cognitive functioning or subjective cognitive impairment a sense of memory loss httphealth.nytimes.comhealthguidessymptomsmemorylossoverview.htmlinlinenytclassifier despite normal memory tests or mild cognitive impairment or M.C.I. a condition often considered a precursor to Alzheimers. About  percent of M.C.I. patients had no amyloid possibly indicating their problems were linked to other dementias. With M.C.I. the first thing you think of is Alzheimers because if you hear hoofbeats youd think of horses not zebras Dr. Rosenberg said. Lo and behold you do a PET scan and a third of the time its negative the M.C.I. is something else like vascular disease or a neurodegenerative state that will not go on to Alzheimers.   https_id Of people without dementia those with more education were more likely to have amyloid than lesseducated people. Rik Ossenkoppele a neuropsychologist at Vrije Universiteit Medical Center in Amsterdam who led the research with Willemijn Jansen and Pieter Jelle Visser said this suggested that lesseducated people developed dementia sooner leaving fewer in studies of healthier people. He and others said this supported the cognitive reserve theory that more educated people have more brain pathways to compensate longer for cognitive decline. The second study httpjama.jamanetwork.comarticle.aspxarticleid of people with Alzheimers or other dementias  frontotemporal vascular or dementia with Lewy bodies  found that  percent of Alzheimers patients had amyloid. Dr. Ossenkoppele said that for the patients who did not have it the amyloid test could have been wrong or the patients could have had another dementia or possibly several diseases making amyloid less prominent. In other dementias amyloid was present but in fewer people. More people with those dementias had amyloid as they aged but in Alzheimers httphealth.nytimes.comhealthguidesdiseasealzheimersdiseaseoverview.htmlinlinenytclassifier patients amyloid presence decreased as they grew older. We dont know why amyloid goes down with age in Alzheimers said Dr. John Hardy a neuroscientist at University College London who was not involved in the research. It could be that the number of nerve cells is going down and amyloid is produced by nerve cells. The research showed the ApoE gene had a bigger effect than some people expected Dr. Ossenkoppele said. Even when people had one copy of a rarer ApoE gene that protects against Alzheimers they were still at high risk of having amyloid if they had the ApoE gene."