The largest analysis to date of amyloid plaques in people’s brains confirms that the presence of the substance can help predict who will develop Alzheimer’s and determine who has the disease.

Two linked studies, published Tuesday in JAMA, also support the central early role in Alzheimer’s of beta amyloid, the protein that creates plaques. Data from nearly 9,500 people on five continents shows that amyloid can appear 20 to 30 years before symptoms of dementia, that the vast majority of Alzheimer’s patients have amyloid and that the ApoE4 gene, known to increase Alzheimer’s 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 Alzheimer’s. Such screening is increasingly used in research. Experts say previous trials of anti-amyloid 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 Alzheimer’s.

“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 Alzheimer’s Disease Center at the University of Texas Southwestern Medical Center at Dallas, who wrote an editorial about the studies.